•Potato Growing In Alaska Curtis H, Dearborn
A-00246
July 1984 Revised February 1990 COOPERATIVE EXTENSION SERVICE University of Alaska Fairbanks and U.S. Department of Agriculture Cooperating
Abstract Potato (Solanum tuberosum) has been grown in Alaska since 1800 and is recognized as an important human food. Most destructive insects and diseases of potato in lower latitudes are not present in Alaska. Potatoes bred and selected for adaptation to Alaskan environments are also useful in lower latitudes. Soils used for potato growing are of many types. Regardless of type, those above 1,000 feet elevation have an environment too cool for good yields. In the preparation of land for planting, delays between preparation and planting most often result in a significant and undesirable loss of soil moisture. A single application of fertilizer in bands at planting provides the crop with its growing season requirements of N, P, and K. Heavy fertilization is a poor practice because it prolongs top growth and delays tuber set, both of which reduce quality and yield. Irrigating should be carefully planned to avoid destroying the effects of chemical weed control treatment and hilling practices. Alaska Frostless can be grown in flat culture in high-density plantings because it sets its tubers deep in the soil. Vine killing with the objective of increasing toughness of tuber skin is not practical, because the harvesting must be delayed about three weeks to obtain adequate skin-set. Field application of a sprout inhibitor is feasible on newly frosted tops or in connection with a top killer that does not destroy the leaves within fortyeight hours of application. Pest control is largely one of weed control, except in the production of seed potatoes where guarding against imported pests is extremely important. Harvesting and handling of Alaskan potatoes requires more attention to prevent bruising than is exercised in regions where potato tops die and tuber skins set before the tubers are lifted from the soil. Vine beating the day before harvest, when coupled with conditioning the newly dug crop in storage, is practical and highly desirable. Conditioning of tubers to promote rapid suberization immediately following storage allows the crop to be held in good condition for nine to twelve months. (Suberization is the process whereby the skin of the potato heals the cuts, nicks, and minor damage caused by harvesting techniques.) Disease-free seed stock that is treated similarly can be held in good condition at 33 to 36 degrees F for even longer periods.
TABLE OF CONTENTS
Foreword History Energy for Survival and Security Plant and Tuber Development Varieties Characteristics of Good Seed Potatoes Seed Tuber Preparation for Planting Soils for Potatoes Land Preparation Fertilizing the Potato Crop Methods of Applying Fertilizers Planting Pests and Their Control , Irrigation Hilling Potatoes ,, Vine Killing, Top Removal, and Sprout Inhibition Harvesting and Handling of Potatoes . , Storage of Potatoes for Table Use, Processing, and Seed . . . Literature Cited . ,
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Foreword
This publication is a tribute to Dr. Curtis H. Dearborn, who passed away May 13, 1982. Dr. Dearborn's professional experience as research horticulturist with the Agricultural Research, Science, and Education Administration, U.S. Department of Agriculture, and University of Alaska Agricultural Experiment Station extended over 30 years. He devoted many years of research and study to the development of potato varieties suitable to Alaska's growing conditions. His contributions to horticultural research have helped to ensure that vegetable and fruit production will continue to increase and succeed throughout Alaska. A publication that was written by Dr. Dearborn and published by the University of Alaska Cooperative Extension Service in 1981, Strawberries in Alaska, continues to be popular and provides the Alaskan grower with information to successfully grow and manage this very popular fruit crop. Dr. Dearborn's contributions will continue to benefit everyone who is interested in Alaska's agricultural future. Wayne Vandre Horticultural Specialist Cooperative Extension Service University of Alaska
History Just when the potato (Solanum tuberosum) was first grown in Alaska and by whom is not likely to be known, Bancroft (2) wrote that personnel of the Billings Expedition in 1790 saw gardens planted with cabbage and potatoes on Kodiak Island at Three Saints Bay. Dall (3), in his travels on the Yukon River in 1867, learned from the Russians that potatoes had been grown and seed carried over from year to year in the village of False Pass since the beginning of that century. He also found that potatoes had been grown at Fort Yukon, but it is not clear if they came via the Russian American Company or the Hudson Bay Company In 1897 Walter H, Evans and Benten Killen, studying
the agricultural situation in the coastal areas of the Gulf of Alaska for the U.S. Department of Agriculture, found potatoes in numerous small gardens. Early records of activities at the Sitka Agricultural Experiment Station (1) show that potatoes were one of the crops worthy of improvement and production throughout accessible areas of Alaska. Potatoes were brought to Alaska from the United •* States during the gold rush at the turn of the twentieth century A few persons in Alaska began to producepotatoes for sale or barter and to realize that varieties^ differed in their productivity
Energy for Survival and Security The potato is a most remarkable food plant for Alaskans. It is the only starchy food plant grown in Alaska whose storage organ, the tuber, is below ground essentially the year-round, either in the field or in manmade storages. It is, therefore, reasonably wellprotected from external contaminants including nuclear radiation fallout. Alaska has the potential to produce far more potatoes than Alaskans can eat in any and all forms. Potato varieties adapted to Alaskan environments will produce 10 to 25 tons of potatoes per acre in ninety to 120 days from planting. For two months after harvest the buds or eyes of the tubers are physiologically in a rest period and will not sprout unless treated chemically. A few seedlings held at 50 degrees F will not sprout for four months. The buds of most potatoes will begin to grow slowly after the rest period if held at temperatures above 38 degrees F. Potatoes can be held in good eating conditions for a year or more at 33 to 37 degrees F (4). Even though man's digestive system does not assimilate much energy from uncooked potato starch, in an emergency one could survive on raw potatoes alone for several months. Raw potatoes have sugars as high as 12 percent; vitamins, particularly Q minerals;
and fiber along with 75 to 80 percent water in the tubers, which provide considerable nourishment. When the tuber is cooked, it provides 17 to 21 percent digestible starch, or 100 percent of the energy food that man needs. Very high quality processed potato products can be made from some Alaska-bred varieties. Potato products made from Alaska-grown potatoes can add a significant measure of security to the diet of Alaskans, who are largely dependent on processed potato products made at lower latitudes and shipped to Alaska. Many people who have never seen a potato growing win search for explicit details on how to grow them in these subarctic and arctic environments. The following summary of information for successful potato growing in Alaska is based on 31 years of research on potato improvement and culture in this state. Emphasis of the information set forth is placed on the practical significance to growers.
Plant and Tuber Development An understanding of the structure and functioning of the potato plant and its tubers gives a grower great insight into solving problems of production and storage that otherwise might be quite baffling. Potato plants of some varieties produce flowers, and their pollinated flowers produce fruit. These small, green fruits (Figure 1) are borne in clusters on stalks of varying lengths in the upper part of the plant. The fruits remain green or finally turn deep purple but are
never edible. Their firm interior has two or more compartments in which small, white or tan seeds develop (Figure 2). These seeds are called "true seeds" and, if planted, will produce potato plants and tubers. These plants and tubers will differ in some respects from each other and from the original plant that produced the green fruit, because each seed has a separate genetic identity acquired partly from the mother plant and partly from the pollen that provided fertilization. In
Figure 1.
Fruits of potato contain true seed resulting from pollination of the flowers. Tuber formation and growth are not dependent on flowering or fruiting.
Figure 2.
Seeds of potato extracted from fruits like those shown in Figure I . Plants grown from seeds that develop as a result of insect pollination usually do not offer many choices for selecting a better commercial variety.
Figure 3,
Stolons radiating from nodes of the stem of potato plants. Note that the lower stolon is first to tuberize. Tuberization should occur on all stolon tips at the same time to provide for uniform tuber size at harvest.
Figure 4.
Tubers have formed on the stem above ground at the junction of leaf stalk and stem and are called axillary tubers or aerial tubers. These green tubers are too bitter to be palatable.
contrast, plants grown from tubers of the original plant will, with rare exception, produce plants and tubers identical to the original plants and tubers. Persons who plant red-skinned, white-skinned, or russet-skinned potatoes in the same row need not fear that the tubers of the several kinds will be modified through crosspollination of the flowers of their respective mother plants. In the production of the tubers, it matters not whether flowers are produced and pollinated, because the tuber is a food storage organ developed on a horizontal, underground stem called a stolon (Figure 3). Enlargement of the tip of the stolon is the beginning of tuberization and is not dependent on flower formation. Stolons originate only at the nodes, or joints, of the plant stem and not from the roots of the potato
plant. Growth from nodes above ground usually develops into lateral branches that form a part of the potato top and foliage. When the stem at the ground line is injured by the black scurf fungus, Rhizoctonia solani, or a large branch is partially split off the main stem, sugars moving down the stem accumulate above the partial girdle. An aerial tuber develops in the axil of one or more of the injured stem's branches (Figure 4). This illustrates the similarity of an apparently dormant growing point in a node of the stem above ground and the tuberization of a stolen from a node below ground. The setting of tubers on stolons of most varieties takes place early in the plant's growth and over a short period of time—seven to ten days (Figure 5). Varieties that tuberize over longer periods or that are sensitive to environmental changes produce an array of tuber sizes
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Figure 5.
Tuber sef has been quite uniform, but it differs from most varieties in that the set is on lateral stolons.
Figure 6.
Large and small tubers on a single plant show that tuberization extended over a considerable period of time. A lull in tuberization stimulus allowed the apical bud of the large tuber to extend a sprout, the tip of which was induced to tuberize following three or four days of high light intensity and high air temperatures.
that may not be desirable (Figtire 6). The variety Alaska Frostless (5), due to its genetic make-up, responds to environmental changes in several ways. Early in the season the tuberizing stimulus is inadequate to stimulate all stolons into tuberization. A few stolons continue to extend horizontally. They finally emerge as new plants 6 to 15 inches from the parent stem (Figure 7). Temporary loss of dormancy of the tuber apical bud (at the tip of the tuber) can result in a second tuber sessile with (issuing directly from) the first and of the same size. Climatic changes that occur a little later in the season may stimulate an eye in the apical bud of a growing tuber to extend a stolon. This soon may be caught by the tuberizing stimulus, which causes a second tuber to be formed beyond the first, as shown in Figure 6. This environmental influence produces gradations from the tuber-stolon-tuber pattern of Alaska Frostless to the dumbbell shape in the variety Green Mountain (Figure 8). Tubers of potatoes are interesting structures (Figure 9). In cross section their anatomy is that of the stem. As the miniature tuber grows surrounded by soil particles, stretching and abrasive action results in the loss of the epidermis, or skin. Tissues below suberize and form a secondary outer layer that we call skin. Buds on the tuber, like the buds on the stem, are arranged in a spiral or in two spirals. Spirals of buds of some clones may be either clockwise or counterclockwise. Specimens of both rotations are present in some hills (Figure 10). So far, studies have not been made to determine if productivity is associated with direction of rotation of the spiral of eyes. A longitudinal cut through a purplefleshed tuber illustrates how the eyes connect to the vascular ring of conducting tissues (Figure 11). Here we
Figure 7. Stolon plants on the variety Alaska Frostless and some other frost resistant potato seedlings appear quite early in the season but do not seem to influence yield, because there is an abundance of stolons on which tubers are produced by these kinds of potatoes.
can see that pith tissue (a centrally located area of stem tissue), constitutes the larger portion of the tuber. Pith streaks branch from the central pith like the limbs of a tree and run through the vascular ring and cortex to the eye. This vascular ring is where ring rot, Corynebacterium sepedonicum, a serious bacterial disease of
potatoes, causes disintegration of the tissues and rotting of the tubers. Generally, ring rot bacteria gets into new tubers through the stolon that connects the tuber to the parent plant. In such cases, one can assume that the seed piece that produced the parent plant was infected.
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Figure 8.
Dumbbell shaped tubers are an indication of a variety's sensitivity to environmental change; usually, only the first tuber formed takes on this shape.
Figure 10. The eyes of the potato tuber from which buds emerge and develop stems or stolons are arranged in a spiral from the basal or stolon end of the tuber to the apical growing point on the other end of the tuber. Buds of some varieties are in two spirals. On the tuber at left, rotation of buds is clockwise, whereas on the other tuber, it is counterclockwise.
Figure 9.
Cross-sectional view of a potato tuber showing the thin skin area, the cortex, and a ring around the tuber between the cortex and pith that is called the vascular ring. Most of the energy is stored as starch in the large central area of the tuber.
Figure 11. Longitudinal cut of a tuber showing that the vascular ring of conducting tissue radiates to the buds.
Varieties Potato varieties can differ in many respects. Many differences are apparent on inspection. Growth of tops, flower color, frost resistance, earliness, tuber size and appearance, keeping quality in storage, and scabbiness of the skin are visible characteristics. Choice of proper varieties by the home gardener, market gardener, or commercial grower may be the most important decision toward assuring that a successful crop of tubers will be produced and marketed. Choice of variety may be determined by climate, use to be made of the crop, soil conditions, and available storage facilities. Tops of Alaskan varieties remain vigorous until frozen, and the skin of most varieties remains tender for three or more weeks after the vines are frozen or otherwise destroyed. Alaska 114 and Alaska Frostless are varieties whose tubers develop a tough skin early in tuber growth; this reduces their chances of being infected by soil bacteria and fungi during and after harvest. At this time Alaska Frostless is the only named variety of white potato that has enough frost resistance in its foliage to be significantly more tolerant to field frosting than any other variety. Growers in frost-hazardous areas now may choose to plant Alaska Frostless, which withstands several moderate frosts and continues to increase tuber yield after the tops of frost-susceptible varieties are killed. Potato earliness in Alaska refers to productivity of tubers, whereas in lower latitudes the term is used to describe the vegetative condition of potato tops. There, early varieties are those whose tops decline in vigor, while the environment still would appear to be favorable for growth. The skins of tubers from these vines toughen as the tops decline in vigor. Varieties that approach their maximum productivity in 85 to 90 days in Alaska are called "earlies." If their yield is high, their dry matter content will be low, because the tops have not had time to fill the tubers with starch. At elevations approaching 1,000 feet the growing season is usually short, so one should choose a variety that •sets few tubers per hill. Those that set many per hill do not have sufficient length of growing season to provide the supply of manufactured food needed to build desired tuber size. The same principles hold for the market gardener who wants potatoes for early market. Late varieties in Alaska are those that require the full growing season to reach their maximum productivity. The "lates" also show marked differences in dry matter (starch and mineral content) that reflect genetic differences in the varieties. As an example, Denali in some seasons contains 25 percent dry matter, whereas Alaska 114 in the same years contains 23 percent. Soil acidity, or pH, may determine which variety one should plant. Streptomyces scabies, the actinomycete that causes potato scab disease (Figure 12), produces the most lesions when the soil reaction is between pH 5.8 and 7.5. Much of Alaska's tillable soil comes within this range. Virgin soils with pH 4.8 to 5.7 may not support maximum growth of potato because the soils do not carry adequate phosphorus to supply the plants'
needs. Soils that have been cropped for many years, manured, and/or limed often produce scabby potatoes. Persons who frequently add lime, wood ashes, shellfish waste, or egg shells to their potato land can expect to intensify scab, because these additives all carry calcium, which makes the soil less acidic. Raising the pH above 7.5 with calcareous materials may reduce scab infection, but nutrient deficiencies also may accompany the pH change. Scab-resistant varieties planted on scab-infested land generally produce crops that are marketable. Other varietal differences not apparent on inspection require mechanical, chemical, and cooking tests to show some of their characteristics. Dry matter content may be the deciding factor on choice of variety. Dry matter of fresh tubers can be predicted from specific gravity readings taken on a representative sample of a variety. High rainfall or continually moist conditions promote succulent growth and produce tubers of a lower dry matter content. Such potatoes are good for pan-frying because they do not crumble, but when they are boiled or baked, their texture tends to be waxy or soggy To avoid this sticky texture condition, one should select high dry matter varieties, such as Denali, Snowchip, and Alaska 114. If production is for the manufacture of potato chips, one must choose high dry matter varieties that have been selected for their light chip color or for a particular chip color, such as red or yellow. If the potato is for French fry manufacture, the processor will prefer the long tuber type varieties that have a minimum of waste in preparation. For the baking trade, uniform 12-ounce tubers are preferred. Storage facilities, or lack of them, may determine the
Figure 12, Potato scab caused by a soil borne * microorganism. It may be carried over winter to the new crop via the ruptured skin pustules and infect newly forming tubers.
varieties to grow, because the storage temperature must be kept at 38 degrees F or lower for such varieties as Snowchip, Sebago, Norgold Russet, and Ontario. Otherwise, these varieties will develop sprouts and be unsuitable for market by mid-January. Without refrigeration, other varieties usually can be held in storage until May before sprouting becomes troublesome. Growers
who do not plan to store their crop, but choose to market it shortly after harvest, should grow varieties that have a tough skin that will maintain good appearance at the market place.
Characteristics of Good Seed Potatoes Seed potatoes should be firm and free from diseases, insects, and nematodes. Although firm tubers are most desirable for seed, wilted and shriveled tubers of disease-free stock are quite acceptable, provided that the spongy condition is not a result of holding the crop for eight or nine months at 50 to 60 degrees F from harvest to planting. Under continuous storage at this temperature range, undesirable physiological changes in the tubers occur in addition to sprout development; some varieties handled in this manner generate one or more small tubers (Figure 13) within three weeks of planting but do not develop a plant. This growth habit is called "sprout tuber" and is essentially a total loss to the grower, except that these new tubers can be used, if necessary, to perpetuate the variety. Like most other newly formed tubers, they require a rest period before they will sprout. Crops grown from sprout tubers are normal and characteristic of the original variety. Only a few varieties of potatoes have been cultured to determine if they are susceptible to sprout tuber. Of twelve well-known varieties tested, Belrus, Nampa, and Ontario were the only ones that developed sprout tuber. Occasionally, potato sprouts are used as starters and are planted much the same as onion "sets." Usually, sprouts develop only two or three tubers per hill. Potatoes have been grown from true seed in the tropics for many years, and the practice is now being adapted as a method for potato production in the temperate zone. If such a practice were adopted in Alaska, the seeds would have to be started indoors and the seedlings transplanted to the field; otherwise, the growing season would be too short for economic yields to be realized. Production from true seeds would lead to more genetic variability in the crop, thereby losing some of the refinements locked in by vegetative reproduction. If all tubers are of the same variety and disease free, the sample is referred to as foundation grade. Such a
Figure 13. Sprout tuber development is undesirable. Small tubers are generated directly from the seed piece without having any potato top above ground. sample is not likely to deviate from this grade in Alaska, unless the seed pieces are mechanically mixed with another variety or are contaminated with maladies imported to Alaska from other potato growing regions. The supply of seed of varieties adapted to several Alaskan environments has improved in recent years. Commercial growers should be extremely cautious of bringing seed potatoes into Alaska from other regions because of the possibilities of introducing serious diseases. Home gardeners should use only Alaska-grown seed, because table stock potatoes from the grocery store may have been treated to prevent them from sprouting, or they could be carrying destructive potato diseases.
Seed Tuber Preparation for Planting The commercial grower and the home gardener are faced with similar conditions as they prepare seed for planting. Whether they use red, white, or russet skin varieties, preparation for planting is the same. Assuming that the commercial grower's seed is in
storage at 38 degrees F and untouched since harvest, it should be warmed in place, if possible, before grading; otherwise, many tubers may shatter crack in handling because they are so crisp (Figure 14). If the potatoes are on pallets, they can be moved to a 50 degree F room
minimum of cut surface. This is well-illustrated by the tuber that was cut into four blocky pieces, in contrast to the other tuber that was cut into four pieces alone the length of the tuber. It's easy to see that there is afof more cut surface on the one cut lengthwise thanjn the one cut crosswise. "..,_' When potato eyes are scooped from a tuber with a melon-ball scoop, the'ball is called a seed eye. Such seed eyes are all right as starters but do not contain enough food reserves to get a crop off to a good start in Alaska. Seed potatoes for planting in Alaska should weigh at least 2 ounces. The stored energy in a seed piece of this size is sufficient to carry the new plant through its early growth. Occasionally, cut seed is planted in soil that is so dry that the seed piece loses moisture to the soil. Under these conditions, cut seed weighing 2 ounces generally has enough moisture in it to support the beginning of growth of a root system, which enables the developing plant to acquire moisture needed to support continued growth. Small, whole tubers planted in dry soil hold their moisture better than cut seed, Moreover, in cold, wet soil, whole tubers are less subject to seed piece rot, because the skin is intact and will prevent microorganisms from entering the seed piece. Small tubers that are small only because Figure 14. Shatter cracks of this kind occur when tubers are bruised after having accumulated high concentrations of sugars in their cells which, in turn, hold high volumes of water. This hydration pressure can occur in potatoes in storage that have converted some starch to sugar. The same cracking occurs at harvest under wet soil conditions and cool temperatures.
and held for two days to warm uniformly. If they are in bins and the storage must be kept at 38 degrees F, the tubers should be handled carefully to avoid shatter cracking. This kind of cracking in itself will not interfere with sprout growth, but it does provide breaks in the skin that allow decay organisms to enter. Another need for minimizing shatter is that the crop might be sold for fresh market potatoes instead of seed, and in the fresh market, the cracking would downgrade the lot. Potatoes for seed purpose should be passed over a grader and examined for the presence of disease. The lot may be sorted by size for convenience in cutting and planting. Tubers that show storage rots (wet or dry) should be discarded. Tubers that weigh more than 12 ounces may pose a problem for machine cutting, and tubers of less than 2-inch diameter need not be cut, so three size classes are obtained. A seed piece containing the stolon cavity should have one good eye in addition to the one in the stolon attachment area, because the latter may not develop a sprout. Whole tubers or pieces that contain an eye cut from tubers are called seed pieces (Figure 15). Seed pieces should be blocky with a
Figure 15. Two tubers cut in quarters for planting. Short, blocky seed pieces are best for planting because they have a minimum of cut surface from which water is lost, a minimum surface for disease entry, and cause less clogging of the planter where mechanical planters are used.
they did not have time to grow larger are acceptable seed. However, small tuber size is sometimes the result of potato diseases. This kind of seed should be avoided because it may produce only small, disfigured potatoes. It is not unusual for large tubers of some varieties to be hollow at the center. If the tissues of the cavity are firm, the cut seed pieces of such a tuber are suitable for seed purposes. If the void extends to the stolon cavity, it may harbor one or more potato diseases, and such tubers should not be used for seed., The composition of a potato tuber changes while in storage from harvest to planting in response to its environment, and some knowledge of what happens under certain conditions can be valuable to all who handle potatoes. When the temperature of stored potatoes declines from 50 degrees F to 38 degrees F, some starch is converted to sugar by enzymes within the tuber. The rate of conversion and accumulation of sugar is more rapid as the decline continues to 33 degrees F, As the temperature rises through this range, the physiological change is reversed, and above 38 degrees F sprouting becomes apparent on most varieties. Sprouting of most varieties occurs first on the lead bud of the apical whorl (circular arrangement of buds). Buds in the eyes on the remainder of the tuber remain essentially dormant. When potatoes are removed from cold storage to a warm room, their accumulated sugars begin to burn away in respiration and growth processes. All potatoes for seed should go through this process before planting for two very important reasons: First, the healing of cut surfaces proceeds rapidly on tubers whose physiology has changed from the cold, dormant condition of storage to the rejuvenation of growth at a warmer temperature; and second, potato tubers that have been warmed for seven to ten days emerge more rapidly after planting than do those that move from cold storage directly to the soil. Rapid emergence shortens the time when the young sprouts are most vulnerable to stem girdling fungi such as Rhizoctonia sola.nl During the warm-up period the skin of tubers exposed to indirect light will become green. Tubers exposed to direct sunlight also lose moisture that may be needed for initial phases of growth. This start of growth, which is called "green sprouting," advances potato plant growth over non-green sprouted tubers by eight to ten days of plant development under normal growing conditions. This is a very desirable treatment
for seed potatoes planted in regions with a short growing season or planted by market gardeners who wish to market their crop early. Small tubers are welladapted to the green sprouting technique. Large tubers that require cutting should be handled differently to accomplish the same purpose. They should be cut before exposure to prolonged sprouting conditions, because the apical bud of most whole tubers starts to grow before all other buds on the tuber. This difference in bud growth prior to planting can result in very uneven plant emergence. Large operators may have to tolerate uneven emergence unless they have facilities for suberizing cut seed-, otherwise, cut seed will lose moisture rapidly, suberize slowly, and rot in the container if planting is delayed several days. Disease free seed that is suberized at 60 to 65 degrees F for a week at 85 to 90 percent relative humidity may be held in 40 degrees F storage for fifteen to twenty days if planting must be delayed. Good air circulation should be maintained during the curing process. Scooped-out, melon-ball-size seed pieces are too small to assure a good crop if planted conventionally. They weigh less than 1 ounce each and do not provide the energy needed to produce maximum yields in short-season areas. It is economical to start a clone from such seed pieces but impractical for maximum production. Home and market gardeners who use cut seed may improve sprout emergence and early tuber yield by promoting more extensive sprout growth prior to planting. This necessitates careful handling of the seed piece to prevent sprout breakage when the seed piece is set in a manner that allows the tip of the sprout to protrude slightly above ground. A potato seed piece handled in this way is the equivalent of a transplant. In the past, chemical seed treatment that contained mercury compounds has been used to control potato scab and black scurf; however, the health hazards associated with such compounds have resulted in its removal from the market. Other compounds have been developed and are helpful under most circumstances. For small plantings, some reduction in the spreading of scab can be obtained by trimming the pustules (raised portion of infected tissue) from infected tubers that are to be planted in clean ground. Currently, disease free seed is the large grower's only practical alternative for avoiding scab spread to his clean land.
Soils for Potatoes Soils for potato growing can be of many types and characteristics. Ideally, one would choose a friable (crumbles easily), sandy loam soil, free of stones, and containing enough organic matter to provide good water holding capacity. With this kind of medium for the developing root system and support of the plant, one can add fertilizer elements and water as necessary to promote adequate top and tuber growth. Potatoes 8
will grow under nearly any mulch or in any soil type. Tuber shape is usually most representative of a variety when it is grown in light, sandy soils. The same variety grown in heavy clay soils tends to produce more flattened tubers. In place of soil, numerous materials can be used for the growing medium as long as the medium does not contain substances in quantities that are toxic to the potato plants. For example, potatoes can
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be grown in straw material where it is dense enough to provide an anchorage for the roots and some support for the stem. Plates of tundra, such as those which can be cut from dry lake beds in the Arctic, are suitable for small garden operations, as are sawdust or shavings. Plantings made in highly organic materials should be provided with more soluble fertilizer salts than potatoes grown in mineral soils, because one must satisfy both the plant requirements and the nutrient requirements of bacteria and fungi that are drawing on the nutrient supply simultaneously. Home garden soils should be at least 6 inches deep; otherwise, drouth conditions are likely to limit plant
growth. Where the potato crop is planted and harvested mechanically, there should be at least 12 inches of soil and good drainage to prevent the crop from being submerged during rainy periods. Soil reaction is not critical to good growth within the range of pH 5.8 to 7.5. However, at either extreme trace elements necessary for plant and tuber development may have to be added to the soil to provide a proper nutrient balance. Acid soils with pH 5.2 to 5.7 and alkaline soils of pH 7.6 and above usually produce scab-free potatoes. Liming of potato land is not practiced because it tends to increase scab problems.
Land Preparation Since the soils on which potatoes can be grown differ tremendously in texture, particle size, and composition, methods of preparation also must differ in arriving at the best tilth (physical condition of a seedbed) for handling the crop. Clearing of timbered land in the past has been mainly by dozer blade shearing action on frozen ground followed by a scarifier blade and root rake. Such clearing leaves much debris that interferes with mechanical potato planting and harvesting. With the advent and general use of the rototiller in land clearing, potatoes can be handled as well on this kind of clearing as they can be on lands cleared by dozer shearing. In either case, there will be sufficient obstructions to potato machine planting and harvesting to force the operator to pick tree roots and broken limbs. Disk harrows are still used to cut up lumpy soil and may continue to be useful. However, the rototilling principle usually leaves the better seed bed. In grassy areas, small clumps of grass tend to remain above ground following rototilling, whereas they are covered by disking. Consideration should be given to compacting the soil surface lightly with a land roller (Figure 16), if the planting operation must be delayed and soil moisture loss by evaporation is likely to be high. Customarily, grain has been the first crop planted after clearing, with the belief that much root picking could be avoided; actually, it has been just a matter of deferring the job. Another disadvantage of the practice is that Alaska's seed laws allow some weed seeds in seed grain, and this brings new weeds to potato land that otherwise would not be there to plague the potato grower. Lands that have been cleared and cropped for several years reflect the loss of organic matter in the pattern of drouth spots and wet spots that persist. A few cardinal rules for maintenance of good soil tilth are (a) avoid working the soil when it is wet; (b) open for planting only as much land as can be planted in a day if the soil is exposed to rapid drying conditions; (c) compact the soil by rolling it with a packer; and (d) when overhead irrigation is used, open the system and wet the soil surface for several minutes. Turn off the overhead sprinkler for five to ten minutes, then proceed with
normal irrigation. This procedure tends to prevent puddling and runoff from soils that absorb water slowly. On a small scale the spade is still a useful tool for preparing a plot for potatoes; however, a small rototiller is ideal for land preparation of plots up to 1 acre in size. Prior to the actual tilling operation, one should examine the plant and insect situation, because some pests can be eliminated before ground breaking. Quackgrass, fireweed, horsetail, and sedges can be treated chemically before tilling the soil, or their presence can be recognized and preparation made to control these weeds after planting. Similarly, during land preparation, control of wireworms, cutworms, slugs, and rodents can be planned. Remember that the crop you plant replaces the previous food source of these pests.
Figure 16. Custom-built cultipacker levels the ridge left by hiller disks, conserves soil moisture, and increases the effectiveness of weed con trol sprays by reducing soil surface area. Lever attached on a pivot point allows the packer to be lifted for travel on rubber tires.
Fertilizing the Potato Crop Potato plants absorb some of their essential elements for growth from the air; these elements include carbon, oxygen, hydrogen, and a little nitrogen and sulfur from raindrops. The major elements nitrogen, phosphorus, and potassium; the secondary elements calcium and magnesium; and the trace elements manganese, molybdenum, iron, zinc, copper, boron, and sodium are absorbed by the roots from the soil solution. A portion of the crop's nutritional requirements may be met by decaying plant and animal matter, but while this decay is proceeding, the potato may be starving for the nutrient element temporarily tied up in the cells of the fungi and bacteria that are working on the plant debris. Trace elements and phosphorus may also be in short supply as pH changes occur and cause the elements to be insoluble in water. Drouth conditions increase the concentration of soluble salts in the soil solution. The plant's capacity to absorb nutrients from the solution decreases as the concentration of salts in the soil solution increases. Composition of the soil in most cases is closely related to the composition and the degree of weathering of the parent rock from which the soil was formed. If the parent rock is from a high limestone region, the soils are likely to be alkaline or near neutral (pH 7.0). Soils may be quite acidic if they are high in iron. Research has shown that potato top and tuber growth on most soils of Alaska are limited by the low available phosphorus in the soil. Thus far, a commercial fertilizer, 8-32-16, containing 8 percent nitrogen-, 32 percent phosphorus pentoxide expressed as P2O5i and 16 percent potassium oxide (K2O) supplies potato crop needs when used at 750 to 900 pounds per acre. A complete soil test will determine total fertility requirements. The nitrogen-phosphorus-potassium ratio may need to be changed in subsequent years as certain nutrients accumulate in the soil. Nitrogen is necessary for lush leaf and stem growth; however, some growers tend to apply more nitrogen than is desirable for high quality and maximum tuber production. The objective is to supply just enough nitrogen to allow the variety to develop a large top rapidly Continuing lush top growth delays tuber formation and reduces yield, because the sugars for tuber growth and starch storage are being used to grow new leaves instead of tubers. Nitrogen-deficient plants move nitrogen from the older, lower leaves to new leaves developing in the top of the plant, and consequently, the lower leaves fade and turn yellow (chlorotic). Phosphorus is essential for good root growth. Potassium in fertilizers that are used for growing potatoes for processing should be in the form of potassium sulfate rather than potassium chloride. The plants are more likely to build a higher starch content in the tubers when they are supplied with the sulfate form. Commercial fertilizers usually contain the chloride form called muriate of potash. When potassium is deficient in the soil solution, the leaves of
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Figure 17. Leaf scorch due to potassium deficiency develops rapidly on some varieties of potatoes and ruins the crop in mid-season.
healthy plants develop a very dark green color followed by "firing" of the margins of leaves midway up the plant (Figure 17). If the potassium shortage continues, the leaves, including those of the growing point, turn brown and crisp and drop to the ground. This generally occurs in early to mid-July and reduces yields to barely more than the return of the seed used for planting. Potatoes grown on soil that is very low in potassium bruise easily at harvest and develop brown, sunken lesions and a cup-like, corky spot under the skin (Figure 18). Tuber starch content is very high under these conditions. Other elements in short supply may limit plant growth. Magnesium deficiency has been conspicuous in the Tanana Valley Magnesium moves up from the lower leaves and causes an interveinal, yellow mottling that is referred to as interveinal chlorosis. The veins remain green with magnesium deficiency, whereas with nitrogen deficiency, the whole leaf becomes yellow. Excesses of the major plant food elements seldom produce destructive symptoms on potatoes in Alaska, but an excess of a minor or trace element can significantly depress plant growth and yield. Boron in the fertilizer or in irrigation water may exceed the tolerance of potato. If this occurs, the plants tend to be spindly in growth habit, and their basal leaves fire along their margins and crop off, much as if scorched by fire (Figure 19). Commercial fertilizer ratios that have been used successfully for potatoes in Alaska are: 8-32-16, 10-20-20, and 16-16-16. (They are listed in the order in which they have given best results.) On new lands, crops benefit from the high phosphorus of the 8-32-16 mixture, whereas on lands cropped for many years, the
Figure 18, Brown, sunken lesions usually appear only in the stolon region of the tuber. These dessicated spots have been associated in the variety Green Mountain with an inadequate supply of potassium for normal plant growth. lower phosphorus content fertilizers can be satisfactory. Commercial fertilizer 6-32-16, applied at the same rate per acre as 8-32-16, supplies the plant with much needed phosphorus and potassium at the same time !'?| that nitrogen is reduced 2 percent.
Figure 19. Leaf scorch of potato plants resulting from more boron in the fertilizer than the plants can tolerate. A fertilizer mixed to supply boron for cauliflower had been unintentionally used as a potato fertilizer in this case. Commercial growers may find it more economical at times to buy the ingredients and mix their desired fertilizer ratios, if the ones they seek are not available from a fertilizer company. High analysis mixtures tend to absorb moisture and harden ("set up") in storage and should not be mixed far in advance of using. However, the high analysis fertilizers are the best buy, because they contain very little filler, or non-fertilizer ingredients, on which shipping charges are paid.
Methods of Applying Fertilizer Fertilizer or fertilizer elements can be applied in several ways: in a band on each side of the seed piece and at or a little below the seed piece level; broadcast uniformly over the entire area; top-dressed or sidedressed on the soil after planting; or sprayed over the crop as a very dilute solution to correct some unexpected element shortage. Good root development depends on an abundance of phosphorus, and the quickest way to get it to the root area is to apply it in bands beside the seed piece at planting. The contrast between an adequate and an inadequate supply of nutrients for good plant growth is illustrated in Figure 20. This field was being fertilized at planting with 8-32-16 fertilizer applied in bands with a mechanical planter. The poor growth shows where someone forgot to put fertilizer in the hopper. Banding at the rate of 5 pounds per 100 linear feet of row is equivalent to about 8 pounds broadcast and worked into the soil. In a band near the seed piece, phosphorus is more readily available to the developing plant than would be the case if the fertilizer were more widely dispersed by broadcasting. The same principle holds ' for the other elements except that they move in the soil solution, whereas phosphorus is soon tied up in the soil complex.
Figure 20, Small plant growth of potatoes in a field of Kennebec (center row) was due to lack of fertilization at planting. Someone forgot to put fertilizer in the fertilizer hopper of the planter. By the time the plant growth indicates this condition, it is too late to remedy it, even with side dressing.
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Figure 21.
Bronze color development in potato leaves is an early symptom of potassium deficiency in the plant. A color change from deep green to bronze takes place on intermediate-age leaves well tip on the plant stem.
If broadcasting is done, all of the fertilizer should be spread before working the soil so that enough will be mixed deeply to supply the crop when drouth conditions prevail. To get the same growth response as banding, one should use at least 25 percent more fertilizer per acre by the broadcast method. Banding appears to be even more significant in the Arctic. Side or top dressing is primarily for adding nutrients after the crop has been established; this has not been found to be worthwhile if the crop has been well fertilized in bands. Unless the fertilizer is added through irrigation water, damage to the root system by side-dress cultivation will most likely offset the value of the added nutrients. Furthermore, side dressing adds
another unnecessary mechanical operation to the enterprise. On sandy soils in heavy rainfall areas, leaching of fertilizer salts may deprive the crop of nutrients; under these circumstances, side dressing may be a desirable practice. Occasionally, potassium deficiency symptoms (Figure 21) can be recognized in potato leaves early enough in the crop's growth to justify spraying the foliage several times at weekly or twice-weekly intervals to correct the deficiency (8). Spraying or side dressing are not practices to be used in lieu of fertilizing the soil at planting.
Planting When to plant? This is a question frequently asked by growers and potential growers. Generally, the time to plant is when the land can be prepared satisfactorily in the spring. Very early planting does not necessarily result in maximum yields. If seed stock for planting is removed from cold storage, cut, and planted immediately in early spring, the soil temperature may be about the same as the temperature of the seed storage, and very little growth will occur in such a cold soil. If this were the only undesirable result of early planting, one could look upon very early planting as a 12
way to get a job done so that other work could be pursued. However, cold, wet soils accompanying early planting frequently harm the crop in two major ways. Poor stands result because soil fungi, mainly Rhizoctonia solani, girdle the stolon and stems and cut some plants off at the soil line. Possibly more important is the fact that girdled stolons produce deformed tubers and lower yields. Healthy seed pieces do not rot incold, wet soils of Alaska; they just remain in a semidormant condition until soil temperatures are favorable for growth. Frequently, varieties in very early
t
plantings emerge no earlier than potatoes planted 10 days later, so the danger of field frosting of early plantings as they emerge is scarcely any greater than that of a later planting. In areas where grasses and perennials are problem weeds, the choice of planting date may be more closely related to the weed control practice used than to the actual climatic factors affecting potato growth. Potato planting may be done in several ways, depending upon the size of the planting and nature of the material that is being used to generate a new crop. Power drawn, mechanical planters carry the seed in hoppers that feed the seed to a turntable type wheel that drops the seed pieces into the furrow at regular intervals (Figure 22). This type is called assist-feed because it requires a person to either fill between the spokes of the turntable or remove seed pieces when there is more than one in a compartment. In another type, a sharp "spike" pierces a seed piece, carries it to the crop position, and retracts, allowing the seed piece to drop at the proper interval. This type has a wheel with numerous "picks". Picker-type planters are more likely to spread diseases contained within the tubers than are the assist-feed types. If a spike pierces a diseased tuber, it is possible that several other seed pieces will be contaminated before all of the diseased fluid is wiped from the pick. Growers of seed potatoes should be aware of this situation and avoid the pickertype planter unless it is equipped with a dip tank that treats the picks between seed pieces. Even this type is more likely to spread diseases than the gravity fed machines.
I
Figure 22,
Depth of the soil cover over the seed piece depends on several environmental factors; the object is to get a green sprout growing into a plant above ground in the shortest possible time. In moist soils of this region a half-inch of soil compacted over the seed piece is enough. In dry soils one must estimate the severity of the drouth and plant deeply enough to prevent the seed piece from being dried out before its root system is developed. One to IVz inches of soil cover should suffice. Deep plantings warm up slowly so sprouts are late in emerging. In areas of short growing season, cut seed pieces planted by hand and positioned in the furrow with their bud group up emerge a little earlier than seed pieces planted with their buds down. If one will be this particular, it is worthwhile instead to allow the seed piece to develop a sprout a half-inch long and then place the seed piece so that the sprout is oriented upward, as with a transplanted seedling. If the new sprout is barely covered with soil, it will soon push through and give the potato an early start. Potatoes planted by hand are covered by hand or by hoe. However they are planted, firming of the soil to make good contact with the seed piece is desirable. Seed pieces that lie on top of the soil turn dark green and develop a rosette of buds, but light inhibits root development, so they do not grow into productive plants. Decisions on direction that rows will be planted, distance between rows, and space between seed pieces in the row can materially influence potato yields. Where possible at higher latitudes, potato rows should 0
W
A conventional single-row planter that simultaneously opens the furrow, distributes the fertilizer, covers it, drops the potato seed pieces, covers them, and finally flattens the ridge with a soil packer. 13
run north and south. During the early part of the growing season, the sun's rays shine under the plant canopy and warm the soil on both sides of the row. Later when the leaf canopy is dense and the tops lodge from the weight of leaves and aging of the stems, warmth of the sun penetrates to both sides of the ridge during the course of the day. When fall frosting occurs, this little accumulation of heat thaws the frozen crust of soil, whereas the north side of east-west rows tend to remain frozen all day. This allows increased frost penetration on the north side, which makes harvest more difficult. If water is likely to erode the potato field, the planting should be on the contour to prevent erosion. Distance between rows is usually 36 inches; however, the possible wheel spread of the tractor and planter control row spacing. Distance between seed pieces in the row should be governed by the growth characteristics of the potato variety, Those that are listed as early types set only a few tubers per seed piece planted and produce more market grade tubers if planted at 7 to 9 inches between seed pieces than at wider spacings. Late types usually have more tubers per hill than will grow to market size at close spacing, so the seed pieces are spaced 11 to 16 iches apart. Failure of tubers to reach full size is frequently due to
Figure 23.
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lack of soil moisture; seed pieces can be spaced closer if irrigation is to be used. Ridge versus flat culture has been brought into focus since the release of the variety Alaska Frostless. Seed pieces of Alaska Frostless planted at the same depth as other potatoes set their tubers deeper in the soil than other varieties—so deep that during their growth they remain protected from sun-greening. This means that the soil need not be stirred after planting until it is lifted in the harvesting operations, providing that chemical weeding is done satisfactorily. Economies derived because of the deep tuber set of Alaska Frostless are numerous. First, any wheel span of the planting, weed control spraying, and harvesting equipment can be used. Second, without the need for hilling, the amount of herbicide required for weed control can be decreased, because the surface area to be covered by a broadcast spray is reduced appreciably, as illustrated in (Figure 23). Flat culture also reduces the surface area exposed to drying conditions. Third, since hilling is not practiced, potato rows can be 12, 18, 24, or 36 inches apart. On this basis, yields per acre due to more plants per acre have been increased as much as 40 percent by using 18-inch row spacings and 11 inches between seed pieces. Fourth, soil compaction is significantly reduced because of these minimum tillage and cultural
Formation of a hilled row can produce a 16 percent increase in surface area of a potato row exposed to evaporation.
practices. Fifth, all labor and fuel costs that would be incurred during hilling are saved. Although some research indicates that potatoes \d on ridges and cultured on mounds are likely to vf be more productive than those grown in flat culture, several factors must be considered. While the ridge warms faster, the warming does not necessarily reach the seed pieces early, because they are buried in a deep pyramid of soil. This is because the sides of the ridge dry faster and develop a dry mantle that insulates the potato seed piece from the warming action. At about the time that the ridge temperature has reached equilibrium throughout the mass, the ridge is knocked
down by cultivating to destroy the emerging weeds. This opening up of the surface soil not only dries it out but cools it through evaporation of exposed moisture. Soil moisture loss from undisturbed ridge culture in the Arctic generally occurs during early plant growth when soil moisture shortage has its greatest depressing effect on final yield. Experiences with shallow planting of green sprouted seed pieces grown in flat culture in the Arctic are not extensive enough to be conclusive. However, the evidence favors this cultural method over planting on mounds or maintaining ridges, not because it is more productive, but because it requires less energy for the same yields.
Pests and Their Control Some serious pests of potatoes in other growing regions of the world are not present in Alaska's potato growing areas. Whether this is due to the unfavorable environment for their survival or because they have not been introduced is not known. It is important that measures be taken to prevent their entry, that growers and others recognize potential problems, and that prevention and control be constantly practiced. For colored photographs of serious diseases and insects of potatoes, the reader is referred to Bulletin No. 60, Some foreign Potato Maladies in Alaska and Domestic Pests of Potato (6). /» Pests that growers should learn to recognize fall into I five categories: insects, diseases, nematodes, animals, and weeds. Insects that are not present in Alaska, but that are troublesome to potatoes elsewhere, are Colorado potato beetle, potato tuberworm, potato stock borer, and June bug (white grub). Diseases that are not present in Alaska are late blight (Verticillfam wilt), and black wart. Nematode injury has not been seen on Alaskan grown potatoes; therefore, the very destructive golden nematode should not be allowed to enter or exist here. Insects found on Alaskan potatoes have not been very destructive to the crop thus far. In the principle potato growing areas, aphids, leafhoppers, and thrips are the most common insects. A few small grasshoppers have been seen in a few areas late in the growing season. Insects have not been numerous enough to warrant control measures, unless one is a grower of seed stock. In that case, the danger of spreading virus diseases transmitted by aphids from undetected diseased plants to healthy plants may be sufficient to justify a spray program. This is particularly true for valleys in interior Alaska, where both day and night temperatures can be favorable for rapid aphid population build-up. Malathion spray has been used successfully to combat aphids. Cutworms are a common pest in Alaska, especially in small plantings? however, stems of potato plants are large in relation to the insect feeding habits and are y seldom cut off. They do sprout again below the damaged area, so the hill is only delayed in growth, not
lost, as with most other crops. Treatment of commercial plantings generally will not be necessary, but cutworm damage in the home garden can be severe. Several chemical insecticides are available for cutworm control. Wireworms are quite a different matter because they feed on the new tubers, gouging out holes y8-hich in diameter and ¥4 to Vz-inch deep. Effective treatments are those that destroy the worms in the soil prior to tuber formation. Wireworm infestations can be expected where sod or grassy areas are planted, because grasses are the host plant of the egg-laying wireworm adult, which is called click beetle. Chemicals listed for wireworm control are Diazinon, Fonofos and Parathion. At the time these chemicals can be worked into the soil to kill wireworms, the soil temperature may be too low to allow the expected results. Some growers have found that Fonofos in cold soils is more effective than Diazinon or Parathion. Slugs are a problem to potato growers on the islands and along the coast where the environment is usually wet. They can be quite destructive by defoliating plants in the home garden. Some relief can be obtained by baiting them with shallow saucers containing beer. Of course, one must shelter the plate from rain? otherwise, the beer will be diluted and ineffective. Slugs feed mostly at night and during low light periods. They crawl under some cover as the day progresses, so one can put out pieces of boards or similar materials and pick the slugs from under them and kill them each forenoon. It seems unlikely that their numbers would be significant in commercial production even if the wet coastal region could be used more extensively. Metaldehyde is listed as a molluscicide for slugs. Birds currently are not very significant pests of potato. Sandhill cranes can be a problem to growers of seed stock-, they frequently peck the seed pieces before sprouts emerge. This activity can result in poor plant stands. Rodents, primarily field mice and shrews, migrate into garden potato plantings in mid-summer, tunnel under the potato plants, and gnaw into or consume the new tubers. They also nest underground in the potato 15
ridge. Rodents have not been a common problem to commercial growers, but they could be in the Arctic where there are great numbers of mice, Good weed control in and around any planting destroys the rodents' cover and discourages their entry into the garden, In regions where mouse damage can be expected, one should keep close watch on the planting and should harvest the crop if there is danger of substantial damage to the tubers. Potatoes can be left in the ground for several weeks in some areas after the tops are frosted, provided that mice are not a problem. This is generally a good practice, because the tubers have a better storage environment in the soil at this time than in a conventional, unrefrigerated storage. Weeds in potato plantings may be native, introduced, or both, depending upon the length of time that the land has been cleared and the kinds of crops grown previously. Native species of horsetail, particularly Equisetum arvense, are among the most persistent perennial weeds in potato fields. They are difficult to eradicate because some of their rhizomes and tubercles are below the plow depth and because potatoes, a crop in which they can be controlled, often are not grown on the same site in successive years. Fireweed (Epilobium angustifolia), is also difficult to eradicate because of its underground spread by rhizomes. Canada thistle (Cirsium arvense), an introduced rhizomatous plant, has already demonstrated its capacity to survive and its nuisance value in potato fields. In the Arctic, tansy mustard (Descurainia sophia) and (D. sophioides), are
Figure 24.
16
vigorously growing weeds that *u u- » ^ to chemical weed control. Beca J they probably will respond to * control treatments. Weeds that when discovered in Alaska are: , carolinense); smallfiower galensoga parviHora)-, common ragweed (Ambrosia artimesiifolia)-, yellow nutsed^ barnyardgrass mallow (Artemesia annua). oroaa-ieavcd ™r« ^ » 1.1 potato growers that have been fnlrodufedTe " '° Common Name Scientific Name Canada thistle ^" arvense Charlock Chickweed Climbing buckwheat Groundsel Hempnettle nempneme Knotweed TLambsquarters » i-> i . Pennsylvania smartweed« ponnwrocc JTCJIHAy>-ACoo T->. t j Pmeappleweed Shepherdspurse Spurry
steliaria ~ ^Po/ySon ^ » i SerSn^r^^^"5 Ga/f vu^ans ^a/eopsjs fefraftjf Po/vdv-,« . , ^ *s u-num aviculare Chenr,^ ^r- «" J( .ujajc ^POdiijrn slbtim Po/vcm« «^"^-< Tut* nutnpensylvanicum J /l/^c»»-N* — . *JJ3spi arvense Mafrfrfl^o > - - -^ _a^car,a ™trrcanoKfes oris
Broad-leaved weeds that dominated ~ » » prior to effective chemical chickweed, fireweed,
Potato foliage showing loss of green color along the veins of leaves that exist d when the crop was sprayed with EPTC. This type of chlorosis may persist f several weeks after spraying, but it is far less damaging than weeds to * production of a good crop of tubers.
t P
.tr
1
lambsquarters, pineappleweed, shepherdspurse, and smartweed. Grasses native to Alaska have not been troublesome , in the areas where potatoes have been grown. Native grasses that form tussocks are difficult to break up in land preparation, but have not established themselves readily after the land has been well-tilled. Native bromegrass species in areas yet untilled may prove to be a problem. Introduced grasses that have been and still are troublesome and expensive to eradicate include annual bluegrass fPoa annua)-, bromegrass (Bromus sp.)-, and quackgrass (Agropyron repens). Barnyardgrass (Echinochloa crus-quali), although introduced twice in the past 31 years and eradicated, may become established if it is neglected. Seeds of barley, oats, and wheat from the previous season's combining operations frequently germinate in potato plantings and require special control measures, because the herbicides customarily used to control broad-leaved weeds do not kill plants of these grain crops. Very satisfactory chemical control of weeds in home gardens and in commercial potato plantings has been obtained with a broadcast spray using linuron marketed as Lorox or Afolan. Metribuzin under the trade names Sencor and Lexone has been used effectively. The skin of red potatoes develops a lighter red color when metribuzin is used. Unlike other weed-control chemicals, metribuzin can be sprayed over the top of both potatoes and weeds if, for some unavoidable reason, the potatoes have emerged before weed control measures have been used. Potato foliage usually shows some injury (Figure 24), but a better crop will be produced than if weeds had not been killed. Quackgrass can be effectively controlled and essentially eliminated by spraying the land with EPTC sold as Eptam. This chemical is most effective when worked into the soil with a rototiller to a depth of 4 or 5 inches immediately after spraying. Glyphosate sold as Roundup is very effective against all grasses and volunteer grains, but must be used in a manner to assure that it does not get on potato leaves. Mopping or brushing grass-type weeds has been extremely effective in killing them (Flgore 25). Normally, a spray applied to quackgrass prior to potato planting does not do maximum damage to the vegetation, because the quackgrass is seldom large enough to absorb quantities that will kill it. Growers frequently do not wait long
Figure 25. Mop for brushing glyphosate on grass, grain, or other vegetation protruding above potato foliage, A portion of a fringed towel laced into a bamboo lawn rake is an effective tool for spot treatment of grasses up to an acre in extent, Glyphosate corrodes a metal rake. enough (eight days) for maximum absorption of glyphosate. Directions for use of these chemicals are on the container labels, and these directions should be followed. Prior to spraying any chemical, the sprayer hoses should be checked to make sure they will not rupture when they are full of herbicide and under pressure. Be sure to follow all safety precautions stated on the label. Since chemicals used for controlling weeds in potatoes only persist for a few weeks, season-long weed control is best accomplished by no tillage until the soil is stirred for hilling. Generally, it is the weeds that emerge with the crop that materially reduce yields and interfere with harvesting, so thoroughness in destroying early emerging weeds is very important.
Irrigation At some time during most growing seasons, irrigation improves potato yields. The question of when to irrigate involves consideration of several other factors and operations. If land preparation for planting
is done in a manner to minimize soil moisture loss, potatoes are not likely to need irrigation water until a small rosette of leaves has formed following sprout emergence. Before the sprouts have reached this stage, 17
a chemical weed control spray should have been applied. The effectiveness of the herbicide is reduced by rainfall or irrigation that exceeds approximately onetenth inch. Under very dry soil conditions, droplets of weed sprays that strike powdery, dry soil "ball up" and do not coalesce to form a film. These minute balls of water evaporate in place, and because there are too few of them to provide full, uniform coverage of the entire soil surface, poor weed control results. It would be better to irrigate to moisten the soil surface before applying the herbicide when these conditions prevail. Under ideal conditions, one would not be able to go onto irrigated land with spraying equipment the day that the land was irrigated. So it becomes apparent that the timing of irrigation, chemical sprays for weed control, and hilling require careful planning to obtain the greatest value of each practice. Once the decision is made to irrigate a crop of potatoes, the practice should be repeated to prevent the soil moisture content from dropping below 50 percent of its water holding capacity. Holding a good moisture content is especially important between June 25 and July 15 when tuber-set and rapid tuber expansion is occurring. Drouth conditions at tuber initiation cause many small tubers to dry up and disappear. Additionally, hollow heart may develop later in tubers that are under moisture stress during their very early growth. Irrigation water is brought to the crop via furrows or by overhead sprinkler nozzles. Water in furrows is difficult to manage in soils of textures as variable as those of most Alaskan soils, and field topography can rule out this method. Sprinkler or overhead irrigation offers the operator versatility, good control, and best use of water, especially on light soils where ditch irrigation water would seep away and carry the nutrients down below the root zone. Sprinkler irrigation is generally most effective when used during the night, because there is less air movement to disrupt the sprinkler pattern and a lower evaporation rate due to coolness of the air. In this regard, many have asked if the low temperature of the irrigation water is likely to
be detrimental to the crop; the answer is no. Cool rains with a little hail in them frequently fall on potato plantings, and no one yet has been able to distinguish growth or yield differences between these and plantings nearby where such precipitation did not occur. In a greenhouse where air temperature was 80 degrees F, application of overhead irrigation water at 42 degrees F resulted in wilting within two minutes. Plants recovered in about two hours, and no ill effects were apparent. Whether the addition of water is on a commercial scale or home garden planting, one should choose sprinkler nozzles that apply water at a rate slightly less than that which causes water to stand in puddles and less than would cause runoff. One way to get the maximum rate of absorption is to sprinkle the soil for several minutes, then turn the water off and let the surface soak up the light application. This procedure allows for minute air and water passages to be developed which conduct air out and water in more readily when the irrigation system is turned on for the main application of water. The question of how often irrigation water is applied depends upon the water holding capacity of the soil and the rate at which it is lost or utilized by the crop. Very fine soils will hold as much as 4 inches of water per foot of soil depth, and on these soils 1 to Wz inches is a fair application. Occasionally, a grower applies irrigation which is followed directly by heavy rains that flood some areas of the field. Such conditions on potatoes in the cool soils of Alaska have not damaged the crop. Nutrient loss by leaching below the root zone is likely to be the most serious consequence of flooding. If flooding occurs after the tubers are halfgrown or larger, it may result in ruptured lenticels and patches of reddish-brown skin coloration on the low side of the tubers in very wet spots of the field, (Lenticels are structures of the skin that permit the passage of gas inward and outward.) On russet-skinned tubers, these lenticels are visible as small, white spots. This tender tissue will suberize in a well-managed storage.
Hilling Potatoes Hilling is the physical process of moving soil toward the potato plants from between the rows so that the plants appear to be growing in the center of a broad ridge, as shown in (Figure 26). The purpose is to add soil cover to the area where new tubers are forming. Pressures of tubers forming near the surface of the ground crack open the soil above the tubers, which allows light to green the exposed tuber. Green tubers contain solanine, a bitter glycoalkaloid that is toxic to humans, but it is so bitter that it is doubtful anyone could eat enough of it to damage his or her health. Two to 3 inches of soil built up around the plants is sufficient to give protection from sunlight. Hilling also provides for runoff and drainage from heavy showers.
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If it is done at the proper stage of plant growth (Figt**« Jure 27), when the tubers are forming, this time frequently coincides with the start of a few weeds that have emerged as the weed control spray loses its effectiveness. This tillage operation destroys late emerging weeds. Hilling, as with other tillage operations, dries the soil, particularly because it leaves much surface exposed. On commercial plantings, it is a good practice to hill at night when there is a minimum of air movement and lower evaporation rates. The leaves of some potato varieties are held more erect at night, and are thus less likely to be covered by soil (Figure 28). This "skirts-up" position allows the soil to be thrown up to the plant
