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For many years scientists and farmers have worked with biological by-products, trace element combinations, plant enzymes and growth hormones in an attempt to enhance crop yield and quality. Dr. Ora Smith in his classic book, Potatoes: Production, Storage and Processing, published in 1968, referred to early work involving gibberellins and indoleacetic acid as potato growth stimulants.

Numerous minor element combinations as foliar sprays have been offered to potato producers as ‘growth stimulants’. In many cases these materials did not offer consistent performance between seasons and growing areas and were soon ignored by commercial producers.

However, in the past few years several firms have developed and are marketing new crop stimulant product lines that appear to consistently affect tuber set, size distribution, yield and/or quality of the potato crop. These product lines have emerged as a result of new technology in the synthesis of plant bio-hormones and enzymes and fermentation technology.

Crop-Set™, the natural growth stimulant developed through microbiological fermentation technology, is one of these exciting new compounds.


The potato crop

Since Spanish explorers in the 1500s found potatoes being grown by the Inca Indians in the Andean mountain highlands in what is now Peru, the potato has spread to every country of the world and is the world’s fourth largest food crop following wheat, corn (maize) and rice. Botanically, there are over 2000 species of the genus Solanum, with up to 180 known tuber-bearing types. Potatoes are grown on over 44 million acres in 125 counties. Russia, other former Soviet bloc nations, eastern European nations and China produce over 60% of world production.

Over 500 common varieties make up the world’s commercial acreage; and new cultivars are being developed almost daily as breeders utilize conventional sexual crosses and genetic modification to improve yield, pest resistance and nutrition of this high energy food product.

The potato is unique in that its varietal stability is maintained by the regeneration of seed through vegetatively produced seed tubers. It is susceptible to numerous soil, seed, insect and airborne pathogens, numerous chewing and sucking insects, nematodes, weeds and to environmental stresses caused by heat, drought or cold. Genetic changes of pathogens and emerging resistance in many insect pests make management of the potato farm an ongoing adventure to achieve maximum yield and quality.

The potato is a herbaceous annual producing a spreading, fibrous root system and a multiple stem, with an erect to spreading plant shape. Tubers arise off underground stems called stolons. The size of rooting systems, stems, stolons and tubers produced is determined by the production environment and varietal growth characteristics.

The aggressive farm manager attempts to minimize stress and maximize the nutrient and water needs of the crop to achieve the highest quality and profitable yields. The nutrient, soil moisture and air quality needs of the potato crop vary by growing area, temperature, length of growing season and variety selected. Potato yields may vary from 4 to 40 metric tons depending on variety, climate and management assets and skill of the producer.

Management of potato health and productivity must include the selection, cutting and planting of the highest quality certified or foundation seed available, the timing and application of adequate rainfall or irrigation to meet crop needs, the provision of adequate major and minor mineral nutrition to meet growth and tuberization demands and the application of pest and weed management materials to minimize root, vine and tuber damage throughout the growing period. If all these variables affecting potato crop growth and productivity could be successfully managed in any growing area, there would be little need for plant growth stimulants, enzyme or plant hormones or specialized minor element spray mixes.


How the potato plant grows

When the potato seed piece is placed in the soil, germination gets underway. The optimum temperature for sprouting growth has been found to be 70-75°F (21 to 24°C), however slow cell division and elongation occurs at lower temperatures.

The number of sprouts or early stems that emerge from the seed piece is dependent on the number of eyes and the physiological age of the seed piece. Generally, the larger the seed piece the more eyes.

Ideally the seed piece should weigh around two ounces. By the time the shoots begin to emerge from the soil, adventitious roots have been formed at internodes and the spreading root system begins to develop. Stolons also arise from these nodes, developing first nearest the seed piece then forming progressively upward as shoots elongate. The number of stolons produced is determined by the genetics of the variety. During stolon development leading to tuberization, the potato plant may become stressed by moisture, heat or cold that may limit stolon development. There is an opportunity for growth stimulants such as Crop-Set™ to influence stolon formation and tuber initiation during this stage of plant development.

Stolon initiation may occur in some varieties well into full bloom and canopy row-closure. As the above ground stem grows in height, nodes are produced and petiole branches emerge, expanding top growth. The rate of growth of the potato plant and the development of tubers is dependent on environmental conditions surrounding the plant above and below the soil surface. Stress conditions that limit or affect growth characteristics can occur at any time during plant development and create an opportunity for crop stimulants such as Crop-Set™ to help the plant weather these conditions with reduced potential for yield and quality loss.





Figure 1. A potato plant developing from a seed piece cut from a tuber. Tubers develop from the enlarged tips of stolons (underground stems). Tubers have eyes (dormant buds), which can develop into shoots with lenticels (pores) through which air can penetrate to interior tissues. Branching occurs from above ground nodes producing terminal leaflets and flower clusters. On some late maturing varieties, stolon formation and growth may extend well beyond flowering but late forming tubers seldom reach marketable size. Multiple stems may arise from larger seed pieces with multiple eyes. The distance that stolons grow away from the mother stem is a factor of varietal characteristics, soil and environmental conditions and crop stress. Soil conditions and fertility levels that favor deep penetration of true roots condition the plant to continue productive growth through periods of drought, high temperatures or extreme weather related evapotranspiration.



GROWTH STAGES OF THE POTATO PLANT

Research scientists have established a classification system for identifying growth stages of the potato plant. Knowledge of these growth stages helps the grower understand when the crop is most likely to encounter stress and when it will respond to management actions to improve potential for yield and quality. These growth stages have been characterized as:

I. Sprout development and emergence
II. Vegetative growth
III. Tuber initiation
IV. Tuber bulking
V. Plant and tuber maturation


Stage I. Sprout development and emergence

Sprouts develop on seed tubers and grow upward to emerge. The seed piece is the sole source of energy for the young plant at this stage, as photosynthesis has not begun. The seedling plant is very susceptible to soil and seed tuber-borne pathogens and to environmental stresses at this stage of growth.

Crop managers use seed treatment and look to ideal soil conditions to encourage rapid emergence and protect the seedling plant. The application of Crop-Set™ in-furrow during planting provides little medicinal support for the seedling plant but may boost development of the expanding root system and strengthen young plant vigor. Growth stage I may last from 20 to 30 days depending on environment and varietal characteristics.


Stage II. Vegetative growth

Leaves and branch stems develop from above ground nodes and rooting systems expand and stolons are formed from below ground nodes. Some nutritional support continues from the seed piece, and photosynthesis gets underway producing carbohydrates as additional energy for plant growth and development. This stage lasts until tubers start to form at the tips of underground stolons.

During this stage stem and foliage growth are rapidly expanding. The tender young leaflets have not developed the heavy, waxy cutin layer that protects leaves from rapid water loss; and the young plant is most receptive to foliar spray uptake of crop stimulants such as Crop-Set™ at this stage.

Growth stage II may last from 30 to 70 days depending on planting date, environment, varietal characteristics and physiological age of seed tubers. Environmental stress during this stage can severely affect tuber initiation, final yield and quality potential and may encourage pathogens such as powdery scab, common scab and Rhizoctonia. Application of Crop- Set™ when plants are 6-8 inches high allows the nutrient and plant hormone components of the product to enter the plant and also to reach expanding root systems as rainfall, irrigation or cultivation mixes the product into the soil.


Stage III. Tuber initiation

Tubers start to develop at the tips of underground stolons, but are not yet rapidly enlarging. The initiation of tubers is controlled by growth regulating hormones produced by the plant and is characterized by active cell division. Most tubers of marketable size are initiated during this period. This growth stage lasts from 10 to 15 days, starts generally 45 to 65 days after planting and usually ends with the onset of early flowering. The application of Crop-Set™ during this growth stage may stimulate tuberization and induce the plant to produce compounds assisting resistance to the onset of soil-borne disease.




Figure 2. The stages of potato plant growth and maturation.



Stage IV. Tuber bulking

During this stage, tuber cells expand as the result of accumulation of water, nutrients and carbohydrates through the plant supply system. Tuber expansion occurs in a linear fashion, if none of the growth factors become limiting. On most varieties, vine growth diminishes and the tuber becomes the primary site for the deposition of carbohydrates and mobile inorganic nutrients. The application of Crop-Set™ during the early phase of growth stage IV has been shown to assist tuber bulking, improve tuber size distribution and enhance marketable yields.


Stage V. Plant and tuber maturation

During this final stage of crop development vines yellow and senesce. Photosynthesis gradually decreases, tuber growth rate slows and vines may die or be desiccated. The dry matter content (specific gravity) reaches its maximum, the skin thickens or ‘sets’ and a gradual death of abscision cells between the stolon and tuber occurs allowing tubers to easily disconnect from the plant during harvest operations. Stage V generally begins about 90 days after planting and may extend from 110 up to 135 days after planting depending on variety, environmental conditions and harvest planning. There would be little advantage to applying a crop growth stimulant at this growth stage.


Nutrient demand by the potato crop

The potato crop requires a broad range of major and minor mineral elements to achieve maximum yield potential. If any one mineral element becomes limiting, major yield capacity can be lost. The volume of mineral nutrients removed from the soil by the potato crop is determined by yield potential, plant spacing, irrigation effect and length of growing season. Most potato farms utilize a soil testing service to determine the soil nutrient supply potential.

In Maine, it is suggested that a 300 hundredweight yield goal (13.6 metric tons) would remove 200 lbs of nitrogen (N), 60 lbs of phosphorus (P2O5) and 300 lbs of potassium (K2O) from the soil. In a longer growing season, as in irrigated areas of the US Pacific Northwest, yield goals in excess of 750 hundredweight (35 metric tons) are common.

However, nutrient supplies must be elevated to reach these higher yield goals. A 600 hundredweight crop (27 metric tons) under irrigated conditions may require up to 400 lbs of nitrogen supplied continually through most of the growing season and much higher inputs of phosphorus, potassium and other nutrients. The application of Crop-Set™ to potatoes grown in this environment may result in significant yield improvement if criteria for application method and timing are met.


POTATO YIELD BARRIERS AND YIELD POTENTIAL

The inherent yield potential of a potato variety is limited by many factors such as disease, insect pressure, environmental stresses, water holding capacity of the soil and length of growing season.

The crop manager strives for rapid emergence, development of strong root and foliage systems, efficient use of inputs (chemical and fertility) and water, optimum rates of photosynthesis, uniform growth and development of tubers and maximum movement of photosynthetic products to the tubers by the end of the growing season. If all of that can happen, maximum yield for the potato crop is a real possibility. However, in the real world, that goal is rarely achieved. Many variables interfere with the crop reaching its full potential.

Potato growers have made steady gains in yield over the past century. The development of new organically produced crop protection materials following World War II and the adoption of new technology in irrigation and water management have been key factors in progress toward higher yields.

Table 1 outlines the yield gains that have been achieved. The yield gains from 1900 to 1950 occurred due to introduction of certified seed production technology, production expansion in irrigated western states, the introduction of row crop tractors and mechanical harvesting, and the development of triple-strength and ammoniated fertilizers. The yield gains through the 1960s, 1970s and 1980s were the result of improved varieties with greater pest resistance, introduction of center pivot irrigation and continued expansion of production in the US northwest.

The sharp gains in yield through the 1990s until the turn of this century are a result of a decline in potato production in low yielding, non-irrigated areas of the eastern, mid-continent and southern US growing areas and the rapid decline in numbers of small farms growing potatoes nationwide.


Table 1. Trends in US potato yields (average of all states).


National Potato Council, 2002



The potential for Crop-Set™

What developments will determine trends in potato yield during the next quarter century? I believe that development of microbiological products that stimulate crop hormone, enzyme and root development systems to help the crop overcome yield barriers caused by stress conditions is the technology of the near future. This technology will help keep farms profitable against rising production costs and narrower contract margins.

By inducing the potato plant to produce greater levels of its own growth stimulating enzymes and hormones, the potato will overcome seasonal stress-related setbacks and promote the continued transport of photosynthesis products, minerals and water to the tuber thus assuring higher yields, improved size distribution and quality.

Crop-Set™ is uniquely suited to use on the potato crop. It can be used as a single application early or as a split application influencing tuber set and tuber sizing. The unique composition of Crop-Set™, containing natural surfactants, available nutrients and microbial factors creates a combination of components that stimulate the potato plant through several phases of development. The product stimulates soil-borne bacteria, active in the solubilization and uptake of mineral nutrients required by the crop, stimulates stolon formation and root development, and boosts tuber filling and type development. Best of all, the product is friendly to handle and compatible with most crop protectants. It is totally natural, leaving no harmful soil or crop residues, and has a very acceptable cost/benefit ratio.


TIMING CROP-SET™ APPLICATION

For the potato crop, timing of application is based on the type of crop response desired. Early application, prior to hooking (tuber set) has great potential for the seed grower looking for maximum tuber set and uniformly sized smaller tubers.

A group of growers in northern Maine who produce small red potatoes for the quartered, broasted-frozen processed market have used the product to increase production of tubers below the 2.5-inch size category, which provides contract incentive premiums. Application prior to full bloom when the crop is entering growth stage IV stimulates total yield, brings smaller tubers into marketable size and contributes to a more uniformly sized pack-out on the grading line. I believe there is great potential for Crop-Set™ to be applied as a seed piece coating or as an in-furrow, at-planting application.

In growing areas such as northern Maine or the Canadian provinces, where length of the growing season is a critical factor in achieving maximum yield, application to stimulate quicker emergence can mean more growing days and greater yield potential.


CROP-SET™ EXPERIENCES IN MAINE

My firm has worked with Crop-Set™ on a number of varieties and types of production. Trials have been conducted in central Maine on chip process potatoes, in central Aroostook County (the primary production area) on red varieties for the quartered, broasted-frozen process market and in Northern Aroostook on foundation seed production farms.

Trials have been conducted in randomized, replicated format and on grower farms as demonstration strips, grower-applied with our supervision of yield sampling and quality analysis.

In a trial conducted in central Maine during the 1998 growing season, four common Maine varieties were grown in a replicated trial. Crop-Set™ application (5.0 ounces/acre) was on July 10th (posthooking) following a normal spring rainfall accumulation when the varieties were filling the rows. The season became very dry during August and all varieties experienced yield shortfalls.

However, the Crop-Set™ application proved very beneficial to total yield, marketable size distribution and percentage of US No. 1, Size A production (Table 2).

Crop vigor at harvest as evaluated by visual rating was superior for all Crop-Set™-treated varieties versus the untreated. All varieties produced greater amounts of marketable yield sizes and generally fewer cull tubers when treated with Crop-Set™ shortly after hooking. At a $6.00 per hundredweight proposed selling price, the Crop-Set™ treated varieties would have averaged $204.00 per acre greater return for a product cost of under $12.00 per acre. I believe the key factor in the yield advantage gained in this trial was the fact that the Crop-Set™ treated potatoes were stimulated to produce despite the pressure of severe drought conditions limiting growth of the untreated control.


Table 2. Effect of Crop-Set™ application on vigor, yield, size distribution and quality of Dark Red Norland, Superior, Shepody and Russet Burbank potatoes grown in central Maine in 1998.


To enlarge the image click here
¹Crop-Set application rate: 5 oz/acre
²Vigor scale: 10 = Plants green, lush and erect, 1 = plants yellow, wilted and flat
³US No. 1, Size A = Tubers 2.25 to 3.5 or 6-10 oz.
NL= NewLeaf (Bt gene inserted CPB resistant potato line)



In a grower demonstration experiment conducted on a certified seed producing farm in northern Aroostook County, Crop-Set™ was applied to traditionally planted and fertilized foundation seed potatoes on June 24th when plants were 8 to 10 inches high. The crop was in the mid-hooking stage of development. At vine kill, no differences were found in crop vigor, stem or tubers set per plant.

However, Crop-Set™ treatment did influence size development resulting in improved size distribution for the seed potato market and higher marketable yield on both Norwis and Snowden varieties being produced (Table 3). The average marketable yield response from the application of Crop-Set™ on the Norwis and Snowden varieties was 29.75 hundredweight per acre. At a traditional foundation seed-selling price of $8.50 per hundredweight, the marketable yield improvement from this Crop-Set™ application returned the grower an additional $250.00 per acre compared to the control plots.

Auxigro, a plant metabolic stimulant developed and marketed at that time by the Auxein Corporation, was also evaluated on the Norwis variety. The Auxigo application produced a slightly higher total yield response; however, the increase appeared to be in the form of a greater development of tubers below marketable size resulting in lower marketable and US No. 1, Size A yield than the Crop-Set™ treatment.

Grower demonstration trials conducted at other locations in Maine and upstate New York have consistently shown that Crop-Set™ application shortly after hooking will result in improved marketable yields. The northeast US and eastern provinces of Canada are characterized by short growing seasons. Frequently, only 100 growing days are available between damaging frost events.

Total yield, marketable yield and quality of most commercial varieties being produced in these areas can be enhanced by the application of crop growth stimulant materials. The properly timed application of Crop-Set™ at rates of 5 to 8 ounces per acre, depending on the type of total yield or marketable size response desired, offers a very reasonable cost alternative for growers trying to produce profitable yields in these short growing season areas.


Table 3. The effect of Crop-Set™ and Auxigro plant stimulants on percent size distribution, total yield, marketable yield and quality of Norwis and Snowden variety potatoes grown for certified seed at Chamberlain Farms, St. Agatha, Maine, 1999.


To enlarge the image click here
Marketable yield = 1.88 to 3.5 inches; US No. 1, Size A = 2.25 to 3.5 inches



References

National Potato Council. 2002. Annual Statistical Yearbook, 1300 L. St., NW, Suite 910, Washington, DC, 20005.

Rowe, R.C. 1993. Potato Hearth Management, The American Phytopathological Society Press, St. Paul, Minnesota.

Basics of Crop Production: Understanding the Potato Plant. 1998. Agri-Growth, Inc., Hollandale, MN.

Smith, O. 1968. Potato Production, Storage and Process, AVI press, Westport, CT.

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