Potatoes

Potatoes (Solanum tuberosum) are members of the Solanacea or nightshade family that also includes tomatoes, peppers and eggplants, and weeds such as the wild tomato and hairy nightshade. Potatoes are a cool-season crop and do well in Saskatchewan's growing season. Saskatchewan’s long, sunny days and cool nights provide locally grown seed potatoes with what is referred to as "Northern Vigor." Seed potatoes coming from Saskatchewan exhibit increased plant vigour and produce higher yields than seed from more southerly regions.

Northern Vigor is a trademark of the Saskatchewan Seed Potato Growers Association.

Soil Conditions

Figure 1: Availability of nutrients to plants in relation to soil pH.
(Chart courtesy of R.J. Cook and R.J. Veseth, Wheat Health. Management, American Phytopathological Society

Soil Types

Potatoes do well on a range of soil types, but lighter textured sandy loam soils are preferred. Heavier soils tend to form clods that bruise the potatoes at harvest time. Heavy soils are also prone to waterlogging that can lead to problems with storage diseases, such as pythium leak and pink rot. The soil should be worked well prior to planting, with no clods or rocks. The potato plant's root system is shallow (less than three inches) but finely branched, making it highly efficient at absorbing nutrients and water.

Soil pH (level of acidity or alkalinity) plays an important role in determining the availability of nutrients to the crop. Figure 1 shows the availability of nutrients at differing soil pH levels.

Salinity is another concern, as potatoes are less tolerant of soil salinity than most cereal crops. Saline soils make it difficult for roots to access water and nutrients. A soil test, including an electrical conductivity test, will give a salinity rating for the soil. Irrigating with high quality (low salinity) water is critical to the prevention of salt accumulation in a field. Subsurface drainage will help in moving the salinity from the soil through the tile drainage. Consult with an irrigation specialist from Saskatchewan Ministry of Agriculture on questions you have pertaining to soil salinity.

Always have soil tested to provide an indication of the soil's condition.

Rotation

Crop rotations of three to five years are recommended for potatoes as a means of managing soil-borne diseases such as scab. Potatoes return little organic matter to the soil, and growers rely on rotation crops to keep potato fields in good condition. Crop rotation also helps to break up insect life cycles.

Rotations that include cereals and forages are ideal for potatoes.

Make sure to have a cropping and pesticide history of land being used for potato production. Certain herbicides have long lasting residues that will adversely affect potato crops. Contact your provincial vegetable specialist for a list of chemicals that are harmful to potatoes.

Potatoes are renowned for leaving large amounts of fertilizer unused, resulting in the potential for high yields from other crops in the following years. This is an attractive feature to land owners who rent land to potato producers.

Seed Selection

Growers must plant certified seed (foundation level or higher), as regulated by The Bacterial Ring Rot Control Amendment Regulations, 2009 of The Pest Control Act of Saskatchewan. Seed selection is a crucial step in growing a quality crop. Select seed lots that are free from disease, have good size and uniformity, are free of debris and stones, have no obvious physical or freezing damage, and are sound with a good skin set.

It is advantageous to visit your seed supplier in the summer when your seed is being grown and in the winter during storage to look at the quality. Make sure you select a variety that fits the market you intend to grow for. Agree on the terms and conditions of your seed purchase well in advance of seeding time.

For potatoes to be eligible to be sold as seed, the fields must be inspected during the growing season by the Canadian Food Inspection Agency. The inspections ensure that the seed fields meet standards for freedom from disease and varietal purity. Growers interested in seed potato production should contact the Saskatchewan Seed Potato Growers Association and the Canadian Food Inspection Agency for details on recommended practices and production standards.

Seed Treatment

Seed potatoes are usually cut into smaller pieces prior to planting. The resulting wounds are open to invasion by organisms that can cause the seed piece to rot before the crop gets established. Treating cut seed is worthwhile as the drying agent in the seed treatment will accelerate wound-healing (suberization) in the seed, which protects it from decay; and the fungicide component of the seed treatment further protects the seed from breaking down. Drop or whole potato (not cut) seed is popular among some growers, as it is more convenient and usually does not need to be treated.

Markets

There are three main potato markets:

• Seed potatoes are grown to be sold for replanting the following year. Canada has a seven-year flush through system designed to ensure good seed quality. The seed system starts with nuclear stock plants that are completely disease-free. Cuttings taken from these nuclear-stock plants are provided to elite growers, who propagate the cuttings by tissue culture or greenhouse methods. The resulting plants produce tubers that are planted in the field. With each subsequent year in the field, the seed drops one level in the certification system. The seed levels, starting with the most disease-free generation, are: Nuclear, Pre-elite, Elite 1, Elite 2, Elite 3, Elite 4, Foundation and Certified. Each year, seed-potato fields are inspected by the Canadian Food Inspection Agency to ensure that disease levels are below the allowable limit for each class of seed. Post-harvest tests are also conducted to further verify that the seed meets established quality standards.
• Table stock potatoes are grown for sale to consumers who will typically boil, mash or bake the potato. All potatoes in Saskatchewan that are sold for consumption are graded to ensure that they meet size requirements and are relatively free of visible defects.
• Processing potatoes are used to make french fries and potato chips. The varieties that are suited to processing have a high dry matter content (specific gravity), the proper shape (long for french fries and oval or round for chips), crispness and colour.

The following varieties are most commonly grown in Saskatchewan:

Table:

• Table Reds – Norland, Dark Red Norland, Sangre, Viking
• Yellows – Yukon Gold, Bintje
• Table Russets – Russet Burbank, Russet Norkotah, Ranger Russet, Goldrush
• Specialty – Caribe, Banana, Russian Blue

Processing:

• French Fries – Russet Burbank, Shepody, Ranger Russet

Seeding

Potatoes are usually planted in early May, as soon as the soil temperature warms to 8°C (46°F) or higher. The higher the soil temperature, the faster the crop will sprout and emerge. Time from planting until emergence ranges from 14 to 28 days. Recommended in-row seed spacing differs, depending on the variety and the grower's access to irrigation. The following are some seeding rates for potatoes:

Seed:

• In-row spacing – six to eight inches
• Between-row spacing – three feet

Table stock:

• In-row spacing – eight to 12 inches
• Between-row spacing – three feet

Processing:

• In-row spacing – 10 to 18 inches (french fries)
• In-row spacing – eight to 12 inches (potato chips)
• Between row spacing – three feet

Between-row spacing depends on your tractor wheel spacing, and the type of tillage, planting and harvesting equipment.

Fertility

Always start a fertility program with a soil test. Due to climatic conditions, differing cultural practices, varying soil conditions and other situations, the crop's response to a fertility regimen may vary from region to region.

General recommendation

N total (applied + residual) = 125 lb./acre dryland; 150 – 200 lb./acre irrigated
P₂O₅ (applied + residual) = 80 – 100 lb./acre dryland; 120 lb./acre irrigated
S - 20 lb./acre
K₂0 (applied + residual) – 400 lb./acre.
Apply a K₂0 starter dose of at 20 lb. associated with the seed, irrespective of the soil test.

Hilling

At planting, a small hill is formed above the potato seed piece. As the plant develops and tubers form, a larger hill is needed to cover the tubers. This extra soil also helps reduce sunburn and frost damage to shallow tubers, and retains soil moisture.

Depending on the size of hill formed at planting and the type of hilling equipment used, growers may hill one to two more times during the growing season. Hilling should be completed well before row closure (plant canopy covers the row spacing), as late hilling damages the crop. Hilling is also an effective method of controlling weeds between the rows.

Irrigation

Potatoes require approximately an inch of water per week during the growing season. Lack of water will reduce yields and quality. An uneven supply of moisture will cause tuber defects such as hollow heart, knobbiness and splitting (see tuber defect section). The soil should be kept at 65 to 70 per cent field capacity to attain optimum yields.

Topkill

A crop's long-term storage potential can be increased by ensuring proper skin set on the tuber prior to harvest. The skin begins to thicken as the crop matures, but this process can be accelerated by topkilling the crop once the tubers reach the desired size. Topkilling involves spraying the crop with desiccants such as or by flailing. Flailing is the mechanical removal of potato tops using a machine that runs over the top of the canopy.

Flailing is not recommended for processing potatoes, as it can cause stem-end browning of the tubers and adversely affects the colour of french fries and/or potato chips. Stem-end discolouration is more likely to occur when the weather is hot, vine killing is rapid and the level of soil moisture is low. Stem-end discolouration is less of a concern for seed and table growers.

Harvesting

Potatoes are ready for harvest once the vines have been completely dead for at least seven to 14 days. The temperature of the potato is critical to a successful harvest. Never harvest when pulp or internal temperatures of the tubers are below 45^°F (7^°C) or above 65^°F (18^°C).

Harvesting at low temperatures leads to bruising, while crops harvested at high temperatures are prone to tuber breakdown from bacterial soft rot and pythium. Soil water capacity should be 65 per cent at harvest, as this will permit the soil to be carried over the primary bed of the harvester, cushioning the tubers as they go up the harvester chains.

Potatoes can be lifted with a one or two-row potato digger and then picked by hand, or they can be harvested with a potato harvester. A potato harvester digs the tubers, conveys them over a series of beds that separates out the soil and places them in a bulk truck. Windrowers are simplified harvesters that travel in front of the main harvester, digging the potatoes and placing them between intact rows. The harvester then digs these rows. Windrowers increase the efficiency and decrease the time needed to finish harvest, but the added handling step results in more damage to the crop.

If frost has occurred, potatoes should not be dug until the ground has thawed, otherwise the crop will be prone to damage and breakdown in storage.

Storage

Potatoes are stored under different conditions depending on their anticipated end use. Seed and table potatoes are stored at 40°F (4°C), french fry potatoes at 45°F (7°C) and potato chip potatoes at 50 to 55°F (10 to 13°C). Relative humidity should be maintained at 90 to 95 per cent. Potato piles should not vary more than 2° to 3°F (1°C) from the top to the bottom of the pile.

Greater temperature differences indicate heat is being generated in the pile, and is often a sign of tuber breakdown.

Once potatoes have been brought into storage, they must be cured. Curing helps heal the wounds that occur during harvest. Improper or inadequate curing will increase weight loss and decay in the stored crop. The curing period typically lasts four to eight weeks. Curing temperature should be 59°F (15°C) with 95 per cent relative humidity, assuming that the crop is in good condition coming into storage. After two weeks the temperature can be lowered by 1/2°C/day until the desired storage temperature is reached.

Crops that are diseased or frozen should be cured at around 50°F (10°C), with less humidity to minimize the development of disease. Airflow must be constant and uniform throughout the pile during the curing and storage period.

Diseased or damaged potatoes should be stored separately from the rest of the crop. If problem lots begin to break down, they can be moved out quickly and either sold or discarded. Storage will not improve the quality of the crop, but good storage and proper management practices will help maintain quality over a long period of time.

Grades

Potatoes are graded based on the specific requirements for seed, table and processing markets. For a complete listing of potato grade standards, contact your local Canadian Food Inspection Agency representative.

Pest Problems and Diseases

Late blight (Phytophthora infestans) is the most destructive disease in potato production. Late blight was responsible for the Potato Famine in Ireland in the 1840s. This fungal disease spreads by wind, machinery, clothes, water splash or from the seed. Once a spore infects a plant, it takes about three to five days for symptoms to appear on the foliage.

The foliar symptoms are water-soaked black lesions on the top surface of the leaf surrounded by a light green halo. The underside of the leaf may be covered with a white, cottony film of spores. The more aggressive strains of late blight also attack the stalk. Tubers become infected if spores fall on the soil and work their way down to the tuber. Tuber decay by late blight shows up as a dry reddish-brown rot.

Tuber decay initiated by late blight can be overtaken by bacterial soft rot or other secondary invaders. Since the seed can be a source of late blight, growers should always purchase disease-free seed. Protectant fungicides, applied at regular intervals to the foliage, will provide protection against late blight. Systemic fungicides can also help to control late blight. See the current Guide to Crop Protection for a list of products or contact the provincial vegetable specialist.

Late blight is a regulated pest in The Pest Control Act. As cull piles represent a potential source of late blight, the Act requires that piles of cull potatoes must be properly disposed of by June 15 of each year.

Early blight (Alternaria solani) is a fungal disease that typically affects potato plants as they begin to senesce (reach maturity) in late summer. The spores of this disease are present in most soils, and are spread by contact and by air currents. Foliar symptoms of early blight involve the development of dark angular shaped lesions with concentric rings. Early blight lesions are slow growing and rarely cross the mid-vein of the leaf. Older leaves are normally attacked first, and problems with early blight worsen as the crop senesces. Early blight is consequently a problem in early-maturing varieties or in situations where crop vigour is lacking.

Early blight primarily attacks the foliage; tuber damage is rare. Tuber infection may occur at harvest as the spores found in the soil enter the small wounds created by harvest machinery. Keeping the crop healthy by selecting good seed, and carefully managing fertility and moisture, will reduce losses from early blight. Early blight protection can be achieved by spraying with protectant or systemic fungicides. Consult the Guide to Crop Protection for registered products, or contact the provincial vegetable specialist. Careful harvest and storage will reduce the risk of tuber damage by early blight.

Black Scurf or Rhizoctonia canker (Rhizoctonia solani) is also known as "the dirt that won't wash off" potatoes. Rhizoctonia arises from inoculum present in all soil or from infected seed tubers. The fungus attacks the emerging sprouts, killing some and weakening others. This results in loss of stand, stunting of the plants and rolling of the leaves. In the fall, the fungus forms small black dots (sclerotia or black scurf) on the tuber. Black scurf becomes more severe if the crop has been topkilled and then left sitting for several weeks in cool moist soil.

Control of Rhizoctonia starts with planting disease-free seed into warm soil. Seed piece treatments, will help reduce rhizoctonia incidence. Growers should harvest within two weeks of topkilling, as soon as skin set is adequate.

Blackleg/Tuber soft rot (Erwinia carotovora) is a bacterial infection that can attack both the tops and tubers of potato plants. When the bacteria attack the tops, the infected stems become blackened and water-soaked near the soil line, giving rise to the term "blackleg." Contact between plants can spread blackleg. Control of blackleg involves planting clean seed and avoiding practices that can spread disease between plants. If roguing blackleg out of seed fields, bag any rogue plants so they do not touch other plants in the field. Crop rotation of at least three years between potato crops is preferred.

At harvest, any damaged tubers can become infected by the soft rot form of Erwinia caratovora. Infected tubers develop a wet, foul smelling rot that may spread from tuber to tuber in storage. Careful harvest, prompt and thorough wound healing, followed by storage under recommended temperatures, will control the development and spread of soft rot in storage.

Common Scab (Streptomyces scabies) involves rough and irregularly shaped lesions or ruptures on the tuber surface. The bacteria that causes scab occurs naturally in most soils, but populations increase with frequent cropping to potatoes. Scab lesions are usually shallow and do not affect the yield, storage potential or table quality of the tubers.

Because scab mars the surface of the tuber, consumers and shippers reject crops that are heavily infested by scab. Hot, dry weather, particularly at tuber set, increases problems with Common Scab. Scab management starts with planting clean seed into fields with a good crop rotation. Maintaining adequate soil moisture levels at key stages in the crop's development will reduce problems with scab. Prompt harvesting is also recommended. Russet varieties are much more resistant to scab than the red or white types.

Powdery Scab (spongospora subterranea) is a soil-borne fungal disease that causes lesions or ruptures on the skin of the tubers that closely resemble Common Scab. Potatoes infested with Powdery Scab can be consumed, but consumers and shippers reject tubers that are heavily infested. Powdery Scab thrives under cool, moist conditions. Management includes buying Powdery Scab-free seed and avoiding fields with a history of problems with scab. Powdery Scab can remain in the soil for six to 10 years even without potatoes being planted. Russet types of potato are more resistant to Powdery Scab than the red or white types.

Bacterial Ring Rot(BRR) (Clavibacter michiganensis subsp. sepedonicus) is a very infectious bacterial disease. If bacterial ring rot is found on a seed farm, the entire farm is decertified and the seed crop must be sold as table or processing stock. Foliar symptoms of ring rot include wilted stems, leaf rolling and early death of the plant. Tuber infection involves rotting of the vascular ring and rough skin cracks on the exterior of the tuber. Some varieties may be contaminated with bacterial ring rot but show few or no foliar or tuber symptoms. The Seeds Act requires that seed potatoes be tested for bacterial ring rot before being sold. Control of this disease hinges on growers purchasing disease-free seed. If a farm becomes infected with ring rot, the storage and equipment must be disinfected with quaternary ammonium or hypochlorite (bleach) solutions. Machinery and storage should be washed and scrubbed to get rid of any excess soil that could be a host for the bacterial ring rot pathogen. In Saskatchewan BRR is regulated under The Pest Control Act and requires that commercial operations plant certified seed of the Foundation Class or better.

Fusarium Dry Rot (Fusarium spp.) is a serious fungal disease in stored potatoes. Fusarium infections originate in the seed or from the soil. When fusarium attacks the seed, the resulting rot reduces plant stands. Fusarium enters the tuber through wounds at harvest and produces a slow growing dry rot during storage.

Control measures include planting clean seed, avoiding bruising at harvest, and storing the crop at low temperatures. Seed pieces can be treated with to help reduce the incidence of fusarium. Refer to the current Guide to Crop Protection for seed piece treatments registered for Fusarium on potatoes.

Potato Leaf Roll Virus is a highly destructive virus spread mostly by green peach aphids. Foliar symptoms of leaf roll infection include a pale green plant and rolled leaves. Tubers from infected plants exhibit net necrosis, which appears as little brown flecks in the tuber.

Severe potato leaf roll virus infection will cause seed fields to be downgraded. Processors will reject potatoes with net necrosis as it causes brown spots in the french fries. Control of leaf roll starts with planting seed that is free of potato leaf roll virus, spraying to control aphids and not planting close to any other potato operations, especially commercial ones. Aphicides that can be used are listed in the Guide to Crop Protection.

Potato Mosaic Virus is the most common virus in potato production in Saskatchewan. The disease is introduced on the seed but spreads within the crop by aphid feeding.

Symptoms include stunting, with light green or yellow mottling of the leaves. Control starts with planting virus-free seed, spraying to control aphids and not planting close to any other operations.

Insects

Colorado Potato Beetles (Leptinotarsa decemlineata) are the most destructive insect in potato production worldwide. Commercial growers in Saskatchewan rarely have problems with the Colorado potato beetles, but it is a pest in home gardens.

These beetles are easily distinguished by the black and yellow/white lines on the backs of the adults and the orange colour of the larval stages. The eggs are also a very distinctive bright orange.

There is usually one generation of Colorado potato beetles per year. The adults over-winter in sheltered areas, such as hedgerows and treelines, and come out in the spring to eat, mate and lay eggs. The eggs hatch within a couple of weeks, and the emerging larvae begin to feed on the leaves of the crop. The larvae go through four moults, getting larger with each stage.

Severe infestations can completely defoliate the crop. Fields should be sprayed once the economic damage threshold has been reached. Defoliation of up to 20 per cent of the plant can occur with no appreciable yield loss. Control products are listed in the Guide to Crop Protection, or call the provincial vegetable specialist.

Wireworms (Agriotes obscurus) are a problem in land that has recently been in pasture or fallow. Wireworms are the larval stage of the click beetle. In the spring, wireworms will tunnel into the seed piece, causing it to rot. Later in the season, the wireworms tunnel into the developing tubers. The resulting holes make the tubers unmarketable. Wireworms may be controlled through cultivation, crop rotation and with some granular insecticides. However, full control is not likely to be achieved.

Aphids (green peach- Myzus persicae and potato aphids - Macrosiphum euphorbiae) are capable of spreading potato mosaic and potato leaf roll viruses. These aphids normally do not over-winter in Canada, but are carried into Canada from the United States on wind currents during the summer. Some green peach aphids may overwinter in greenhouses. Control includes spraying (see potato leaf roll virus and potato mosaic virus). Seed growers can use yellow pan traps to monitor aphid numbers as an aid in timing spray applications.

Weed Control

Weed control is very important for the first 60 to 70 days after planting potatoes. Once the crop is established, there is enough shade and cover from the potato plants to suppress most new weed growth. Working the soil in the spring prior to planting will eliminate winter annuals. Hilling the potato crop also helps control any weeds that may have emerged after planting.

Check the Guide to Crop Protection for herbicides registered for weed control in potatoes and always read the product labels for rates and weeds controlled.

Non-infectious disorders

Management of hollow heart includes: consistent irrigation, closer row spacing and planting varieties that are resistant to hollow heart. Russet Burbank and Viking are susceptible, while Ranger Russet is highly resistant

Growth cracks are large, irregular cracks that form on the tuber. The causes and recommended management practices are the same as for hollow heart.

Blackheart occurs most commonly in storage but can also occur in the field. Blackheart is due to a lack of oxygen in the potato pile or in waterlogged areas in the field. Symptoms include uneven black discolouration of the tissue in the tuber and a strong odour. Management includes adequate ventilation in storage, decreasing storage temperatures and avoiding excessive moisture prior to harvest.

Greening is the accumulation of chlorophyll in the tuber caused by direct exposure of the tubers to sunlight or artificial light. Greening increases glycoalkaloid levels in the tubers, resulting in a bitter flavour. Greening can be controlled by proper hilling, avoiding varieties that set high in the hill, adequate moisture to avoid soil cracks in the hill, and by avoiding light exposure during storage or in the retail chain.

Knobbiness refers to secondary growths that occur on the tuber. Causes are the same as hollow heart. See above for management practices.

Frost causes cells in the tubers to rupture, resulting in breakdown of the tuber. The extent of frost damage to the crop depends on both the degree and duration of the frost event. It takes a few days after the frost event for the tubers to show signs of frost damage. If possible, harvest should be delayed to allow some of the tubers to break down, allowing easier grading.