Crop Profile for Potatoes in North Dakota

Prepared December, 2000

General Production Information

North Dakota ranks 6^th in the United States in potato production:

• Approximately 5 % of the nation’s potatoes are grown in North Dakota
• In 1996 producers planted 134,000 acres of potatoes and harvested 131,000 acres with a yield of 220 cwt/acre. The value of the 1996 crop was estimated at $135 million.
• In 1997, producers planted 125,000 acres and harvested 105,000 of those acres with a yield of 205 cwt/acre.
• In 1998, producers planted 126,000 acres of potatoes and harvested 122,000 acres with a yield of 235 cwt/acre.
• North Dakota produced 32,079, 29450, 24041 acres of certified seed potatoes in 1995, 1996, 1997 respectively.

Production Region

The main potato producing region in North Dakota is the Red River Valley. The northeast, east central, central and north central portions of the state comprising the major potato producing region. Irrigation is a common practice on the lighter soils of North Dakota. The primary cultivar grown under irrigation is the Russet Burbank Approximately one quarter of North Dakota’s potatoes are grown on irrigated soils. North Dakota is also a producer of seed potatoes.

Cultural Practices

Producers are usually finished planting potatoes by June 1. Irrigated potatoes are planted by mid May. It is generally considered best in nonirrigated production to precede potatoes with small grains or fallow. Sugarbeets and potatoes should not follow each other in a rotation because of the depletion of soil moisture. This would substantially reduce yields of both crops. Common practice is to precede potatoes with a grain crop and a grain crop the two years directly after the potato crop, entirely excluding fallow from the rotation. Common practice in the Red River Valley is to fall plow, followed by two or more field cultivations and frequently a harrowing in the spring. Producers harvest potatoes from September to the end of October.

Irrigated potatoes of North Dakota are generally grown on coarse textured soils low in organic matter. Typically, these soils are characterized as sandy loams or loamy sands, low in native fertility, and quite acidic. These acres typically have a higher yield compared to non-irrigated potatoes.

Insect Management

There are many key pests of potato, fortunately relatively few occur in North Dakota and the Red River Valley. The Colorado potato beetle is the most serious pest found in North Dakota. Other potato pests occur sporadically. The aster leafhopper is of concern for producers who raise potatoes intended for chipping. A seed potato crop can be deemed unsuitable for seed potatoes because of the viruses transmitted by aphids. Wireworms potentially could be a problem for potatoes on light soils. Other insect pests would cause problems if it were not for the control measures being taken against the key pests. The major pests are discussed below. Insecticides used for controlling these pests and the estimated number of treated acres in 1996 are summarized in Table 1 (Zollinger, et al. 1998). Table 2 summarizes recent data for insecticide use obtained from a USDA-NASS survey on agricultural chemical use in fall potatoes (USDA-ARS, 2000).

Colorado Potato Beetle (Leptinotarsa decemlineata)

The Colorado potato beetle (CPB) is a constant threat as a defoliator in potato fields. Both the adult and immature forms feed on the foliage of the potato. CPB overwinter as adults and emerge in the spring with the developing potatoes. Overwintered adults feed, then lay eggs for the first generation. First generation larvae feed from early June to mid-July. Larvae develop into adults around mid-July. In North Dakota and the Red River Valley there is a second generation of adults that usually emerge towards the end of August. Insecticide control measures target the small larvae. They are the most susceptible to the insecticides, reducing the number of small larvae before they become large larvae. The larger larvae cause the greatest amount of defoliation. Resistance to chemicals is of concern to the producers of North Dakota. There are documented cases of CPB being resistant to the pyrethroid (Asana and others) and carbamate (Furadan) insecticides in North Dakota. When used, these classes of insecticides should be limited to once per season to reduce the possibility of a resistant population. Cultural control practices for CPB control are to distance potato fields from fields of the previous year by crop rotation. Current biological control practices that are commercially viable are the use of genetic engineered plants that contain Bt (Bacillus thuringiensis).

Leaf Hoppers

Potato Leafhopper (Empoasca fabae)
Aster leafhopper (Macrosteles fascifrons)

There are two leafhoppers commonly associated with potato fields in North Dakota, the potato leaf hopper and the aster leafhopper. The leaf hoppers overwinter in southern United States and are carried to the northern states by upper level winds. Aster leafhopper also overwinters in the egg stage, producing a local population. They build up their populations in alfalfa or grain fields before the potatoes have emerged in the spring. The potato leafhoppers can cause significant yield loss by feeding on potato sap. Most years they arrive late enough not to be of concern; other years, such as 1990 and 1999, they arrived early and needed to be managed with insecticides. Unlike the potato leafhopper, only the aster leafhopper adults are found on the potato. The aster leafhopper does not reproduce on the potato. The aster leafhopper is a vector of the potato purple top which affects chipping potato and is why it is a species of concern for many producers. Biological control has not been a viable option to control these insects. Because these insects cause damage in different ways it is important to make the correct identification of the species.

Aphids

Green peach aphid (Myzus persicae)
Potato aphid (Macrosiphum cuphurbiae)
Buckthorn aphid (Aphis nasturtii)

Aphids are a major pest of seed potatoes. These aphids are the primary vectors of viruses which can lead to the rejection of the seed lot. For this reason, seed producers must keep aphid numbers much lower than what can be tolerated in table stock potatoes. Control measures are targeted specifically against aphids in an effort to keep virus spread to a minimum in seed production fields. Control is seldom necessary in normal commercial production, though even this may be changing in recent production years. The treatment threshold for aphids in seed stock is 10 aphids per 100 leaves; the threshold to prevent yield loss in table stock is 30 aphids per 100 leaves. The two most important viruses transmitted are the potato leaf roll virus (PLRV) and potato virus Y (PVY or mosaic). Although there are several species of aphids on potatoes, the green peach aphid is the most common and the most efficient vector for PLRV. The potato aphid and buckthorn aphid can transmit PLRV. Green peach aphid is an efficient PVY vector, but there are many other aphids that act as a vector for PVY.

Potato Flea Beetle (Epitrix cucumeris)

The potato flea beetle is often the first pest to attack the potatoes during spring. The overwintering adults can cause economic damage to the newly emerging potato plants. The damage is identifiable by small shot holes in the leaves. There are usually two generations of this beetle in North Dakota. Mid-season damage is uncommon because the plants are larger and are often treated for other insect pests. Fortunately, the tuber beetle, Epitrix tuberis a much more destructive pest, does not occur in North Dakota.

Other Damaging Insects

Grasshoppers, especially redlegged grasshopper, Melanoplus femurrubrum; black blister beetle, Epicauta pennsylvanica; and numerous other defoliators occur on potatoes and occasionally cause conspicuous injury. The economic consequence of these pests seldom justifies treatment. Potatoes are relatively tolerant of some defoliation especially if the attack is not sustained. Cabbage looper and other caterpillars are probably never of economic importance on potatoes.

The soil insects, wireworm and white grub, are occasional pests of potato in the region. Soil insecticides have provided some protection from damage when populations are small. Unfortunately, infestations may go undetected until tuber damage is found at harvest

Table 1. Registered insecticides and their usage in North Dakota to manage potato insect pests in North Dakota.

Table 2. Registered insecticides and their usage during 1999 in North Dakota to manage potato insect pests.

New Advancements in Insect Control

When managing Colorado potato beetle and the green peach aphid, insecticide resistance to currently available products is a major concern. New insecticidal discoveries are providing some attractive alternatives for use in managing these pests. The goal should be to preserve these new alternatives through industry coordinated resistance management programs.

One major group of insecticides being used and developed are the Neonicitinoids. This class of insecticidal compounds includes imidacloprid and thiamethoxam. Imidacloprid (Admire and Provado) has seen a significant increase in use in the region since its section 3 labeling in 1995 (Table 2). The insecticide has provided effective control of both these pests within the region. The Admire formulation is applied in-furrow or to seed pieces; the Provado formulation is applied as a foliar spray. Use of imidacloprid is restricted to a total of 0.31 pounds active ingredient per growing season, or the high label rate for soil application with Admire. However, this does not restrict the use of the Provado formulation when lower rates of Admire are utilized. Thiamethoxam (Platinum and Actara) has registration pending, possibly for the 2001 season. Platinum is a soil use formulation; Actara is a foliar formulation. Labeling for this insecticide will recommend not using the foliar formulation after the soil application is used, but will not restrict the use. With two effective compounds available from the same chemical class, greater consideration in the development of a comprehensive resistance management program among manufacturers should occur. Without a common strategy, these insects could face season long exposure to one or both of these insecticides which share their chemistry, a scenario that could enhance the risk of insecticide resistance developing.

Spinosad (SpinTor), a reduced risk pesticide, is being used to manage Colorado potato beetle (Table 2). The section 3 label was issued in 1999. Spinosad has been classified as a NaturalyteŽ insecticide product, being biologically derived from the fermentation of Saccharopolyspora spinosa, a naturally occurring soil organism. Spinosad has insecticidal activity against coleoptera, lepidoptera, and others. It is recommended for use against small larvae of the Colorado potato beetle. The label restricts use to two applications per generation, and not for use against consecutive generations. Spinosad does not have activity against the other major potato pests in the region.

Pymetrozine (Fulfill) is registered in potato for managing green peach and potato aphids. The active ingredient belongs to the insecticide class Pyridine azomethine, a class of compounds not previously used as pesticides and where no resistance or cross-resistance is known. Pymetrozine inhibits neural control of feeding behavior in aphids and is expected to aid in the reduction of virus transmission. The compound has no significant impact on other major potato insect pests and has demonstrated only low levels of toxicity to beneficial insects.

Another insecticide of interest is fipronil (Regent), which is being considered for registration on potato to aid in the management of wireworms and white grubs. Fipronil belongs to the chemical class Fiproles. It is registered for use on field corn to manage the soil insect pests mentioned. Currently, the only insecticides registered in the state for managing wireworms in potato are phorate, ethoprop, and diazinon, all organophosphate compounds. No insecticides are labeled specifically for managing white grubs in potato.

Disease Management

Diseases are present at every stage of the potato life span and are known to cause severe economic loss to potato growers. There are potato diseases that reduce seedling stands, cause leaf spots, cause wilts, cause storage rots and are carried by the seed tuber from one crop to the next. Any improper cultural practice used in growing the potato crop can result in disease. Management is probably the most important tool in controlling, avoiding, or reducing crop losses in North Dakota. Disease resistant varieties are the best way to reduce disease occurrence, but resistance does not equate to immunity to diseases. The best cultural and sanitation practices should be integrated into the growing system to reduce the spread of diseases. The different diseases are discussed below.

Fungal Diseases

Fungicides are used to control the fungal diseases in North Dakota. The two main types of fungicides are contact or systemic fungicides. Contact fungicides, commonly called protectant fungicides, work on the surface of the plant to protect against infection. In contrast, systemic fungicides are absorbed by the plant tissue and are active from inside the plant.

Protectant fungicides kill germinating fungus spores to prevent infection, but cannot protect against systematic infections, such as wilts and virus diseases. Protectant fungicide are used for seed piece treatment and to protect against foliar diseases, primarily early and late blight. They cannot prevent disease development once infection has occurred and must be present on the leaf before infection occurs. Common protectant fungicides include chlorothalonil (Bravo, Echo), copper hydroxide (Kocide, Champion), mancozeb (Dithane, Manzate, Penncozeb, Manex II) maneb (Maneb 80, Maneb Plus Zinc F4), and triphenyltin hydroxide (Super tin, Agri Tin). Most are broad spectrum fungicides.

Systemic fungicides work after the plant has been infected. Many systemic fungicides target certain fungi and have limited broad spectrum activity compared to protectant fungicides. Systemic fungicides commonly move to the edges of the leaves when applied. Most move upward if they are absorbed into the conducting tissues of the plant. Metalaxyl (Ridomil), a acylalanine fungicide, moves downward, to some extent. Other systemic fungicides are thiabendazole (TBZ, Mertect) and thiophanate methyl (Topsin M). Cymoxanil (Curzate) and azoxystrobin (Quadris) are effective systemic fungicides against late blight fungus. The benzimidazoles are primarily effective against protecting against infection of seed piece pathogens. Metalaxyl (Ridomil) is active against the lower fungi, namely Pythium and Phytophthora (which includes the late blight fungus), and has both protectant and curative properties. Fungi have developed resistance to certain fungicides rather quickly, so some fungicides come as a package pre-mix. The purpose of the package pre-mix is to delay the development of resistant fungi. Table 3 and 4 summarize the estimated amount of foliar fungicides used in 1996 and 1999 on North Dakota potato acres. Table 5 summarizes the estimated amount of seed treatment or soil applied fungicides used in 1996 on North Dakota potato acres.

Verticillium Wilt

There are several species of Verticillium fungus that occur in the soil. Verticillium alboatrum and Verticillium dahliae are the two species associated with "early dying" or Verticillium wilt of potatoes. The occurrence of either species is temperature related. Verticillium alboatrum is associated with cooler temperatures, and V. dahliae with warmer temperatures. These fungi survive in potato fields through infected potato debris that remains in the harvested field. Verticillium wilt may remain in the field for several field seasons as small sclerotia or dark mycelium. The loss that results from a wilt is reduced tuber size (quality and yield) and loss of quality because of tuber discoloration. Wilt may become more severe with the presence of the lesion nematode (Pratylenchus penetrans).

Short crop rotation systems may not be a useful management tool because of the survival system of the wilt fungus. However, if possible a rotation system where potatoes would not occur every other year should be helpful. Of more importance would be the physical removal of disease-infected potato stems after harvest. Several new varieties have a high degree of disease resistance. These varieties should be grown where possible, along with using the recommended sanitation practices. In heavy infestations with severe yield losses, soil fumigation may be required to reclaim the land.

Fusarium Wilt

Fusarium sambucinum is common in soil and in infected crop debris. The fungus survives as spores or mycelium in soil. Infection is favored by high soil temperatures and moisture conditions and infection occurs mainly through the roots. Fusarium wilt is similar to Verticillium wilt in diagnosis but Fusarium wilt will have a yellowish brown discoloration of the stem tissue and may be more fibrous. It appears that Fusarium wilt fungus has not infected North Dakota potato fields to the same degree as Verticillium wilt. There is no practical control at this time. Disease free seed and crop rotation are recommended, but are not solutions for this disease.

Early Blight

Early blight is caused by the fungus Alternaria solani. Symptoms develop in July or August on the lower foliage. Early blight is favored by warm weather, especially if it is alternating wet and dry. It is more serious on early maturing varieties. Immature tubers are readily infected, but mature tubers are infected only through wounds. Mature tubers may be infected through wounds or bruises made at harvest.

Crop rotation helps reduce levels of overwintering early blight fungus. Effective fungicides include azoxystrobin, mancozeb, maneb, chlorothalonil, and triphenyl tin hydroxide. Copper fungicides are less effective. Fungicides applications should begin when the first lesions are visible. These fungicides will not kill existing leaf spots but will protect against new infections from spores produced on early lesions. Complete coverage with fungicide applications is necessary for good control. To prevent tuber infection, allow tubers to mature prior to harvest. Vine killing before harvest will allow the tubers to mature and reduce the potential for tuber infection. Use harvest practices and equipment that minimize wounding and bruising.

Late Blight

Late blight is caused by the fungus Phytophthora infestans. The current P. infestans genotype present in the region is resistant to the fungicide, metalaxyl (Ridomil). Late blight is a serious problem in potato growing areas where cool, humid weather is common. Outbreaks in the warmer and drier areas of North Dakota tend to be sporadic, occurring only when weather conditions are favorable. Late blight is more common in irrigated areas, especially near the pivot. Late blight can infect leaves, stems and tubers. Soft rots may follow late blight infections resulting in destruction of the entire tuber.

Late blight survives the winter in infected tubers. Primary infection comes from spores produced on the infected sprouts of diseased seed pieces or tubers. Secondary infection comes from the spores produced by late blight on the crop. These spores can either infect tubers or leaves of other potato plants. The best management against late blight is to use blight-free seed and destroy cull piles and volunteer plants. Crop rotation is used to avoid or reduce carryover of the late blight fungus. If the weather has been cool and damp and more cool and damp weather is predicted, spraying for late blight should begin immediately. The management program relies heavily on the use of protectant fungicides such as cymoxanil (Curzate), dimethomorph (Acrobat), and propamocarb (Previcur). Fields should be inspected carefully for late blight. Another practice is to hill potato rows to prevent the spores from reaching the tubers. Kill vines at least two weeks before harvest so that the tuber skin is mature and any infected tubers will have disintegrated before digging.

Fusarium Dry Rot

Fusarium fungi can cause seed decay, wilt and dry rot decay in storage. Most varieties do not have resistance to dry rot. Fusarium sambucinum infects potatoes through wounds or injuries associated with harvesting and handling operations. Manage and control dry rot by assuring that the tuber skin is mature before harvesting. This is accomplished by vine killing. Avoid injuring or bruising tubers at harvest and promote a storage environment for suberization. Thiabendazole may be applied post-harvest as part of a management program for dry rot control. The dry rot fungus has developed resistance to thiabendazole in many parts of the United States and Canada; thiabendazole efficacy is reduced where resistance is present. To treat for seed decay dry rot before planting warm seed tubers for a week before planting or cutting seed pieces. Also treat with a fungicidal dust. Sanitizing equipment will reduce dry rot spread between seed lots.

Rhizoctonia Canker / Black Scurf

The Rhizoctonia fungus is found in the soil and on tubers wherever potatoes are grown. Rhizoctronia infection causes cankers on young plant stems prior to emergence, resulting in uneven stands or gaps in the potato rows. Avoid planting in cool wet soils, avoid irrigation until after emergence, and do not hill until plants have emerged. This disease is very important in irrigated potatoes where the very susceptible Russet Burbank is the primary variety grown for use in processing. Harvest within two to three weeks of vine killing to limit the amount of sclerotia formed on the tubers as sclerotia form on senescing plants.

Silver Scurf

Silver scurf, caused by Helminthosporium solani, is most noticeable on red-skinned potatoes. No rot develops and symptoms are relatively superficial. Infected areas become somewhat wrinkled in storage due to excessive water loss. This fungus survives in the soil and on tubers. Diseased seed pieces are the most common source of infection. Manage silver scurf by using disease-free seed and harvesting tubers as soon as they mature. A seed piece treatment with fludioxonil can reduce the amount of silver scurf going into storage. Preventing condensation by reducing the humidity and temperature in storage may reduce symptom development but not reduce the occurrence of the disease.

Common Scab

Common scab is caused by the fungus Streptomces scabies on rapidly developing tuber tissues. In addition to tuber infections, stems, rhizomes and roots may develop milder symptoms. It is favored on alkaline soils, which are common in the Red River Valley of North Dakota. Dry soil condition also favor scab development. Resistant varieties are the most effective means of control in the region. Manure should not be applied to fields where potatoes may be grown, as this may increase the severity of scab. Increase the length of rotation between potato crops to reduce scab. Use of sulfur to reduce soil alkalinity has been recommended in other potato regions but is not feasible in highly alkaline soils. No chemical seed or soil treatments have been effective in the Red River Valley. Partial control under irrigation is possible by maintaining adequate soil moisture.

Pythium Leak

Leak is one of two water rots that can cause serious storage rot problems during years when harvest conditions are unfavorable. The Pythium fungus, present in the soil, invades the tubers through wounds made during the digging operation. To manage leak do not dig potatoes in unusually warm temperatures and reduce tuber injuries. If leak develops, lower pile temperature, increase the air flow and dry down the diseased tubers with dryer forced air. Dry air is not conducive to wound healing or suberization, so the storage should be watched closely.

Pink Rot

Probably the most prevalent of the two water rots is pink rot caused by the soil_borne fungus Phytophthora erythroseptica. This disease can be found in the field before harvest and is characterized by rotted tuber tissues that turn pink after exposure to air for 20 to 30 minutes. Another important diagnostic trait for pink rot is that the rot will usually appear to start from the stem end of the tuber and will then progress through the tuber in a very uniform manner, often with a nearly straight line between the healthy and the diseased portions of the tuber. Pure pink rot is not a slimy soft rot, but infected tissues are easily, and often invaded by soft rot bacteria which will cause this symptom. In a tuber that is infected with the pink rot fungus alone, the rotted tissues will retain some structure and firmness but not nearly as much as the healthy portions of the tuber. The texture of the infected tuber tissue is much like that of a boiled potato and is referred to as a "cooked potato" texture. Another feature of pink rot is the characteristic smell of ammonia that is given off by pink rot infected tubers. This odor can frequently be detected in potato storage structures prior to the development of visual symptoms.

Pink rot and pythium leak can both be managed efficiently using applications of metalaxyl (Ridomil) during the growing season. One well timed application, when the largest tubers are the size of a nickel to a quarter, is generally sufficient. However, water rot susceptible varieties may require a second application. Irrigation managers should avoid excess watering late in the growing season.

Table 3. Registered foliar fungicides and their usage in North Dakota to manage potato diseases.

Table 4. Registered fungicides and their usage during 1999 in North Dakota to manage potato diseases.