Potato Diseases, Pests, and Disorders

The potato seems to be subject to more diseases than many other vegetables, but the severity of any one of them depends largely on the section of the country and on weather conditions. Pests and physiologic disorders create more problems for potato producers. Some pests can damage plants and stunt production; all pests can spread disease. Physiologic disorders can ruin a commercial crop.

Diseases

Late Blight Fungus (Phytophthora infestans) has historically been the most generally destructive. The Irish famines of 1843-47 were caused when this disease wiped out successive potato crops which, unfortunately, were relied upon by the Irish peasantry for 80% of their diet. The fungus is airborne, waterborne and seedborne. The innoculum sources are seed, cull piles, and volunteers. The disease first appears as water-soaked areas on the leaves, with a white mold on the underside. Controls include using resistant varieties and disease-free seed; spraying with fungicides; eliminating cull piles; and killing potato foliage 10 days to two weeks before digging.

Early Blight Tuber Blemish (Alternia solani) is a fungus that enters the tuber through cuts and bruises. It is airborne and waterborne. The fungus innoculum is carried in soil and plant debris. Shallow, necrotic sunken blemishes will appear after a period of storage. Controls include spraying with fungicides, practicing crop rotation and good sanitation.

Black Scurf and Stem Canker are two different forms of the same fungus (Rhizoctonia solani) . The first, black scurf, looks like brownish-black dirt on the potato skins, which can't be removed. The spore bodies of the scurf (a corky sclerotia) do not affect cooking quality. But if the tubers are used as seed, the sclerotia are the source of innoculum for a more damaging form of Rhizoctonia, stem canker. This airborne and soilborne fungus causes cankers which can girdle underground sprouts, stunt or kill stems, and infect developing stolons leading to lower tuber set. The result can be a much poorer crop. Controls include treating seed tubers, avoiding planting in cold soils, and using disease-free seed.

Common scab (Streptomyces scabies) is a fungus chiefly affecting the appearance of tubers, producing rough, scabby spots. Scab is caused by an actinomycete that spreads rapidly in dry alkaline soils. Keeping the soil more acidic, planting resistant varieties, practicing medium-to-long crop rotations, avoiding infested fields, treating seed and maintaining uniform moisture all control this disease

Black leg (Erwinia carotovora var. atroseptica) is a bacterial soft rot of the base of the stem. Rotted areas of infected tubers are cream to dark tan with a distinct, dark border between healthy and infected tissue. The rotted tissue is soft, mushy and can be easily be rinsed away with water. The bacteria is seedborne, soilborne and spread by unsanitary equipment. Soft rot has no odor. Controls include using disease-free seed, preventing bruising of tubers, treating and suberizing seed pieces, warming seed and practicing good sanitation.

Bacterial Soft Rot (Erwinia carotovora) is a bacterial soft rot affecting tubers. Areas of infection are cream to dark tan with a distinct, dark border between healthy and infected tissue. The rotted tissue is soft, mushy and can be easily be rinsed away with water. The bacteria is soilborne and spread by contact. Soft rot has no odor. Controls include allowing tubers to suberize after harvest; drying washed potatoes; and minimizing wounding and bruising.

Fusarium wilt (Fusarium solani) is a fungus which causes yellow leaves and brown rings inside the stems and tubers and, most importantly, results in tuber rot during storage. It is soil- and seedborne. Controls include using disease-free seed and practicing medium-to-long crop rotations.

Dry rot (Fusarium spp.) is a soil- and seedborne fungus which is easily spotted on tubers. It produces lesions which are fawn to dark chocolate brown. The rotted tissue is relatively firm. As the lesions age, the tissues become dry and punky, and may contain areas that are yellow, pink or orange. Medium-to-long crop rotations are a particularly good method of reducing potato losses. Other controls include preventing wounding at harvest, treating tubers with fungicide, allow tubers to suberize and then storing at low temperatures.

Bacterial ring rot (Clavibacter michiganense ssp. sepedonicum or Corynebacterium sepedonicum) can be identified by the cracking of the tuber's skin. The tissues may have a vascular ring discoloration. The bacteria is seed-borne and spread by infected equipment and storage sheds. Controls include planting disease-free seed, using whole seed, and disinfecting storages and equipment.

Silver scurf fungus (Helminthosporium solani) does not usually result in losses in yield, but causes cosmetic defects which impact commercial sale. It is soil and seed-borne. Controls include using disease-free seed, practicing crop rotation, and using resistant varieties.

Some prevalent viruses are Rugose mosaic (potato virus Y), which causes spotted leaves and stunted growth, and Potato leaf roll and Net necrosis. Controls for these include controlling aphids, using resistant varieties, roguing diseased plants, and practicing good sanitation. Spindle tuber, a viroid, is spread by diseased seed and infected equipment. Controls include using disease-free seed, and reducing machinery use in fields after plants emerge. Yellow-dwarf (a virus) affects both vines and tubers. It is carried in seed, and spread by leaf-hoppers and using infected equipment. Controls include using disease-free seed and controlling leafhoppers.

Serious Pests

The most well known pest is the Colorado potato beetle. The adult beetle is quite colorful with convex, yellow, hard shells, which have five black stripes. The beetles originally came from Mexico and fed on buffalo burs. But they switched their food source when potato production became widespread. During the winter the beetles hide in the ground. In the spring they emerge to lay their eggs on the leaves of the potato plant. When the eggs hatch, both the larvae (soft, dark red with black head and spots) and the adults feed on the leaves. Together they can strip a plant bare. Handpicking is best but growers resort to sprays to control them.

Some other pests are the very small, shiny, jet-black potato flea beetle, which eats small, round holes in the leaves until they look like sieves; and the potato aphid. Aphids and leafhoppers carry both bacteria and viruses and can compound problems. Potato tuber moths lay their eggs in cracks in the ground and can be controlled by keeping the fields well watered so no habitat for eggs can be created. Wireworms tunnel in potato stems, roots and tubers and can do a great deal of damage. They are not really worms, but beetle larvae.

Disorders

Brown Center, Hollow Heart, and Translucent End are physiologic potato disorders resulting from vagaries in soil and climate, planting practices, or from potato variety susceptibility. Good crop management practices can control these disorders. Brown Center and Hollow Heart both describe the tissue appearance of the potato and have serious commercial impacts for potato processors and retail customers. Translucent End describes tubers with translucent ends, ends high in reducing sugars and low in starch, or tubers with either a dumbbell shapes or pointed ends. The translucent end potatoes produce french fries that are dark on one end. This creates an unsatisfactory product.

Conclusion

Many potato diseases are carried in the tubers. Most fungus disease spores are too tiny to be seen and in small amounts will not usually make a potato inedible. But when the tubers are used as seed potatoes, the fungus will begin to spread as the potato plant begins to grow. One infected potato can infect all the other plants in a field. Viruses are an even more serious problem. The answers to controlling both problems are two-fold. First is continuing the use of "certified" or approved high-quality seed potatoes, and planting in properly-disinfected, disease-free fields; the second lies in continued research into pest management.

In Idaho, the majority of seed potatoes used are grown in Montana and North Dakota. Seed potatoes are inspected by the Idaho Crop Improvement Association (ICIA) before they are certified and sold. The inspector looks for the presence of zero-tolerance diseases such as Bacterial Ring Rot, Corky Ring Spot, and Root-knot Nematode. If any of these diseases are found, the seed potatoes will not be certified. Fusarium Dry Rot, Rhizoctonia Canker, Late Blight infection and Soft Rot must be below established tolerances. Levels of other defects, such as Hollow Heart and Growth cracks, are also established.

In Washington, the State Potato Commission funds seed lot trials which are conducted by scientists at Washington State University. Both seed potato and commercial potato growers provide seed samples which are then planted in the state's commercial growing area. The results of the seed trials are made available to the public. Commercial growers can then select the highest quality seed.

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