Diseases of Potatoes

Donald M. Ferrin, Hadziabdic, Denita, Hollier, Clayton A., Overstreet, Charles

Late blight is caused by Phytophthora infestans. All plant parts are susceptible to infection. Late blight is a major problem in potato production worldwide. The Irish potato famine of the 1840s was caused, in part, by this disease.

Late blight. Leaf symptoms include necrotic lesions surrounded by chlorotic tissue. Under favorable conditions (cool and wet weather), petioles and stems may become infected. Infected tubers are especially susceptible to invasion by soft rot bacteria.

Common scab is caused by the bacterium Streptomyces scabies. This disease can significantly decrease tuber quality and reduce marketability. Symptoms are variable and may include browning of the tubers in the early stages.

Black leg is caused by the bacterium, Pectobacterium carotovorum subsp. carotovorum (previously Erwinia carotovora subsp. carotovora). The bacteria enter the tubers through stolons, creating black sunken lesions at the stem end. Note the infected tuber with soft rot in the center. Often secondary bacteria attack already weakened plants, resulting in total plant collapse.

Rhizoctonia canker on potato is caused by the soil-borne fungus, Rhizoctonia solani. The disease is characterized by brown to black sclerotia (fungal bodies) that develop on the tuber surface. Cool and moist soils are optimal for infection.

Fusarium wilt is caused by various soil-borne species of Fusarium. Disease can reduce both quality and yield of potato tubers. Vascular tissues of the stems and tubers exhibit a tan discoloration, characteristic of this disease.

Black heart is a non-pathogenic disorder of potato tubers caused by oxygen deprivation. Sharply defined purplish, blackish areas in center of tuber cavities are characteristic of black heart. Disease can occur in the field and storage.

Potato leak (watery wound rot or shell rot) is caused by soil-borne pathogen, Pythium ultimatum. Disease affects only tubers in which dark lesions spread fast, resulting in soft rot and foul odor. A distinct dark brown margin may separate diseased from healthy tissue. As disease progresses, infected tubers become almost black and exude a dark, watery liquid.

Pink rot is caused by soil born pathogen, Phytophthora erythroseptica. When cut and exposed to air, infected tuber tissues change from cream to pink color in period of 20-30 minutes and then progressively darkens to brown and finally black. Color change is good diagnostic tool to distinguish between this disease and potato leak. Disease is favored by cool weather and wet soils.

Root knot nematodes (Meloidogyne spp.) cause the formation of galls on potato roots and tubers. Severely infected plants may turn yellow, resulting in stunting and wilting. Root knot nematodes can cause significant damage to potato production, reduce the quality, yield and marketability of the product.

12/9/2005 2:21:46 AM
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