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Figure 1.  Blackleg on potato

SYMPTOMS

Black leg is a common disease of potato in Kansas. It is most severe in the early part of the growing season during cool, wet weather. Black leg is a leading cause of poor stand establishment and "skips" in the field. The pathogen may kill sprouts from tubers before they emerge, resulting in a poor stand. The plants also may be affected shortly after emergence. One characteristic symptom of the disease during this period is the development of an inky black decay which usually begins on the stem near the soil surface. Affected plants are stunted and eventually wilt and die. Tubers produced by infected plants may show symptoms ranging from vascular discoloration to a soft rot.

The disease may also affect tubers in storage, resulting in a soft rot. The disease normally appears at the stolon end of the potato and progresses through the center of the tuber. As secondary bacteria enter the rotting tissue, they create a foul odor.

DISEASE CYCLE

The bacteria survive in or on seed tubers. They may persist for a short period in the soil, but soil borne inoculum is not an important factor in the development of the disease.

Nevertheless, bacteria may overwinter in soil inside infected tubers or other plant debris. The disease is most severe under cool, wet conditions at the time of planting followed by high soil temperatures after plant emergence.

CONTROL

Control of blackleg depends largely on sanitation. Plant only certified seed. While cutting seed pieces, frequently disinfect cutting tools in a 10% solution of household bleach or some other type of disinfectant. Treating seed pieces with captan dust will help prevent other tuber problems that in turn will help prevent black leg. Avoid planting seed pieces in poorly drained soil. If black leg is noted in the garden, remove and destroy all infected plants. Make sure the tubers are mature before harvesting and avoid excessive bruising of the tubers while lifting. Most commercial growers in Kansas do not store tubers for long periods, but homeowners who wish to store tubers should provide good ventilation during storage and avoid any surface water buildup on the tubers resulting from condensation.

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Web updated 9/01/06
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