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By Tom Woods
When the tops of the potatoes start to die down, you will know it is time to harvest. One issue you may see on newly harvested potatoes is brown scab-like wounds (see photo). This is a disease called scab. The potatoes are still fine to eat. The causes of the problem and how to prevent it in the future will be discussed.
This disease is caused by the bacterium Streptomyces scabies, which persists in the soil for long periods. Brown corky scabs or pits occur on potato tubers. These spots enlarge and merge together, sometimes covering most of the tuber. Leaves and stems are not affected.
Scab is most severe in dry soil with a pH above 5.5, and in soil low in nutrients. Tubers infected with scab are edible; however, when blemishes are removed, much of the tuber may be wasted.
Prevention and treatment
Soil pH should be between 5.0 and 5.3; therefore, avoid alkaline materials such as lime and wood ashes. Scab is favored under low soil moisture conditions, so the garden soil must be kept moist during the active growing period of the tubers (particularly 4-9 weeks after planting). Although potatoes are heavy feeders, high nitrogen levels can increase scab severity.
Additionally, high ratios of calcium to potassium can increase disease. Do not use manure on potatoes, because the bacterial spores can pass intact through the digestive tracts of animals.
Plant potatoes in the same area only once every 3-4 years. Do not plant Irish potatoes after beets, carrots, radishes, parsnips, rutabagas and turnips, which are also susceptible to scab. Use certified seed pieces that are resistant to scab.
The following potato varieties are scab tolerant: Superior, Goldrush, Red LaSoda, Red Gold, Caribe, Dark Red Norland, Butte, Carola, Russett Burbank, and Sebago. There is no reliable chemical control.
Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.