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Jesica Conrad/Master Gardener: Dealing with those potato diseases

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Columns Bemidji,Minnesota 56619
Jesica Conrad/Master Gardener: Dealing with those potato diseases
Bemidji Minnesota P.O. Box 455 56619

The common diseases of potatoes that plague gardeners are early blight and late blight. By the way, tomatoes and potatoes belong to the nightshade family (as well as eggplant and peppers). Rotate your tomato and potato crops to avoid spreading from one crop to the other.


With early blight, the fungus Alternaria solani overwinters in infected residue in the soil and on other weedy solanaceous (nightshade) plants. Susceptible leaves are those that are older and under stress with nitrogen deficiency and cool moist conditions. When the leaves touch the ground or when spores are splashed or blown, the leaves are infected directly.

Appearing near the end of the season, be alert for small (1-2 mm) brown lesions that are dry and papery on older, lower leaves spreading up toward new growth. There may be characteristic dark, circular rings of raised and necrotic tissue. Leaf tissue often turns chlorotic (yellow) at the edge of the lesion. As the disease progresses, the entire leaf can become chlorotic and then necrotic (brown). Infected potato leaves usually do not fall off. The infected potato tubers develop dark, sunken lesions that are often surrounded by a purplish raised border. Lesions will increase in size in storage, though they will remain superficial.

Multiple control plans include planting disease-free and resistant seed potatoes. Test your soil as nitrogen and phosphorus deficiencies increase susceptibility. Rotate potatoes, tomatoes, pepper and eggplant for three to four years. Water in the morning at the base of the plant to allow them to dry out during the day. There are chemicals that are effective against early blight. Contact the University of Minnesota Extension for advice if you suspect you have early blight.

Late blight (the Irish potato famine of 1845) is caused by the fungus Phytophthora infestans. This can occur anytime during the season but typically during autumn. Ideal conditions are when the temperatures are between 60 to 70 degrees with night temperatures between 50 to 60 degrees with humidity nearly 100 percent.

This blight survives the winter by living in potato tubers left in the ground. As the fungus grows in these leftovers, the spores are carried by water to the tubers. Initially, blight appears as irregularly shaped dark green lesions on the lower leaves of the plant. One may see a white cottony growth (fungal mucelia) on the underside of the leaf. Turning brown, the infected leaves die. Eventually, the entire plant dies. The tubers develop irregularly shaped, firm, sunken, dull brown to purple lesions around the eyes. This can infect the entire tuber, which basically rots. This is what happened in Ireland. And boy, does it smell. It happened to me. Do not plant potatoes in that spot for about five years.

Prevention includes purchasing healthy certified seed potatoes, rotating potatoes and tomatoes, keeping your tomatoes and potatoes away from each other, planting in rows parallel to prevailing winds to ensure airflow, watering early in the day and at ground level, removing all tubers at season end, controlling weeds, removing any volunteer plants and keeping your eye out for any new plant disease resistant varietals.

Scab, an unsightly lesion like a raised corky area on the home gardeners’ potatoes, is not a problem. It is quite edible but some prefer to peel the affected area away. It is caused by Streptomyces scabies. It can be managed by keeping the soil pH at less than 5.2, careful crop rotation and planting disease-resistant varietals.

To find reliable information about gardening and other horticultural topics, go to the University of Minnesota Extension website,

> Local master gardeners will also answer your gardening questions via a voice-mail service. Call 444-7916, leaving your phone number, name and the nature of your question. A volunteer master gardener will give you a call.