Several diseases affect potato plants, but the most common diseases are blight, verticillium wilt and rhizoctonia canker. These diseases are easily identified and if treated early enough, the plants may be saved. If the diseases are not caught early enough, the entire plant should be removed. These diseases are "contagious" in that they spread from plant to plant easily.
Blight starts out as light or dark green spots on the leaves. The spots sometimes have halos. If the spots are still green, the disease is in its early stage. If not cared for immediately, the spots turn dark-brown or black with a purple tint.
To help prevent blight, make sure the starter potatoes are free of disease. Choose cultivars that are resistant to blight. Some resistant cultivars are Sebago, Kennebec and Elba. If you fertilize the potato plants, use a fertilizer that has a low nitrogen content. Potatoes should be grown in hills, not sown in rows. The hills help prevent blight.
Verticillium wilt usually attacks the potato plants later in the season. One of the main and easiest symptoms is that the lower leaves on the plant begin to wilt. The stem remains strong, but more leaves begin to wilt and eventually turn yellow. Verticillium wilt is a fungus that lives in the soil, on infected plant waste or in infected seed potatoes.
If you notice verticillium wilt, plant the potatoes in a different spot the next year. Rotate the planting area with other legumes or grains such as corn, oats, peas or rye. Common causes of verticillium wilt include high temperatures and high moisture soon after the seed potatoes are planted. Implement extensive weed control around the potatoes and do not over-irrigate.
Rhizoctonia canker is seen as brown-black lesions on the underground stems of the potato plant. The plants might look spindly or weak. Pruning the plants, if they are affected early in the season might save the plants. Eventually, if not caught, the leaves curl upwards and get a purple or yellow tint.
To prevent rhizoctonia canker, be sure the seed potatoes are disease free. If the potatoes do contract rhizoctonia canker, it may be treated with fungicides. To help prevent this disease from attacking the potatoes, make sure the soil is at least 60 degrees Fahrenheit before planting.