Potato Growing

Potato Problems

Did you grow potatoes in your garden this summer? It's not uncommon to come across a few problems when growing potatoes.

One thing you may notice is green patches on the skin. This is caused by exposure to the sun. This green portion is poisonous and can make you sick, so it should be cut away. [IMPORTANT: A farmer has written and states that for 50 years, his family has kept back the green potatoes for their own use (rather than selling them), and has never experienced indigestion or poisoning from their consumption.] Storing your potatoes in a dark place will prevent this problem.

Fluctuations in the moisture of the soil can cause growth spurts and stops which can result in knobby potatoes or over large potatoes with hollow centres. Balanced moisture will help there.

Scab is extremely common in potatoes and other root crops. It's caused by a soil borne fungus that can survive in the soil for years. High volumes of fresh manure, wood ash, and lime seem to worsen the problems with scab. If you use certified seed potatoes and water the soil regularly you can help your potatoes resist scab. "Superior", "Russet Burbank", "Chieftain", and "Belleisle" are a few examples of scab resistant varieties.

Wireworms feed on underground plant parts including potato tubers. You can help prevent problems with wireworms by preparing the garden the year before you plant, because this pest tends to be present in areas recently covered with sod.

Cornell Plant Pathologists Urge Gardeners and Farmers to Watch for Potato and Tomato Late Blight

CU Press Release (05/04/95)

ITHACA, N.Y. - Blighting, caused by the same fungus blamed for the great Irish potato famine, is expected to wreak havoc among home gardeners this season, warn Cornell University plant pathologists.

"Many home gardeners had late blight last season in their potatoes and tomatoes. It was caused by a new genotype of the old fungus species that caused the Irish potato famine," said William E. Fry, Cornell professor and chairman of plant pathology.

Fry is particularly worried for the organic gardeners, farmers and home gardeners. Late blight last year brought severe problems to many commercial New York farms, so serious that some farmers lost their entire income. Although fungicide is not Fry's favorite option, gardeners and farmers may want to consider using it to save the crop.

Irregular or circular lesions on the foliage or stems are symptoms of late blight. Young lesions may appear water-soaked. Older lesions can be surrounded by a halo of collapsed tissue that is yellow or brown. Infected tubers may have a discolored reddish- brown granular rot. Sometimes in humid weather, a white fuzz or mildew-like growth appears on the foliage or the stems, which indicates the fungus is producing spores. It can occur at any time during the growing season, said Fry.

"The most effective strategy for managing last blight is to avoid sources of spores," said Diane Karasevicz, Cornell plant pathology extension associate. "Do not save potato seed from last year; use only health-certified potato seed." She cautions that certified tubers are not a guarantee against late blight: Examine the tubers to make sure they are blemish free. Fully destroy any rejected tubers by burning them, discarding them in a plastic bag or deeply burying them far away from the garden. Late blight fungus apparently does not survive on tomato seed.

Resistant varieties of potatoes are available. They slow down the disease, but they do not prevent its development. These varieties include: Elba (the most-resistant available), Kennebec, Sebago, Allegany and Rosa. No late blight resistant tomato varieties exist now.

Karasevicz suggests scouting gardens at least weekly, searching for signs of late blight. Gardens should be scouted more often if the weather has been wet. If a plant becomes infected, the infected parts must be destroyed to prevent further disease development. Because lesions do not become apparent until three to six-days after the spores have arrived on the tissue, new lesions can develop daily - even after the tissue is destroyed. Fungicides available to gardeners will not prevent the development of presymptomatic lesions into visible lesions. Thus, if a fungicide is to be effective, it must be on the foliage and stems before the fungal spores arrive.

If the plant must be destroyed, get rid of it in the same manner as the tubers. (Be sure to destroy all diseased stems and foliage to avoid post-harvest tuber rot.)

Other methods for staving off the fungus include watering the ground, rather than the foliage. If avoiding wet foliage is impossible, water the garden in midday, so the leaves dry quickly. Also, keeping the garden weed-free allows for better air circulation and drier foliage.

If a fungicide is needed, be sure to explicitly follow the label directions and be aware that once the plant is blighted, it cannot be cured by a fungicide. It is only used as a protectant. Fungicides include: mancozeb, maneb, chlorothalonil and copper. Check with a local gardening retailer or local extension office for further information on its use. Karasevicz cautions to use the appropriate product for your plant. Some fungicides are designed only for tomatoes or potatoes. She also said: "Labels on these products are considered legal documents; if you do not follow the label directions exactly, you will be breaking the law."

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