Grocery stores have set up their seed potato displays, and many gardeners are getting ready to plant. However, some have expressed dismay about last year's crop: disfigured, dark, rough, pitted and corky, their potatoes were infected with potato scab, a disease caused by a soil borne bacteria.
Potato scab bacteria lives on organic material in the soil and infects young tubers through pores or wounds. The infection produces reddish-brown spots that grow into large corky lesions. Spores develop in these lesions and are shed into the soil, just waiting to do more damage next year. Infected potatoes that are used as seed potatoes the following year can re-infest the soil where they are planted.
Although potatoes infected by potato scab are unsightly, the tubers are edible, and the scabby areas can be peeled away. If stored in dark, cool, dry locations, the potatoes keep reasonably well.
There are no chemical controls recommended for home gardeners, but there are some cultural practices that will help to control the disease. Make sure you purchase certified, scab-free seed potatoes. Select varieties such as Norland, Superior, Norgold Russet or Burbank Russet, all of which have shown resistance to potato scab.
Make sure to rotate potatoes in your garden. Do not plant them in the same area more than once every three to four years. Do not plant them where you have recently planted other root crops such as beets, carrots, parsnips, rutabagas, turnips or radishes, as these can also be infected by scab. Remove and destroy any potato debris and cull tubers at the end of the season.
Lowering the soil pH will significantly reduce scab. A pH of 6.5 is about as low as you should go, any lower will be too acidic for potatoes and most other vegetable plants. Have your soil tested to determine your acidity. To increase soil acidity, apply an acid producing fertilizer like ammonium sulfate.
Keep potato plants well watered, especially during the time the new tubers are forming. However, do not keep the soil saturated as this will encourage root rot and other diseases.
How do you plant potatoes? Just follow these easy instructions for a successful potato patch:
·Seed pieces can be planted as soon as soil warms, generally in mid-April.
·Plant seed pieces with at least one eye or bud per piece, no smaller than about 2 ounces in size.
·Cut seed pieces at least one day before planting to allow cut surfaces to dry.
·Plant seed pieces cut side down, 10-12 inches apart and about 3-5 inches deep, in rows 30-36 inches apart.
·Start hilling plants when stems are about a foot tall, and once or twice more during the growing season.
"New" small potatoes can be harvested about 7-8 weeks after planting. Mature tubers can be harvested after leaves have dried or when tubers have reached full size. Hope you have great success in your potato patch! Until next time, happy gardening!
Robin Trott is a Horticulture Educator with University of Minnesota Extension.