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Q. This fall when I dug up my sweet potatoes, many of them had black spots on the skin but they still tasted fine. Also, the skin got wrinkled and rough during storage. What is the problem?
- J. G., Kings Mountain, NC
A. "There are a number of fungal diseases that cause black skin on sweet potatoes. Black rot causes circular, dark depressions on the tubers. Scurf creates small, round, dark spots on the surface, but doesn't affect the eating quality. Stem rot results when plant stems are injured. To minimize problems with your future crops, plant in a different garden spot, plant only healthy slips, choose disease-resistant varieties, and provide the best growing conditions possible. Problems with scurf can be minimized by growing in an acid soil. Don't over fertilize the vines, especially with a nitrogen-rich fertilizer, as this can cause tough, stringy roots.
The wrinkling skin may have been due to improper curing. After harvesting, dry the sweet potatoes for 2 or 3 hours, then spread them out in baskets lined with newspaper. Set them in a dry area where the temperature will remain between 80 and 85F for about 2 weeks. Once they're cured, store them at 55 to 60F with a humidity of about 85 percent. Check the stored sweet potatoes occasionally and remove any that show signs of decay."
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Safer Brand® 3 in 1 Garden Spray
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Q. Many of our potatoes have rotten spots that spread, eventually turning the whole potato into a soft pulpy rot. What is causing this and what can we do to prevent it from happening next year?
- N. Family, ON
A. "If your potato plants appeared healthy all season, then the symptoms you describe can those of bacterial soft rot, caused by a bacteria called Erwinia. It's difficult to tell which of the remaining tubers might be infected because the bacteria can enter through wounds, injuries or even through natural openings (lenticels) in the skin. Check each potato for injuries or wounds and discard those that seem suspect.
Also, potatoes that have suffered from late blight (the infamous Irish Potato Famine blight) often turn to slime, too. If the entire plants blackened and died before their time, blight is probably the culprit. There are several potato varieties that show resistance to late blight; Elba is currently the most resistant potato variety available. Potato varieties with moderate levels of resistance include Kennebec, Sebago, Allegany, and Rosa.
To reduce problems with disease, buy only potatoes that are certified disease-free; don't save potato seed from year to year. Examine your seed potatoes carefully before you plant, and plant only sound, blemish-free tubers. Rotate crops in the potato family (which includes tomatoes, peppers, and eggplant) on a 4-year cycle in your garden plot. Pull up spent plants and clean up garden debris in the fall and do not compost diseased plant material."
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Q. What makes the skins on my harvested potatoes scabby?
- B. P., Legrand, IA
A. "Common scab is caused by a naturally-occurring soil microbe called Streptomyces scabies. The problem tends to be most severe in neutral (pH 7) or slightly alkaline soils. (The optimum pH for scab development is from pH 6.0 to 7.5. In general, scab is not a serious problem in soils with a pH of 5.5 or lower.) The scab organism is very sensitive to soil moisture, pH, and temperature. The incidence of the disease is generally greater in sandy or gravelly soils than in heavier soils. The amount of soil moisture at the time of tuber development is very important.
Several practices can limit the severity of scab.
1. Planting resistant varieties is the most feasible and practical control. Varieties showing some level of scab tolerance include Norchip, Norgold Russet, Reliance, Russet Burbank, Shurchip, Norland, Hi-Plains, Pungo, Redskin, Russet Sebago, Shoshoni, and Superior.
2. Avoid planting seed exhibiting scab lesions.
3. Rotate crops so that potatoes are planted in the same area only once every 3 or 4 years. Avoid rotating with root crops; instead plant grains, grasses, or legumes.
4. Maintain good moisture levels in the soil while tubers are developing.
5. Avoid spreading fresh manure just preceding the potato crop."
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