Potato scab is caused by a bacteria-like organism called Streptomyces scabies, which can infect most varieties of potatoes (Solanum tuberosum). Infected potatoes have raised lesions that resemble cork, warts or tree bark. Though scab is an ugly disease, potatoes with scab are still edible. Unfortunately, potato scab is incurable. There are no chemical treatments, and infected fields stay infected for years. Precautions, however, can minimize the chances of your potatoes becoming infected.
Scab tends to be a problem of "sweet" soils -- in other words, alkaline soils with a pH higher than 5.5. The Ohio State University Extension recommends a pH between 5.0 to 5.5 for growing potatoes. If a soil test reveals a higher pH than that, you can add elemental sulfur or ferrous sulfate to lower it. Spread the sulfur evenly over the surface of the soil and then till it in to a depth of about a foot. If your pH is only slightly high, you can try a continuous mulch with oak leaves or pine needles. As the mulch begins to break down it will begin raising the pH. At that point you will need to rake the old mulch away and lay down a fresh layer. Also avoid using lime or wood ashes in your garden. They will raise the pH.
Avoid using manure on your potato patch. Manure can be a carrier of potato scab. It is not uncommon for animals to ingest the potato scab pathogen. The pathogen can then stay alive in the digestive tract long enough to still be contagious in the animal's manure.
Potato scab does eventually die if it doesn't have potatoes to feed on. Fields, however, can remain contaminated for years. If you see potato scab on your potatoes, don't grow potatoes in the same spot for at least three years. In the absence of potatoes, the pathogen has been known to infect other root crops such as turnips (Brassica rapa), radishes (Raphanus sativus) and carrots (Daucus carota), so avoid planting them in the potato field as well. Red clover (Trifolium pratense) also tends to increase the infection. Rotating potatoes with grains works best.
If you are having problems with potato scab, the best solution is to be very careful about the seed potatoes you plant. Potato scab can enter a field on infected seed potatoes. Buy your seed potatoes only from reputable dealers and buy resistant varieties. According to the Clemson Cooperative Extension, the following potato varieties are scab resistant: Superior, Goldrush, Red LaSoda, Red Gold, Caribe, Dark Red Norland, Butte, Carola, Russett Burbank and Sebago.
- Ohio State University Extension Fact Sheet: Scab of Potato Tubers
- Ohio State University Extension Fact Sheet: Growing Potatoes in the Home Garden
- Michigan State University Extension: Common Scab of Potato
- Horticare, Inc.: Acidifying Your Soil
- Ohio State University Extension Fact Sheet: Mulching Landscape Plants
- Clemson Cooperative Extension: Irish & Sweet Potato Diseases
- University of Nebraska-Lincoln Extension: Common Scab of Potatoes
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