By Erin Marissa Russell
The plant disease potato scab gets its name from the scablike lesions it leaves on the tubers of the potato plants it afflicts. Crops infected with potato scab result in profit loss for gardeners who are growing their potatoes to sell. There is no way to completely cure potato scab in the garden, but there are ways to prevent the disease or control and minimize its spread. Keep reading to learn about how to identify potato scab if it shows up on your plants as well as how to prevent the disease and treat it if it does appear.
Potato scab is also called common scab and Irish potato scab, and it is caused by a bacterium called Streptomyces scabies and other soilborne Streptomyces bacteria. The disease spreads through the soil, through water, on nematodes or insects, and on spores attached to seeds. If manure is taken from animals that have consumed the Streptomyces bacteria, the manure can also spread the disease when it is applied to fields.
The bacteria enter the potatoes they infect through the openings created by wounds or lenticels. Inside the potato, the bacteria release a toxin that causes cell death. The dying cells produce corky cells that move outward, leaving lesions on the skin of the potatoes. The lesions will spread and grow larger as more of the corky cells are created and the disease progresses. The earlier in their growth cycle potatoes are infected, the deeper their lesions will be.
How to Identify Potato Scab
The symptoms associated with potato scab can help in diagnosing the disease if it appears in your garden. The primary symptom is the lesions that appear on tubers, which may be raised, pitted, or only superficial and will either be brown or black. Stems and stolons that grow underground can also show lesions that range in shade from tan to brown. However, it can be hard to tell the difference between crops infected with potato scab and those infected with powdery scab.
The only way to be certain of exactly which disease your plants have contracted is to save samples and send them to your local agricultural extension office to be tested. You can look up your county extension office using this map from the National Pesticide Information Center.
How to Prevent and Treat Potato Scab
- Potato scab thrives when soil moisture is low (65 to 70 percent or lower) at the time of tuber initiation. To prevent potato scab, increase the moisture of the soil to fall between 80 and 85 percent when tubers are initiated, and keep the moisture at this level until potatoes grow to between an inch and an inch and a half long.
- Crop rotation can help to prevent potato scab when potato crops are followed by green manure crops, including buckwheat, canola, millet, oat, and rye.
- Studies have shown animal manure to both increase and decrease the risk of potato scab. Researchers believe that animal manure may cause more incidences of potato scab when the soil pH level is high.
- Scab-resistant potato varieties are available on the market. Choosing to grow scab-resistant potatoes is one of the best ways to minimize your risk of dealing with a potato scab infection. Popular resistant potato varieties include Atlantic, Dark Red Norland, Kennebec, Norchip, and Superior.
- Alkaline soil makes the likelihood of potato scab infection higher and also increases the severity of the disease. To lower the risk of potato scab infection or decrease its severity, gardeners should work to keep the pH level of their soil under 5.5. You can decrease the pH level of your soil with applications of sulfur or acid-forming fertilizers.
- A high ratio of calcium to phosphorus in the soil increases the risk of potatoes contracting potato scab. Adding more phosphorus or minimizing the calcium in the soil may help lower the risk of your crops becoming infected with potato scab.
- Examine your seed potatoes while you are planting them, and do not plant any pieces that show signs of potato scab. Although seed potatoes may have been treated against potato scab, they are not guaranteed to be free from the disease.
- Keep the area where potatoes are growing as well as the area surrounding potatoes free from invasive weeds, which can be an alternate host for potato scab. Pigweed and other fleshy-rooted weed plants are of particular concern, and their presence near potato crops should be kept at an absolute minimum.
Although there is no cure-all when it comes to treating potato scab, the environmental and cultural controls described in this article go a long way toward lowering your risk of the disease. And by familiarizing yourself with the symptoms and disease progression, you’re making it more likely that you will catch an infection early, making it easier to treat so you can minimize its spread. There’s no reason to feel helpless against potato scab when there is so much you can do to prevent and control the disease.