Some garden words just beg to be written about. Black scurf strikes me as one of those.
Black scurf is a fungal disease of potatoes most easily recognized by lots of small, irregular, dark bumps on your potato skins. These bumps look like soil but they don’t wipe off. There is also a silver scurf of potatoes and a different type that infects sweet potatoes. We will discuss those some other time.
Black scurf is caused by Rhizoctonia solani. This is the same fungus that causes belly rot, collar rot, damping off, Rhizoctonia blight, root rot, and several turfgrass diseases. Black scurf is mostly cosmetic if you’re talking about the edibility of a potato. This disease will reduce your crop size, weaken any plants that survive, and may kill off your seed potatoes.
If you’ve never grown potatoes, it helps to know that they are grown from seed potatoes. Seed potatoes are not seeds. They are simply potatoes, or pieces of potatoes, that have at least one of those “eyes” that sprout in your refrigerator or pantry. When you plant those pieces of potato, you put them in a trench that is 6–8” deep, with the cut side down and the eye pointing up. Those raw edges and new growing tips are vulnerable to several types of attack, including black scurf infections.
Symptoms of black scurf
The tell-tale dark bumps seen on potatoes infected with black scurf are clusters of resting fungal mycelium known as sclerotia. These sclerotia do not damage the still-edible potato, but they do perpetuate the disease. As temperatures rise, these sclerotia germinate. The mycelium may begin growing again, too.
As the disease progresses, you may also see reddish-brown lesions on stolons, stems, and roots. When the lesions move to above ground stems, the disease becomes known as Rhizoctonia canker. These lesions mature into rough, brown cankers that can girdle the stem, killing everything above it.
If a black scurf infection hits critical mass and starts its sexual phase, you may also see a white, powdery mold just above the soil line. This is especially common after it rains or been foggy. Heavily infected seed potatoes are unable to produce stems, in a condition known as “no top”.
Black scurf look-alikes
Diagnosing plant disease can be tricky, especially when you’re new to gardening. I like to look at it as a puzzle, searching for clues. In the case of black scurf, there are a few other diseases that look similar enough to warrant closer inspection. If stems look mushy, it’s probably blackleg. If seed potatoes rot in the ground, it’s probably caused by Fusarium. If you notice leaf curl, the potato leaf curl virus may be causing the problem.
Sadly, you often won’t see signs of this disease until later in the season, long after the damage is done. If your potato crop is mostly small, green tubers (aerial tubers), it’s probably black scurf. This occurs because the invading fungi interrupt the flow of nutrients through the vascular bundles, making life difficult for your potato plants.
Black scurf control
Since many soil-borne disease fungi seem to be able to hang in limbo for years, preventing disease ends up being easier than curing it. Some people suggest a 3-year crop rotation but this is only marginally effective. Those years mean nothing to resting fungi, but it can’t hurt.
Commercial potato growers use fungicides containing chemicals with names like azoxystrobin, flutolanil, and mancozeb to treat black scurf. These chemicals range in side effects from mild skin and eye irritation to bird toxin to a possible carcinogen, respectively. I don’t want any of them in my garden or my food. Instead, there are other ways to prevent black scurf from infecting your potato crop.
The easiest way to prevent black scurf is to only use certified disease-free seed potatoes. Seriously. As tempting as it may be to plant those refrigerator/pantry sprouts, don’t do it. Those potatoes can carry potato blight, the disease responsible for Ireland’s Great Potato Famine, along with several other difficult to get rid of diseases. Buying healthy seed potatoes is a small price to pay. Plus, once you start your potato bed, it can go on forever.
When planting, wait until soil temperatures reach at least 60°F. This gives your seed potatoes a better shot at being healthy and strong enough to protect themselves. Also, Rhizoctonia does not compete well with other soil microorganisms, so maintain plenty of organic matter in the soil to improve overall soil health and microorganism populations. As always, avoid overhead watering, space your plants out with an eye to good airflow, and dispose of infected plants in the trash.
Researchers have found that mulching your potato bed with a green manure made from members of the cabbage family helps prevent black scurf (Sexton et al., 2007). Apparently, compounds released by the Brassicas actively kill the sclerotia. Unfortunately, both potatoes and Brassicas host white mold. If white mold is a problem in your garden, you may want to avoid that particular treatment.
Harvesting your potatoes as soon as they are mature also reduces the chance of infection, along with bruising and insect damage.
Potatoes are an easy crop to grow. They look great in towers, containers, and raised beds. And, like their near-sibling, the tomato, fresh is far better than store-bought.
You can grow a surprising amount of food in your own yard. Ask me how!
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