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Black scurf is a fungal disease that causes black patches on the surface of tubers that can be rubbed off. Although unsightly it is not destructive, but infected seed potatoes can lead to plants with stem cankers.
Potato stem canker
Black scurf is caused by the fungus Rhizoctonia solani, which can exist in the soil in the absence of potatoes. It does not feed on the tubers, and stored tubers with black scurf on them will not decay or deteriorate. The black marks are readily removed by peeling and scrubbing. Heavily-affected seed tubers should not be used, however, as the tips of the emerging sprouts, and later the stems can be more seriously affected.
Stem canker is caused by the same fungus but symptoms appear on the developing shoots instead.
Black scurf is easily recognised:
The fungus can also cause a symptom known as stem canker on potato sprouts (new shoots) and stems:
No chemical controls are available to gardeners.
Rhizoctonia solani is found very commonly in soil, but exists as a number of different strains, not all of which can attack potatoes. Infection can arise from the soil or by the planting of affected seed tubers. The strains that attack potatoes are likely to increase rapidly in the soil if the crop is grown in successive years or in a short rotation. The black scurf is a resting structure of the fungus, known as a sclerotium.
Attacks on the sprouts are usually most damaging if their growth through the soil is delayed by cold, dry weather. Stem canker and black scurf also tend to be more common on lighter soils. Development of black scurf on the tubers increases rapidly as the crop dies down naturally at the end of its growth.
Potato and tomato blight
Potato tuber rots
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