Symptoms
Raised black spots, irregular in size or shape, appear on the surface of the potato tubers (scurfs). These black marks can be rubbed or scraped off easily. With the help of a hand lens, white fungal material can be observed around these spots. The fungus also causes symptoms resembling those of stem canker on new shoots and stems. Brown, sunken patches develop on the root, often surrounded by white fungal growth. If the rot girdles the stem and blocks water and nutrient transport, the leaves become discolored and withered.
Recommendations
Organic Control
Apply the biological fungicide Trichoderma harzianum, or nonpathogenic Rhizoctonia species in the furrows. This might reduce the incidence of black scurf in fields and the number of infected tubers. Another possibility would be to apply cattle manure to the furrows or bio-fumigation with green mustard residues.
Chemical Control
Always consider an integrated approach with preventive measures together with biological treatments if available. Seed treatments with fluodioxinil or a mixture of thiophanate-methyl and mancozeb are effective against the spreading of a variety of fungal diseases, among them black scurf. In-furrow treatments at planting with fluotanil or azoxystrobin also help to control the progression of the fungus.
What caused it?
Black scurf is caused by the fungus Rhizoctonia solani. At temperatures ranging 5 to 25°C the fungus survives in the soil for long periods of time, even in the absence of potatoes. The infection can arise from the soil or from the use of infected tubers as seeding material. The fungi actually do not cause rot, but the tubers should not be used for propagation. The infection can be exacerbated by cold and wet weather. Warm temperatures during early stages of plant development reduce the impact of the disease. Black scurf and stem canker also tend to be more common on lighter, sandier soils.
Preventive Measures
- Use seeding material from healthy plants or from certified sources.
- Avoid planting early in the season.
- Plant seed tubers in warm soil (above 8°C).
- Consider the use shallower furrows to allow for earlier shoot emergence from the soil.
- Practice crop rotation.
- Leave non-host plant residues in the soil after harvest.
- Water the plants adequately, especially during dry periods.
