Black Scurf = Stem Canker
Rhizoctonia solani
Causative agent and transmission
Brown scurf is caused by the fungus Rhizoctonia solani, which develops from black sclerotia attached to the mother tuber or in the soil. These sclerotia are a means of preserving the fungus.
Description of symptoms on crops
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At the start of growth, when weather conditions are unfavourable (cold, damp), attack by Rhizoctonia solani is expressed by irregular or late emergence of the plants : buried sprouts, stolons and lateral roots show deep brown blemishes which lead to their slow growth or death.
During the growth period, the contaminated plants show a straight habit and small aerial tubers can be observed in the axils of the leaves.
Uprooted plants are observed with brown and dry necrosed areas. Under damp conditions, a whitish mycelial mantle can develop in the soil.
Black scurf is also characterized by curling up and yellowing of the foliage, a meagre appearance of the plants and tuber development grouped at the stem-base.
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Description of symptoms on tubers
Small very hard black clusters, called sclerotia, can be found on contaminated tubers, and are clearly visible on washed tubers.
Tubers coming from affected plants are small, misshapen, angular and sometimes with desquamations resembling the common scab. In certain cases, lenticular necroses or small corky plugs ("dry core") may be observed.
A closely-related fungus, Rhizoctone violet R. crocorum, is sometimes encountered in damp soil after certain crops such as beetroot.
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Control
- Use of healthy seed potatoes;
- Long crop rotations;
- Planting in heated and well-prepared soil;
- Delay in haulm defoliation - harvesting should not last too long;
- Use of fungicides for treating seed potatoes, and occasionally the soil as well: Monceren (pencycuron), Dithane (mancozeb), Iota (flutolanil), Oscar, etc.
