Black scurf/Stem canker – Rhizoctonia solani

Symptoms    Variable sized, black/brown particles on the surface of the tuber.  In unplanted seed tubers the disease can kill the developing sprouts, while after planting it can attack the underground parts of the young plant, delaying emergence, causing rolling of the leaves and loss of vigour.  Tubers from affected plants will often be variable in size, netted skin and affected by growth cracks.  At maturity, tubers often display Black scurf and may be rejected for either seed or packing.

Sources and Factors Favouring Development

Seed and/or soil-borne, especially on light soils.

Close rotations and delayed burn-down/harvest increase the incidence of black scurf arising from soil infection.  Dry, cold soil conditions appear to be most favourable for disease development; environmental conditions, which slow early crop growth, can increase the severity of stem canker symptoms.

Controls       Chose varieties that are more resistant to Rhizoctonia solani.  Increase length of rotation.  Use healthy, chitted seed.  Avoid early planting; soil temperature should be above 8°C. Treat seed tubers with known infection with an approved fungicide.