Pathogen

Fungus

Hosts

Potatoes

Symptoms

Black scurf is the tuber phase of stem canker. Superficial dark brown / black particles (sclerotia) of variable size and shape that can be removed by rubbing or scratching with the thumb nail.

Development

Infection by the seed and soil-borne fungus leads to the development of black scurf on tubers at the end of the growing season and during storage. Stem canker is also likely to develop in infected potato stocks which are planted.

Black scurf can cause stolon pruning of the roots and shoots which leads to reduced tuber-set and consequently a wider range of tuber sizes at harvest.

Favourable Factors

Dry soils, cold springs conditions and planting unsprouted seed.

Importance

Black scurf is found in all potato growing areas in the UK.

Tubers with black scurf on the skin surface have an unsightly appearance and reduced value, but storage and cooking qualities are generally unaffected.

Rhizoctonia is comprised of a range of distinct anastomosis groups (AG`s) which are genetically diverse and have differing characteristics with regard to host range, survival and fungicide sensitivity. These can be identified by soil testing prior to planting.

Control

• Information on less susceptible varieties is available on the National List.

• Plant non-infected, well sprouted seed, especially if planting early.

• Crop rotation of up to 4 years

• Avoid planting too deeply

• Use fungicide seed treatments as well as in-furrow fungicides

• Early harvest with mature skins can help to suppress disease development

Mishapen tuber due to Rhizoctonia (Photo S. Wilson)