Black scurf

Rhizoctonia solani

Present across potato growing regions in Australia, black scurf, also known as rhizoctonia root rot, is a seed and soil-borne fungal disease that attacks tubers, stems and stolons of potato plants. Black scurf is a significant disease, causing reductions in crop growth, yield and quality. There are a number of different strains of rhizoctonia root rot affecting crop and pasture species.

Refer to the link below for more information on rhizoctonia root rot in other crops.

Description

Black scurf exists as mycelium on decaying organic matter, or sclerotia which are found on the surface of tubers. Black or brown masses of sclerotia form on the tuber’s surface, giving the disease its name, black scurf. These masses range from small, flat, barely visible black spots through to large, raised black lumps. While these masses don’t damage the tubers, they assist the development of the disease if these infected tubers are planted and the disease cycle continues.

Rhizoctonia also survives on organic matter in the soil and attacks the sprouts before they emerge.
Initially, dry, brown, slightly sunken elongated spots occur on the stolons and stems before potentially developing into cankers on the stems. These cankers restrict the flow of carbohydrates from the leaves to the tubers, reducing tuber growth.

Above the surface, emerged stems may be stunted with curled leaves. As a result of the cankers restricting starch flow, small green tubers may form above ground on the stem.

Cool, wet weather after planting favours the development of the disease. It can also be spread by machinery, shoes or contaminated seed.

Control

There are a number of cultural practices available to reduce the likelihood of disease development including;

Crop rotation

Planting certified disease-free seed

Avoiding cold and wet soils

Controlling volunteer potatoes and weeds prior to sowing

Harvesting as soon as possible to restrict sclerotia development

Removing plant debris or allowing residues to breakdown prior to planting.

Fungicides are available as either seed treatments or as in-furrow sprays to control the disease.. Emesto Prime is registered for in-furrow application at planting and Serenade Prime is also registered in-furrow for the suppression of potato black scurf.

Related Pest

Rhizoctonia root rot

Rhizoctonia solani

A fungal soil-borne disease that damages cereals and some other crops by attacking their roots. Much more widespread and damaging to crops since the adoption of minimal tillage.

References

Agriculture Victoria (2013), ‘Potatoes – Strategies for pests and diseases’, http://agriculture.vic.gov.au/agriculture/horticulture/vegetables/vegetables-a-z/potatoes/potatoes-strategies-for-pests-and-diseases

Jackson, G (2017), ‘Potato black scurf (295)’, Pacific Pests and Pathogens – Fact Sheets, Pestnet,
http://www.pestnet.org/fact_sheets/potato_black_scurf_295.h