Stem and Stolon Canker (Black Scurf)

Aboveground symptoms of stem canker include uneven stands, weak shoot growth, and aerial tubers. Foliage may develop yellowing, purpling, and upward curling of leaves. Aboveground symptoms alone are not diagnostic, however, because other diseases can cause similar symptoms. On belowground stems and stolons, Rhizoctonia solani typically causes reddish-brown lesions that often develop into sunken cankers. Stolons can be girdled and killed, resulting in a pruning effect and malformation and abortion of tubers. If tubers in affected fields are left in the ground after vine death, they often develop black scurf, an accumulation of irregular black sclerotia of R. solani on the tuber surface. This is especially visible on fresh market red and white-skinned varieties and is an economically important problem for organic growers.

The strain of R. solani that typically affects potatoes, called AG-3, is specific for potatoes and generally does not cause damage in or reproduce on other species. Close relatives of potato, such as black nightshade and tomato, may be exceptions. Rhizoctonia solani is a soilborne fungus, but inoculum of the fungus on seed tubers (visible as dark, irregularly shaped sclerotia) is sometimes more important for disease development than inoculum in soil. In Kern County, AG-3 types of R. solani apparently do not survive in soil between crops of potatoes; whereas in Tulelake districts, R. solani AG-3 may overwinter in soil. The fungus only infects juvenile tissue. Disease development is favored by relatively wet, cool (55° to 60°F) soils.

Where R. solani AG-3 survives in soil between potato crops, rotate out of potatoes for two to three years to reduce soilborne inoculum. Sugarbeet, however, has been associated with increased severity of stem canker in subsequent potato crops.

Reduce initial inoculum by using certified seed tubers that are free from sclerotia of the fungus.

Fungicide treatment of seed tubers can reduce R. solani inoculum borne on the seed pieces. The significance of this benefit may be small in fields where heavy soil infestations of R. solani AG-3 persist between potato crops.

Rhizoctonia stem canker can be further reduced by favoring rapid emergence, such as warming seed tubers before planting, planting tubers at a relatively shallow depth, avoiding early planting dates when soil temperatures are cool and avoid overwatering.

Apply fungicides to seed pieces and/or in-furrow at planting to reduce the disease incidence on belowground stems and stolons. Research indicates that fungicide applications after planting can reduce tuber blemish on daughter tubers. Daughter tubers may also be protected by harvesting quickly after vine desiccation rather than holding tubers in soil for extended periods. However, this may lead to insufficient skin formation and increases the risk of bruising and skinning problems.