What is Rhizoctonia Solani?
Rhizoctonia solani, also known as stem canker and black scurf, is an important soil-borne pathogen (fungus) that attacks the potato plant at various stages; the stolons and stems underground as well as the tubers prior to harvest.
The source of the inoculum can be either seed-borne or soil-borne, which can remain present in the soil for many years, even in the absence of potatoes.
Diseases associated with rhizoctonia solani are black scurf and stem canker, which is the most damaging as it develops and affects the crop underground.
Both diseases can cause extensive crop husbandry issues, resulting in reduction of marketable yield and therefore crop profitability.
How to identify
Affected roots, stems and stolons show reddish brown necrotic patches called cankers. Above ground, symptoms exhibited can be stunted plants with a rolling of the upper leaves. Brown, slightly sunken lesions with distinct edges develop on the stem base and on stolons. If severe, lesions can merge completely around the stem, which in severe cases, can lead to stem “pruning” with uneven emergence and crop growth.
Appears on tubers close to, and at harvest. Appearance of black scurf is characterised by relatively small irregular dark brown blemishes on the tuber skin, often referred to as looking like peat, which can be rubbed off, often in flakes.
Disease development is favoured by cool and damp / wet conditions, where plant growth is particularly inhibited.
Impact of rhizoctonia solani
Stolon damage will reduce the number of tubers. Stem pruning, even in moderate cases, can lead to uneven crop establishment and development throughout the season.
Harvested tubers can be uneven, in shape and size, reducing marketable yield. Furthermore, uneven crops can make management decisions for desiccation timing difficult as the tubers on each plant can be at different stages of maturity
Management involves a combination of cultural and chemical methods, as no single measure is effective.
There are no resistant varieties, however healthy and disease free seed is important. Management practises to promote early establishment and emergence is paramount, such as planting in warm, non-compacted well-drained soils.
The use of a rhizoctonia solani specific fungicide, such as RhinoDS and a soil-applied fungicide at planting will help to protect the seed tubers during the growing season.