Rhizoctonia solani

(Stem canker = Black scurf)

• Causal agent and transmission

Stem canker and black scurf of the potato are the two common names for the same disease due to Rhizoctonia solani, a soil-borne fungus. Black scurf refers to the presence of small to large, dark black dots on the tuber surface, which correspond to sclerotia, i.e. survival stage of R. solani.

Stem canker is the symptom visible on sprouts and young growing shoots.

This fungus is also pathogenic to other cultivated crops such as corn, beetroot, carrot, rape, cereals, etc.

Soil and infected seed tubers are the main sources of inoculum. However little is known about the relative importance of soil borne and tuber borne inoculum.

• Significance 

In most European countries, stem canker and black scurf are serious concerns for potato production and both stages of the disease have to be maintained below the economic threshold level. For almost all sectors of production, fresh-ware and seed sectors, black scurf (sclerotia on tubers) is the major cause of tuber blemish causing downgrading or rejection of affected lots.

Due to the pathogenicity of R. solani on other major cultivated crops, overall control management, especially crop rotation, is difficult at field level.

• Fungus varaiability

R. solani is a highly variable group of different fungal populations which can be divided into “anastomosis groups” (AG) defined by the ability of the strains of a given AG to anastomose (fuse) with another of the same AG. There are at least 13 different AG’s with different levels of pathogenicity and host range. The most predominant AG on the potato is the AG 3 which can be isolated from black sclerotia, present on the tuber surface at the end of the growth cycle.

AG2-1, AG4 and AG5 have also been sporadically isolated from potato tubers but their respective level of pathogenicity has still to be confirmed. Some of them are also pathogenic to major crops like sugarbeet and corn (AG2-2). The close relationship between strains of this soil-borne plant pathogen highlights the complexity of disease management at the field level. There is some evidence that potatoes, known to be infected only by AG3, could be a symptomless host for other AG’s.