Causal agent(s) 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.
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.
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.
Symptoms on foliage
Early in the growing season and with cold and humid climatic conditions, infection due to R. solani is distinguishable by poor, irregular or late emergence of seed tubers. Dark brown spots, referred to as stem cankers, can be visible on the different organs present in the soil at this stage (e.g. sprouts, elongating shoots and stolons: photos 1 to 4).
The plant tissues necrotises as a results of the infection by the mycelium of R. solani and, in case of a severe infection, the new sprouts will not emerge.
Later, and resulting from the infection at the stem base, altering the sap movement, a typical symptom on the aerial part of the affected plant (photos 5 and 6) consists of: leaf roll, purplish edges on the leaves and yellowing (sometimes wilting) of the foliage.
Another consequence of this nutritional imbalance is the presence of aerial tubers developing at leaf axils (photo 7) and later, the presence of small daughter-tubers grouped around the stem base.
In periods of high humidity, a mantle of whitish mycelium can sometimes be observed at the stem base; this is the phase of active growth and sexual reproduction of the fungus (photos 8 et 9).
Symptoms on tubers
The occurrence of the fungus R.solani on the tuber is variable and its incidence depends on the time of the attack during the growing stage.
An attack at the time of tuber initiation may cause several types of characteristic symptoms on tubers:
- deformations, cracks and damage to the skin (irregular corky lesions or desquamation) that may look similar to symptoms of netted scab or skin russeting.
When these symptoms are present, the tuber size is not necessarily affected, but the quality can be downgraded as the tubers are often deformed, cracked and have surface blemishes (photo 9).
The association of various symptoms such as corky lesions and tuber deformation is often related to Rhizoctonia infection but, in some cases, distinguishing it from symptoms due to light common scab or environmental factors may be difficult.
- sclerotia: at the end of its life cycle and prior to harvest, the fungus causes its most typical damage. When the conditions are unfavourable, the fungus goes into its survival stage by forming black mycelium masses called sclerotia on the surface of the tubers (photos 11 and 12). The sclerotia are tightly attached to the surface and will remain on tubers, even after washing.
- a symptom called “dry-core” may sometimes be observed on tubers infected by black scurf. These brown dry-core areas are just a few mm in diameter and form on the lenticels without penetrating further into the flesh (photo 13). This symptom is sometimes associated with the presence of wireworms.
Stem canker in potatoes is favoured by a cool climate after planting as well as by any other factor that slows emergence.
This increases the period of susceptibility of the plant to the disease: for example, avoid early deep planting in cold soil with unsprouted and unheated seed tubers.
Short rotations greatly favour infection, as does a long delay between haulm destruction and the harvest for black scurf on tubers.
- Use adequate rotations taking into account potential soil contamination and the other crop species cultivated in the field.
- Plant in heated and well-prepared soil.
- Use healthy seed tubers to ensure a homogeneous emergence and a low risk of sclerotia on the tubers at harvest.
- Some cultivars might be less prone to sclerotia formation on tubers at harvest. Until now, current cultivars have not been adequately assessed.
- Chemical protection of the tuber before planting is an additional guarantee of safety; soil treatment at planting is permitted in some countries.
- Destroying haulms by uprooting them may reduce the formation of sclerotia. It is essential that the interval between haulm destruction and harvest should be as short as possible.
- Other control measures such as biological control with antagonistic fungi (Verticillium, Trichoderma) are being studied as well as the use of green manure crops.