Symptoms

• Symptoms on foliage, stolons and roots

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.