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Rhizoctonia solani Kühn [teleomorph: Thanatephorus cucumeris (Frank) Donk.] is an important fungal pathogen widespread in all potato growing areas of the world that causes stem canker and black scurf of potato (Solanum tuberosum L.).
The aim of this study was to find a simple and reliable technique for determining the pathogenicity of Rhizoctonia solani isolates. Sixty (60) isolates of R. solani obtained from sclerotia on potato tubers, collected from different market of Agadir and Casablanca regions (Morocco), were studied for their morphology, pathogenicity and molecular characteristics. They were morphologically characterised by the production of sclerotia and moniloïd cells, and by the mycelium growth capacity at 15, 20, 25, 30 and 35°C. This morphological characterisation leads to three groups of isolates. The first group contained P01 and P03 isolates, which were able to develop under 35°C. However, under 25°C, they did not develop sclerotia. The second group, only formed by L17.1 isolate, did not form sclerotia under 25°C and was not able to develop under 35°C. The third group, formed by several isolates, developed sclerotia under 25°C conditions and were not able to grow under 35°C. Also, a positive correlation was consistent between the production of sclerotia and moniloïd cell formation. The anastomosis reaction revealed that P01, P03, L17.1, and L4.1 isolates were identified as AG-4 and for the other isolates as AG-3. The pathogenic characterisation has shown that P01, P03, L4.1, and L17.1 isolates caused important damping off of radish, tomato, beans, zucchini, and melon. However, the other isolates showed only a minor damping off rate. Molecular characterisation confirmed the classical anastomosis grouping of the isolates into AG-3 and AG-4 Anastomosis Groups. The molecular characterisation is the most rapid and reliable technique to determine the anastomosis group of unknown isolates. The three tests including the pathogenicity, the cultural anastomosis grouping, and the molecular method helped to separate the studied isolates to two groups AG-3 and AG-4.