Rhizoctonia solani AG5 and Its Offspring – Morphology and Sensitivity to Fungicides

The objective of these studies was to identify differences and similarities within the progeny of Rhizoctonia solani AG5, which arose from basidiospores produced by the maternal strain ID23. The following characteristics were analyzed: appearance of the mycelium (color, structure, zonation, and presence of sclerotia), growth rate (at 10°C, 20°C, and 30°C), fungicide sensitivity, and hyphal structure.The mycelial color of R. solani AG5 ranged from white/cream to light and dark brown. The structure of the mycelium may be compacted or flattened with visible zoning or fluffy with dark brown sclerotia on the colony surface. Homokaryons and heterokaryons derived from homokaryons were analyzed by constructing a phylogenetic tree using morphological data. Single basidiospore-grown isolates formed a separate subclade, the majority of which were grouped with a maternal isolate; however, heterokaryons derived from them created a separate subclade. In addition, isolates grown in basidiospores germinated at low temperatures created their own group, but with some exceptions. This shows a divergence in the morphological parameters of the subsequent generations and within generations. The optimal temperature for growth was found to be between 20°C and 30°C. The exceptions were strains obtained from basidiospores that germinated at refrigerated temperatures. For these samples, 10°C was found to be the optimal growth temperature. The hyphae of homokaryons were characterized by the presence of branching at an almost right angle and the presence of a septum at the site of constriction of the branch itself. The mean diameter of hyphae ranged 2.93–15.60 μm, depending on the age of hyphae. The fungicidal compounds at a concentration of 10 ppm had no significant effect on the activity of the tested strains, whereas a tenfold increase in the dose reduced the growth ability of the tested isolates. The activity of fungicides containing azoxystrobin, thiuram, or thiophanate-methyl on R. solani resulted in a reduction in the mycelial growth rate only in the case of azoxystrobin and thiuram, and in some cases, it was completely inhibited (thiophanate-methyl).