First morpho-molecular identification of Rhizoctonia solani AG-7 from potato tuber-borne sclerotium in Saudi Arabia

Losses in yield and potato tubers quality caused by Rhizoctonia solani in Saudi Arabia was investigated in 2007. A number of sclerotia of R. solani were isolated from potato tubers collected from Saudi Arabia. Based on morphological observations, hyphal fusion compatibility and pathogenicity tests, most isolates collected belonged to AG 3, but 13, 5 and 2% of the isolates were AG-4, AG-5 and AG-7, respectively. What is more, hyphal branching at a right angle with a constriction and septum near the origin, multinucleate and monilioid cells and hyphal diameter were investigated. Cultural appearances of R. solani AG-7 growing on malt extract agar (MEA), dextrose sabroud agar (SDA), Dox agar (DOX), potato-dextrose agar (PDA) were brown or dark brown colony with aerial mycelia and sclerotia, but without clear zonation on DOX and PDA. Culture of AG-7 had abundant pitted sclerotia with dark brown exudates on SDA. Currently recovered isolates of R. solani shows hyphal anastomosis with AG-7 tester isolates. Koch's postulates were applied by re-isolation of the fungus from inoculated tubers. The microsatellite-primed polymerase chain reaction (MP-PCR) was used to identify R. solani AG-7. The reference laboratory tester and R. solani isolates (AG-7) produced highly homologous fingerprints (level of homology, 86%). By comparing specific MP-PCR fingerprints of unidentified R. solani isolates with those of AGs testers, R. solani isolates (AG-7) could be identified to the group level even if they could not be identified by routine morphological methods. This is claimed to be the first report of R. solani AG-7 in Saudi Arabia.