Thanatephorus target spot is one of the most important diseases of the rubber tree (Hevea brasiliensis) in the Amazon region of Brazil. In addition to rubber tree, the fungus Thanatephorus cucumeris (anamorphase = Rhizoctonia solani) causes several foliar diseases on other agricultural crops or native species in the region. Based on the lack of information on which anastomosis groups (AG) of R. solani are associated with rubber tree and other hosts in the Amazon, we tested the hypothesis that these isolates belong to distinct AGs. There is no information about the cross pathogenicity (to rubber tree) of isolates from other hosts. The AG was determined based on phylogenetic analyses of the ITS region of the rDNA. We concluded that a new subgroup of R. solani (the AG-2-2 Hb) is the main pathogen causing the rubber tree leaf spot. However, isolates from other AGs (AG-1 IB and ID, AG-4 HGI) and from Ceratobasidium sp. (binucleate Rhizoctonia sp. AG-R) from distinct hosts can also infect the rubber tree. This implicates in diversity of inoculum sources for the survival and spread of the leaf spot pathogens. For the very first time, we report the occurrence of a new AG of R. solani infecting citrus in Acre, Brazil, which is distinct from all the others already described worldwide (probably AG-14).
Rhizoctonia solan; anastomosis grouping; cross pathogenicity; rubber tree; citrus
