Virulence of Rhizoctonia oryzae and R-solani AG-8 on wheat and detection of R-oryzae in plant tissue by PCR

Rhizoctonia oryzae and R. solani anastomosis group (AG)-8 both cause root rot of wheat and barley, but R. oryzae has been considered secondary in importance to R. solani AG-8 on these cereals in the U.S. Pacific Northwest. Of 19 isolates of R. oryzae, 12 caused both preemergence damping-off of wheat and a significant reduction in root biomass of 21-day-old seedlings in natural soil at 12 degrees C, whereas 7 isolates induced minimal or no damage to wheat under these growth conditions. R. solani AG-8 had no effect on seedling emergence and seminal root development, but four of eight isolates tested caused severe root rot of wheat. Thus, R. oryzae and R. solani AG-8 may cause distinctive and different damage as pathogens of wheat, and their relative importance may vary among field sites and with the developmental stage of the host plant. The nucleotide sequence of the rDNA internal transcribed spacer (ITS) regions was divergent between the two species; therefore, the oligonucleotide primers RO1 and RO2 were developed from sequences within ITS1 and ITS2, respectively, that are unique to R. oryzae. These primers amplified a 511-bp fragment from DNA of R. oryzae but not DNA from any intraspecific group of R. solani or from binucleate Rhizoctonia spp. A polymerase chain reaction protocol with the RO1 and RO2 primer set was used to detect R. oryzae in wheat roots and is a suitable method to diagnose this fungus.