Yield loss in rice caused by Rhizoctonia oryzae and R. oryzae-sativae in Australia

Rhizoctonia oryzae-sativae and R. oryzae are the causal agents of aggregate sheath spot and sheath spot of rice, respectively, in Australia. Both diseases are well distributed within the Australian rice growing area. The effects of six fungicides (metalaxyl-methyl, azoxystrobin, kresoxim-methyl, pyraclostrobin, propiconazole and toclofos-methyl) on mycelial growth of R. oryzae-sativae and R. oryzae were tested in vitro. Pyraclosotrobin and propiconazole were strong inhibitors of both pathogens and all isolates tested were sensitive to these two fungicides. Both fungicides significantly reduced disease development in field tests but failed to increase rice yield. As well, aggregate sheath spot caused yield losses as high as 20.3% and sheath spot reduced yields by up to 10%. This is the first report quantifying potential yield losses caused by aggregate sheath spot and sheath spot of rice under Australian conditions.