Inoculation techniques and effect of viruses on the development of rice sheath rot caused by Sarocladium oryzae

Among the four inoculation techniques tested, spraying a spore suspension in beef extract peptone broth (BEPB) or potato marmite broth (PMB) mixed with carborundum (grit 320) at 1% gave the highest disease incidence, severity, lesion length and seed discoloration. This method is easy to use, does not need skilled labor. A large number of plants can be inoculated in a short time as compared to other techniques. The sheath rot pathogen reduced the flag leaf sheath length, panicle emergence, grain weight, and increased the percentage of partially emerged panicles, and unfilled spikelets. S. oryzae caused the plant to reduce adventitious roots from flag leaf sheath node and nodal branches from the second node from top. S. oryzae inoculation in tungro-viruses infected and virus-free plants increased sheath rot incidence compared to the uninoculated controls. Rice tungro spherical virus (RTSV) infected and virus free plants inoculated with S. oryzae showed no difference in sheath rot incidence. Disease incidence was high in rice tungro bacilliform virus (RTBV) infected plants inoculated with S. oryzae. The RTBV might have changed the physiology of rice plant, and predisposed to sheath rot pathogen. So there was increased in sheath rot severity and lesion length. RTBV and S. oryzae acted synergistically which resulted in increased seed discoloration, percent partially emerged panicle, and caused reduction in panicle emergence. In RTBV + RTSV infected plants inoculated with S. oryzae, sheath rot incidence was also high which may be due to involvement of RTBV. RTSV in combination with S. oryzae increased the sprouting of nodal branches (second node from top) and emergence of adventitious roots (from flag leaf sheath node), while RTBV, alone or with RTSV had little or no effect. RRSV-infected and virus-free plants inoculated with S. oryzae increased sheath rot incidence over the control. The highest SR incidence severity and lesion length were observed in RRSV-infected plants inoculated with S. oryzae indicated the predisposition effect of RRSV to S. oryzae