RHIZOCTONIA-SOLANI INCITED DAMPING-OFF IN RADISH SUPPRESSED BY PLASMID AND NONPLASMID CARRYING STRAINS OF R-SOLANI THAT ENHANCE HOST CHITINASE ACTIVITY

Thirteen Rhizoctonia solani isolates belonging to anastomosis group fo ur were examined. All of the isolates carry plasmids except isolates C HU344 and CHU345. Plasmids were analysed by agarose gel electrophoresi s and observed by transmission electron microscope. The plasmid DNA is linear, having a size of 2.6 kbp. The termini of the plasmid have a h airpin structure. The replication of plasmid DNA of R. solani CHU341 ( pCHU341) was demonstrated by electron microscopy. Virulence of the fun gi was tested on radish seedlings and expressed according to percentag e of radish seedlings that damped-off. Strains CHU341, CHU344, CHU345, CHU346, CHU347, CHU348, CHU350 and CHU353, suppressed damping-off of radish seedlings by the virulent isolates, CHU342, CHU349, CHU351 or C HU352 and was expressed according to a suppressiveness index. The chit inase activities of radish seedlings increased from 10 to 20 times aft er inoculation with suppressive isolates, CHU341, CHU344 and CHU345. T he results indicate that damping-off of radish seedlings caused by R. solani can be protected by related strains with or without plasmids. T he protection of the seedlings may result from the enhancement of chit inase activities induced by the suppressive strains of R. solani.