Rhizoctonia solani Incited Damping-off in Radish Suppressed by Plasmid and Non-plasmid Carrying Strains of R. solani that Enhance Host Chitinase Activity

Thirteen Rhizoctonia solani isolates belonging to anastomosis group four were examined. All of the isolates carry plasmids except isolates CHU344 and CHU345. Plasmids were analysed by agarose gel electrophoresis 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 hairpin structure. The replication of plasmid DNA of R, solani CHU341 (pCHU341) was demonstrated by electron microscopy. Virulence of the fungi was tested on radish seedlings and expressed according to percentage 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 CHU352 and was expressed according to a suppressiveness index. The chitinase activities of radish seedlings increased from 10 to 20 times after inoculation with suppressive isolates, CHU341, CHU344 and CHU345. The results indicate that damping-off of radish seedhngs caused by R, solani can be protected by related strains with or without plasmids. The protection of the seedlings may result from the enhancement of chitinase activities induced by the suppressive strains of R, solani