A nonpathogenic isolate of Rhizoctonia solani (No. 521, AG-4) induced increased growth in a variety of crops. In field experiments, it was expressed in increases of plant weight, cotton fiber weight, or grain yield. The increases for treated compared with untreated plants were as follows: radish, 13.4–19.8% fresh weight and 28.4–36.0% dry weight; carrot, 30.0–97.6% fresh weight and 55.0–150.5% dry weight; lettuce, 58.4% fresh weight and 61.8% dry weight; cotton, 28.7% fiber weight; wheat, 10.6–25.3% weight per grain and 15.4–36.5% grain yield. For the potato crop, although an increase in leaf, shoot, and tuber weight was induced by R. solani 63–70 days after planting, it was not expressed in yield at harvest time.

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The hypovirulent (HV) isolate (No. 521) densely colonized the outer surface of the hypocotyls and roots of radish and cotton seedlings but did not penetrate into the cortical parenchyma, whereas the virulent (V) isolate (No. 82) penetrated the root and hypocotyl tissues (except for the xylem vessels) to the center of the pith parenchyma. The HV isolate did not cause any degradation of the cell wall material except for the cuticular layer that had disappeared in the regions of close contact between the hyphae and the epidermal outer surface. During the prepenetration stages of the V isolate, the cell walls of the seedlings underwent a significant, visible degradation. The cuticle between the hyphae and the epidermis was detached and degraded. The HV isolate seems to densely cover the outer surface of the seedlings, and it may occupy the possible infection sites, rendering recognition and occupation of such sites unavailable for the virulent pathogen.

Among several hypovirulent isolates of Rhizoctonia spp. tested, one isolate (No. 521) provided significantly higher protection (76–94%) to radish and cotton seedlings against infection by virulent isoltes of Rhizoctonia spp. and also induced a higher increased plant growth response than the others. The hypovirulent strain did not compete for root exudates. The protected plants were not induced to produce new polyphenols, melanins, more lignins, or phytoalexins. Also they did not produce inhibitors of pectinases or cellulases released by the pathogen, nor did they produce cell wall lytic-type enzymes, such as chitinase and β-(1,3) glucanase. Ca2+ content was significantly higher in seedlings colonized by the hypovirulent isolate. The hypovirulent isolate densely colonized the surface of roots and hypocotyls, and removal of its hyphae nullified the protection provided by this isolate. Key words: Rhizoctonia solani, damping-off, hypovirulent isolate, biological control.