DECLINE OF RHIZOCTONIA ROOT-ROT ON WHEAT IN SOILS INFESTED WITH RHIZOCTONIA-SOLANI AG-8

Soils collected from two sites were used for up to six successive plantings of wheat in the greenhouse. Soils were infested before each of the first four plantings with Rhizoctonia solani AG-8 or R. oryzae, or left non-infested. R. solani stunted seedlings during the first planting. After a second or third crop, depending upon the soil origin, shoot weights were significantly higher in infested soils than in controls. Shoot growth was never suppressed when soils were infested with R. oryzae. Disease suppressiveness tests performed during a fifth crop demonstrated that successive plantings of wheat into soils infested with R. solani AG-8 caused a decline of the disease. Both a susceptible host (wheat) and virulent pathogen (R. solani AG-8) were necessary to achieve disease suppressiveness. In contrast, R. oryzae did not induce soil suppressive to R. solani. Induction of Rhizoctonia root rot decline occurred earlier in tilled than nontilled soil, and rates of applied nitrogen had little effect on decline.