Effect of soil moisture on activity and dynamics of Rhizoctonia solani and Trichoderma harzianum

The effect of soil moisture on activity and dynamics of the soilborne pathogen Rhizoctonia solani and of its antagonist Trichoderma harzianum were studied in soil under greenhouse conditions. Pots filled with a mixture of soil and sand were inoculated at the same time with R. Solani AG-4, produced on rice grains, and with T. Harzianum, produced on wheat bran. Soil moisture was periodically monitored and kept at four levels varying from − 0.0007 to − 1.03 MPa. To test the fungal activity, bean seeds were sown immediately after soil infestation and 20, 60, 180 and 360 days after soil infestation (DAI), and 3, 6, 12 and 18 DAI in a complementary experiment. The pathogen effectively survived in the soil in absence of host tissue at least one year, although the severity of root rot and the damage to plants were lower in the test at 360 DAI than in the previous ones. Soil moisture did not affect the severity of root rot. The pathogen could more easily be recovered from drier soils, but in the presence of T. Harzianum population density of R. Solani was reduced, except in the driest soil. T. Harzianum could sometimes compensate for negative effects of R. Solani on emergence and plant weight, which was especially observed in the first activity test and in the test carried out 180 DAI. However, when the pathogen was well established in the soil, antagonistic protection was lower. Antagonistic effects were hardly detectable at 360 DAI. The antagonistic ability and activities of T. Harzianum were greater in soils held at intermediate soil moisture levels than in wet or dry soils.