THE PERSISTENCE OF THIRAM IN SOIL AND ITS RELATIONSHIP TO THE MICROBIOLOGICAL BALANCE AND DAMPING-OFF CONTROL

Abstract

A means of determining the thiram content of treated soil was developed, applying the paper disk bioassay technique with Glomerella cingulata as the test organism. It was found that thiram persisted in sandy soil for over two months but disappeared from compost soil within one week. Thiram treatment changed the microbiological balance in the soil, the number of bacteria being increased and the fungi decreased for some time. Thiram was shown to be selective in its action against fungi; Penicillium and Trichoderma, being resistant, increased in number after soil treatment. Thiram treatment rendered soil more difficult to infest artificially with Pythium ultimum and also prevented a natural increase in the population of damping-off organisms resulting from repeated cropping. This protection for seedlings persisted longer than did the fungicide in the soil.