Effectiveness of Coniothyrium minitans and Trichoderma atroviride in suppression of sclerotinia blossom blight of alfalfa

The effects of the mycoparasites Coniothyrium minitans and Trichoderma atroviride on the suppression of alfalfa blossom blight caused by Sclerotinia sclerotiorum were evaluated under indoor and field conditions. When T. atroviride (9.0 x 10(4) conidia/floret) + S. sclerotiorum (6.0 x 10(3) ascospores/floret) or C. minitans (9.0 x 10(4) conidia/floret) + S. sclerotiorum (6.0 x 10(3) ascospores/floret) were applied to detached young alfalfa florets, T. atroviride effectively inhibited saprophytic growth of S. sclerotiorum, whereas C. minitans showed no inhibition under the same conditions. When T. atroviride (6.9 x 10(4) conidia/floret) + S. sclerotiorum (6.0 x 10(3) ascospores/floret) or C. minitans (6.9 x 10(4) conidia/floret) + S. sclerotiorum (6.0 x 10(3) ascospores/floret) was applied to young alfalfa petals in vivo just after pollination, the percentage of pod formation was higher for T. atroviride+S. sclerotiorum than that for C. minitans+S. sclerotiorum, and the percentage of pod rot was lower for T. atroviride+S. sclerotiorum than that for C. minitans+S. sclerotiorum. However, when they were applied to senescent petals attached to developing pods of alfalfa at 9.2 x 10(4) conidia/floret together with S. sclerotiorum at 4.5 x 10(3) ascospores/floret at 14 days after pollination, C. minitans was more effective than T. atroviride in suppressing sclerotinia pod rot and seed rot of alfalfa. Field experiments showed that three applications of C. minitans (5.4 x 10(6) conidia mL(-1)) or T. atroviride (5.4 x 10(6) conidia mL(-1)) at a 7-day interval to blossoms of alfalfa effectively suppressed sclerotinia pod rot in two out of three annual trials. Coniothyrium minitans effectively suppressed sclerotinia seed rot in all three years, whereas T. atroviride was not effective against seed rot in any of the trial years. The efficacy of C. minitans was not significantly different (P > 0.05) from benomyl (250 mu g ai mL(-1)). This study suggests that C. minitans has potential as a biocontrol agent to control blossom blight of alfalfa caused by S. sclerotiorum.