Survival of the mycoparasite Coniothyrium minitans on flower petals of oilseed rape under field conditions in central China

The mycoparasite Coniothyrium minitans is an effective biocontrol agent of stem rot of oilseed rape ( Brassica napus) caused by Sclerotinia sclerotiorum. This study was to investigate survival of C. minitans on flower petals of oilseed rape under field conditions in central China using the mutant strain SV-5-2 which is tolerant to the fungicide vinclozolin, and the two bactericides benzylpenicillin sodium (BPS) and streptomycin sulfate (SS). Results showed that potato dextrose agar (PDA) amended with vinclozolin (500 μg ml −1), BPS (500 μg ml −1), and SS (500 μg ml −1) was suitable for detection of conidial germination and mycelial growth of C. minitans strain SV-5-2. Results of the five field experiments (A–E) conducted in 2004 and 2005 showed that C. minitans strain SV-5-2 applied to flower racemes of oilseed rape could be detected for five days in four experiments and three days in one experiment. The rate of survival of C. minitans decreased with time and at the last sampling date, the survival rate was 17.3, 14.3, 35.0, 62.6, and 1.9% for experiments A, B, C, D, and E, respectively. Moreover, survival of C. minitans was affected by weather conditions. Relatively, high rates of survival of C. minitans were observed under sunny weather in the experiments C and D, implying that conidia of C. minitans are tolerant to sunlight irradiation for a period of at least five days in the experiment C and three days in the experiment D. Low survival rates of C. minitans were observed under light rain weather in the experiment A, and heavy rain weather in the experiments B and E. The low recovery rate of C. minitans under rainy conditions suggests a possibility of rain wash of the inoculum applied to the florets. Methods for improving efficacy of C. minitans in the field are discussed.