Selection of mycoparasites of sclerotia of Sclerotinia sclerotiorum isolated from New Zealand soils

Seventy-four isolates of fungi including Trichoderma spp. (18), Gliocladium spp. (12), Coniothyrium minitans (39), Chaetomium globosum (3), Chrysosporium luteum (1), and Fusarium sp. (1) were screened for ability to parasitise sclerotia of Sclerotinia sclerotiorum. Seventeen isolates killed all sclerotia after 4 weeks of incubation, with a further 15 isolates reducing sclerotial viability to less than 20%. These 32 isolates were screened in a second sclerotial parasitism test, where solid substrate (1% kibbled maize:perlite colonised with antagonist) was incorporated into soil. Twenty-four isolates significantly reduced sclerotial viability compared to the untreated control, with C. minitans Conio, CHI, T5R4 2g, A69, and T5R4 2i giving the greatest reduction. C. minitans isolates showed considerable variation in their ability to parasitise sclerotia with some showing high activity but others little or no activity. Two New Zealand isolates (A69 and T5R4 2i) and two overseas isolates (CH I and Conio) were compared further. Increasing the length of the incubation period after treatment of sclerotia with C. minitans resulted in a decrease in the viability of sclerotia and an increase in the infection of sclerotia by C. minitans. This effect was significantly greater for isolates CH1 and Conio compared with A69 and T5R4 2i. Incorporation of spores of the four isolates into soil (1 x 10(6) spores/g soil) reduced sclerotial viability, with all isolates of antagonists surviving in the soil for the 2-week duration of the experiment. Variation in the susceptibility of three S. sclerotiorum isolates to sclerotial parasitism by C. minitans CH1 and A69 was observed, with S. sclerotiorum isolate S9W1 the most susceptible and isolate S35 the least.