Quantitative aspects of infection of Sclerotinia sclerotiorum sclerotia by Coniothyrium minitans -: timing of application, concentration and quality of conidial suspension of the mycoparasite

White mould disease leads to production of sclerotia, which subsequently survive in soil and may be responsible for future epidemics. The effect of the mycoparasite Coniothyrium minitans in decreasing survival of sclerotia of Sclerotinia sclerotiorum was studied. Infection of sclerotia of S. sclerotiorum by C. minitans can be achieved by a single conidium. Under optimal conditions, 2 conidia per sclerotium produced 63% of the maximum infection (ca. 90%) of sclerotia produced by up to 1000 conidia. Similar results were observed on the infection of stem pieces infected by S. sclerotiorum. In field trials, application of conidial suspensions of C. minitans to a bean crop soon after white mould outbreak led to a higher percentage of sclerotial infection than later applications. Ninety per cent infection of sclerotia was obtained within 3 weeks of application by C. minitans suspensions in the range of 5 x 10(5) and 5 x 10(6) conidia ml(-1) at 1000 l ha(-1). The concentration of the conidial suspensions and the isolate used were of less importance. The result was marginally affected by the germinability of the conidia (75% against 61% infected sclerotia at 91% and 16% viability of isolate IVT1, respectively). Less apothecia of S. sclerotiorum developed in soil samples collected after 2 months from plots sprayed immediately after disease outbreak than from those treated 11-18 days later. It is concluded that a suspension of 10(6) conidia ml(-1) in 1000 l ha(-1) (=10(12) conidia ha(-1)) sprayed immediately after the first symptoms of disease are observed, results in >90% infection of sclerotia of S. sclerotiorum. The infection of sclerotia, which prevents their carry-over, occurs within a broad range of inoculum quality.