Coniothyrium minitans on Apothecia of Sclerotinia trifoliorum

During a study of apothecial dynamics of Sclerotinia trifoliorum at the University of Kentucky Spindletop Farm at Lexington, an apothecium with small black patches on the surface of the hymenium was found. The affected apothecium was incubated in a moist chamber at room temperature. After 3 days, white, cottony mycelium was observed on the surface of the hymenium; pycnidia formed in the mycelium and around the stipe of the apothecium several days later. The apothecium eventually decayed and shrunk. Pycnidia measured 168 to 520 μm (mean 311 μm). Pycnidiospores were dark brown en masse; they were ovoid to ellipsoid, measuring 3.1 to 8.2 μm (mean 6.0 μm) in length and 3.1 to 4.1 μm (mean 3.7 μm) in width, and were faintly verrucose. Fresh sclerotia of S. trifoliorum were produced in vitro and then inoculated with pycnidiospores produced on potato dextrose agar. Inoculated sclerotia were incubated in a moist chamber at room temperature. After 7 to 10 days, inoculated sclerotia shriveled and decayed, pycnidia formed on their surfaces, and the same fungus was isolated. The fungus was identified as Coniothyrium minitans Campbell. Among 58 apothecia examined in the field on 1 November, three were apparently parasitized; pycnidia developed on one of these following a 3-day incubation. Weather conditions during the preceding 2 weeks had been generally humid with above-normal temperatures (daily mean air temperature range and interquartile range were 4.0 to 20.0 and 8.9 to 16.1°C, respectively), which may have favored activity of the mycoparasite. C. minitans was reported by Campbell (1) in California on sclerotia formed in cultures of Sclerotinia sclerotiorum. It causes decay of sclerotia of several Sclerotinia spp., some Botrytis spp., and Sclerotium cepivorum in soil. Consequently, it may have considerable biological control potential. It has been recorded in 29 countries and on all continents except South America (2). The fungus previously has been isolated from only sclerotia or, in a few instances, directly from soil. This is the first report on C. minitans parasitic on apothecia collected from the field. References: (1) W. A. Campbell. Mycologia 39:190, 1947. (2) C. Sandys-Winsch et al. Mycol. Res. 97:1175, 1993.