Optimising sporulation and virulence in Drechslera avenacea

Studies were conducted on agar media to optimise sporulation of Drechslera avenacea, a fungal pathogen being evaluated as a biological control agent for Avena species (wild oats). Conidium production was affected by nutrition, pH, temperature and light conditions. Of the agar media tested, Czapek Dox agar (CZA) and half-strength oatmeal agar (OMA) were the only media where sporulation occurred at all temperatures tested under a 12-h light:12-h dark photoperiod (L/D). The optimum temperature for conidium production was 20°C on OMA, whereas there was no optimum temperature on CZA. Under a 12-h near-ultraviolet (NUV):12-h dark photoperiod (NUV/D), similar numbers of conidia were produced on CZA at 6.66, 14.56, and 22.78 W m-2, whereas on OMA conidium production was the highest at 14.56 W m-2. When NUV/D and L/D conditions were compared, similar numbers of conidia where produced on CZA, whereas OMA conidium production was superior under the NUV/D photoperiod. Considerable variation in sporulation and degree of violence of D. avenacea was detected among isolates from different geographic areas. The most virulent conidia were obtained on OMA at 20°C incubated under continuous illumination NUV light. Therefore, the most suitable conditions for conidium production of D. avenacea were growth for 1 week on OMA at 20°C under continuous NUV at an intensity of 14.56 W m-2. Under these conditions, 1.1×105 conidia mL-1 were produced which is the highest sporulation yet reported for any Drechslera spp., which are traditionally poor sporulators.