Yield and germination of the entomopathogenic fungus Beauveria bassiana when grown on different rice preparations

Experiments on two isolates of Beauveria bassiana (Balsamo) Vuillemin (IMI 389521 and IMI 386243) investigated whether modifying the solid-substrate phase of mass-production, namely the par-boiling, water and washing regimes of the rice substrate, affected conidial yield and germination. Germination was assessed on progressively drier media (0.987, 0.978 and 0.968 aw); the effects were also studied before and after a period of short-term cool storage. There was little difference in conidial yield between treatments and/or isolates. The only significant difference was IMI 389521 on unwashed then par-boiled rice with 600 ml water/kg rice, which yielded more conidia than IMI 386243 on unwashed and not par-boiled rice. There were significant treatment effects on the germination of both isolates. In particular, the germination of conidia produced on rice that had been washed under running tap-water for 5 min and not par-boiled, was lower than all other treatments, for both isolates. Highest conidial germination for IMI 389521 was from the unwashed then par-boiled rice with 600 ml water/kg, at 96%; highest germination for IMI 386243 was from the unwashed and not par-boiled rice, at 72%. As the growth media was made progressively drier there was no change in the germination levels of IMI 389521, for any of the rice treatments. In contrast, germination of two treatments of IMI 386243, namely washed under running tap-water for 5 min and not par-boiled, and unwashed then par-boiled with 300 ml water/kg, fell significantly (by 29 and 35%, respectively). After 8-weeks storage at 5 °C, germination had fallen significantly for three of the four treatments of IMI 386243 and for one treatment of IMI 389521; the effect was greatest for IMI 386243 produced on unwashed and not par-boiled rice, with a 62% loss in germination. Inherent differences between isolates and limitations of the methodology are discussed.