Loss of virulence in an isolate of an entomopathogenic fungus, Nomuraea rileyi, after serial in vitro passage

Serial in vitro passage of Nomuraea rileyi (Farlow) Samson (UFL 78-6 pathotype) altered both its growth and development on in vitro and in vivo substrates. After only six conidial transfers on Sabouraud maltose and yeast extract agar (SMAY) plates, a reduction in the ability of these cultures to produce the yeastlike, hyphal body stage was observed. Conidia derived from transfers 6-10, although virulent to neonatal Anticarsia gemmatalis Hubner larvae, failed to produce progeny conidia on larvae killed by N. rileyi. Continued serial conidial transfer (greater than 10) of this pathotype on SMAY resulted in conidia with reduced virulence against neonatal larvae. By the 16th conidial transfer, progeny conidia were avirulent to larvae. The attenuation of N. rileyi was associated with the sporulation process. After multiple vegetative transfers (greater than 80) of the hyphal body stage on SMAY plates, no decrease in virulence to A. gemmatalis was observed