Effects of physical and nutritional stress conditions during culture on conidial germination speed, adhesion to host cuticle, and virulence of Metarhizium anisopliae, an entomopathogenic fungus
Growth under stress may influence pathogen virulence and other phenotypic traits. Conidia of the entomopathogenic fungus Metarhizium anisopliae var. anisopliae (isolate ARSEF 2575) were produced under different stress conditions and then examined for influences on in vitro conidial germination speed, adhesion to the insect cuticle, and virulence to an insect host, Tenebrio molitor. Conidia were produced under non-stress conditions [on potato–dextrose agar plus 1 g l−1 yeast extract (PDAY; control)], or under the following stress conditions: osmotic (PDAY + sodium chloride or potassium chloride, 0.6 or 0.8 m); oxidative [(PDAY + hydrogen peroxide, 5 mm) or UV-A (irradiation of mycelium on PDAY)]; heat shock (heat treatment of mycelium on PDAY at 45 °C, 40 min); and nutritive [minimal medium (MM) with no carbon source, or on MM plus 3 g l−1 lactose (MML)]. Conidia were most virulent (based on mortality at 3 d) and had the fastest germination rates when produced on MML, followed by MM. In addition, conidial adhesion to host cuticle was greatest when the conidia were produced on MML. Media with high osmolarity (0.8 m) produced conidia with slightly elevated virulence and faster germination rates than conidia produced on the control medium (PDAY), but this trend did not hold for media with the lower osmolarity, (0.6 m). Conidia produced from mycelium irradiated with UV-A while growing on PDAY had somewhat elevated virulence levels similar to that of conidia produced on MM, but their germination rate was not increased. Hydrogen peroxide and heat shock treatments did not alter virulence. These results demonstrate that the germination, adhesion and virulence of M. anisopliae conidia can be strongly influenced by culture conditions (including stresses) during production of the conidia.