Effect of artificial culture media on germination, growth, virulence and surface properties of the entomopathogenic hyphomycete Metarhizium anisopliae

Culture media influenced the germination of conidia, appressorial development and mycelial growth of Metarhizium anisopliae. Although the addition of KCl to Sabouraud dextrose agar (SDAM) significantly reduced germination of the conidia of three isolates in vitro and lowered germination and appressorial development on the cuticles of Myzus persicae and Meligethes aeneus, conidia grown on SDAM and minimal medium (MM) were more aggressive than conidia derived from SDA or yeast extract agar. No relationship was found between either germination or appressorial development and LT, values, suggesting that virulence depends on the speed of infection. Calcofluor staining and lectin binding showed that culture conditions also influenced the adhesion of Metarhizium anisopliae to insect cuticles through alteration of surface carbohydrates such as 1-4-beta-glucans. Dark, pigmented conidia from MM bound less lectins and calcofluor whereas the paler conidia from the other media fluoresced more intensely. Conidia from SDAM fluoresced 20-fold brighter and adhered more readily to insect cuticle compared with conidia derived from MM. Although inocula from both media showed elevated pathogenic activity against aphid and pollen beetle, MM-grown conidia were more aggressive since high mortality was induced by comparatively fewer conidia. When fluorescent intensity of calcofluor-stained conidia was compared to the number of conidia adhering to the insect cuticles, the relationship was significant, indicating that this dye has practical implications for the assessment of fungal strains in screening programmes for commercial development.