The biology of the acaropathogenic fungus Hirsutella kirchneri

The acaropathogenic fungus Hirsutella kirchneri (Rostrup) Minter, Brady and Hall grew best and produced most mycelia on a medium containing yeast extract, dextrose and agar; conidial production, however, was maximal or potato dextrose agar (PDA). The best growth on both media was at 25°C and conidial germination was high within a wide range of temperatures (10-35°C). Colony growth, mycelial mass production and conidial yield were best under alternating dark and light regimes. Maximal germination occurred under dark conditions. When grown in continuous light the fungus produced synnemata (compacted conidiophores) which remained viable for 22 weeks. Of six species of phytophagous mites assayed, three spider mites and a rust mite became infected by the fungus, as did, to a limited degree, a parasitic mite. Another four mites, including two pests, a scavenger and a predator, were unaffected. The fungus grew on and sporulated from heat-killed cadavers of a dead mealybug (Homoptera). Conidial germination and penetration into live mites, under saturation conditions, were affected little by temperatures. Intra-host growth was temperature dependent, with mite death beginning on the second or third day post-infection. Maximal sporulation from infected mites took place at 25°C. Mortality was quickest at 25°C when mites were held under saturation conditions, but some death also occurred at lower relative humidities. These data are discussed with a view to using H. kirchneri in the biological control of plant mites.