Susceptibility of the biocontrol fungi Metarhizium anisopliae and Trichoderma asperellum (Ascomycota: Hypocreales) to imbibitional damage is driven by conidial vigor

Imbibition damage (ID) is a phenomenon characterized by the death of dehydrated conidia when immersed in water at temperatures⩽ca. 25°C, as a result of the transition in the conformation of the plasma membrane in the presence of free water. In the present study we evaluated the susceptibility to ID of eight major species or varieties of fungi used in biological control programs in several countries (Isaria farinosa, I. fumosorosea, I. javanica, Nomuraea rileyi, Pochonia chlamydosporia var. catenulata, P. chlamydosporia var. chlamydosporia, Purpureocillium lilacinum, and Trichoderma asperellum). A commercial isolate of Metarhizium anisopliae sensu stricto, known to be sensitive to ID, was also included. Within all tested taxa, significant reductions in germination of dehydrated conidia after immersion in cold water (15°C) compared to immersion in warm water (33–37°C) were reported, due to delayed germination and/or ID. Additional tests with dehydrated M. anisopliae and T. asperellum conidia produced on different substrates revealed that susceptibility to ID was greatly impacted by vigor (% dehydrated conidia with rapid germination). Low mortality (1–7%) of dehydrated conidia immersed in cold water was observed when vigor was ⩾95%. Conversely, mortality was ⩾20% when vigor ranged from 50 to 90%. We hypothesize that, in vigorous dehydrated conidia, the cell wall is capable of slowing down water influx, allowing rehydration of the cell membrane by water vapors before contact with free water, thus mitigating ID. The marked effect of conidial quality on susceptibility of M. anisopliae and T. asperellum to ID highlights the need for production and post-production procedures (drying, harvesting, formulation and packaging) that ensure high vigor of conidia used as active ingredients in mycopesticides with low water contents.