Effects of drying on the survival of conidiosphores of Metarhizium anisopliae var. acridum Driver & Milner

Abstract

Metarhizium anisopliae var. acridum Driver & Milner (= Metarhizium flavoviride Gams & Rozsypal), isolate CG423, is being developed as a mycoinsecticide against grasshoppers in Brazil. Conidia were harvested and stored in a drying chamber during a study period of 260 d, in a room at ∼25°C. Water content (dry weight) of fresh and dried conidia was 70 and 4%, respectively. Conidia from fresh and dry preparations produced similar in vitro germination patterns and were both infective of the American grasshopper Schistocerca americana Drury. The drying process greatly enhanced the survival of CG423 conidia, as witnessed by nearly 100% germination rates after more than a 100-d desiccation period. Prolonged 135 and 176-d storage in the drying chamber at room temperature decreased viability to 72.5% and 28.5% respectively. In contrast, fresh conidia sealed in plastic bags and stored at ambient conditions for 52 d were inactive (<10% viable). The low germination rate (28.5%) of conidia exposed to 176 d in dry storage was altered by exposure of the dry conidia to high humidity (96% RH). Dry conidia exposed to a 4-h rehydration period had germination rates of ∼50%, whereas 24-h preincubation at high humidity resulted in ∼90% germination rates. However, the germination rates were reduced at 204 (81%), 323 (70%) and 260 (23%) days in storage. In addition, preincubation of the CG423 dry conidia in water produced similar positive effects on germination as compared to germination of fresh conidia. At 24h post-inoculation, dry conidia produced significantly lower germination rates (21%) than rehydrated conidia (germination rates > 91.7%, p < 0.05). However, after 48h on SDAY plates, the germination rate of dry conidia (88.3%) was comparable to the germination of conidia rehydrated in water, water Tween-20, or in high humidity.