Debilitation in conidia of the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae and implication with respect to viability determinations and mycopesticide quality assessments

Germination of Beauveria bassiana (Bb) and Metarhizium anisopliae (Ma) conidia determined from a fast-rehydration (FR) protocol were compared to those obtained when dry conidia were subjected to slow rehydration (SR) by holding under high humidity conditions prior to aqueous suspension. Differences in viability estimates obtained using the FR vs. SR protocols increased markedly after conidia were exposed to various stress factors in storage (high aw, temperature, and O2 concentrations), with the SR protocol producing higher estimates of viability in all cases. After Bb conidia were stored under moist conditions for 21 days at 25 °C, the SR estimate of viability was >21% greater than the FR estimate. In jars flushed with different O2 concentrations and stored at 50 °C for 34 days, proportional differences between protocols varied, depending on water activity, from 18–44% in jars flushed with 0% O2 (100% N2) to as high as 63–93% when treated with 21–22% O2. For conidia stored over a broad range of moderate to high temperatures in the absence of O2, SR–FR differences were ⩽9% at 25–40 °C but 30% at 50 °C. Germination of stressed Bb and Ma conidia increased substantially when incubation time on the germination substrate was increased from 24 to 72 h, whereas germination of non-stressed conidia showed little change. Conidia debilitated by stress were characterized by hypersensitivity to lethal imbibitional damage (damage that is mitigated by slow rehydration) and slow germination. Viability protocols that may provide more reliable assessments of overall mycopesticide quality are discussed.