Conidial vigor vs. viability as predictors of virulence of entomopathogenic fungi

•Speed of germination of fungal conidia is a highly significant virulence factor.•LC50s of conidia are directly related to the proportion of slow-germinating conidia.•LC50s based solely on fast-germinating (vigorous) conidia tend to be constant.•Vigorous conidia lead to shorter survival times of Spodoptera frugiperda larvae.•Conidial vigor is a better predictor of mycoinsecticide quality than viability.

We tested the hypothesis that debilitated conidia exhibiting slow-germination (requiring > 16 h to germinate) are less virulent than vigorous conidia exhibiting fast germination (requiring ⩽ 16 h to germinate). Preparations of Beauveria bassiana s.l. strain CG 1027 with variable ratios of vigorous to debilitated conidia were assayed against third-instar larvae of Spodoptera frugiperda. As the proportion of debilitated conidia in test preparations increased, LC50 expressed in terms of total viable conidia increased, while LC50 expressed solely in terms of vigorous conidia remained constant, indicating that vigorous conidia were responsible for nearly all mortality observed in the assays. Larvae treated with conidia from low-quality batches (with high proportions of debilitated conidia) survived consistently longer than those treated with comparable doses of conidia from high-quality batches. These results confirm our previous hypotheses that inclusion of debilitated conidia in viability assessments can lead to overestimation of the quality (potency) of mycoinsecticide preparations and support our recommendation for use of short incubation periods for assessing viability whenever viability is relied upon as an indicator of product quality.

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