Influence of some parameters on the germination assessment of mycopesticides

Abstract

The substantial negative impact of some parameters on the germination of low-quality conidia (high proportion of slow-germinating propagules) was demonstrated, whereas for high-quality batches their effect was small or even absent. Germination was increased as the initial hydration status of conidia immediately prior to suspension preparation was increased, being ca. 33% and 80% for dehydrated Metarhizium anisopliae propagules (water activity ⩽0.314) from low- or high-quality batches after an 18h incubation period, respectively, and 63% and 95% for hydrated propagules (water activity=0.933). Germination of low-quality propagules also increased as the time dry conidia were kept in aqueous suspension prior to inoculation onto culture media (15min, 3 or 24h) or the incubation time at 25°C before counts (18, 48 or 72h) was increased. Depending on treatment conditions, average germination of low-quality conidia varied from 53% to 98%. On the other hand, germination for high-quality conidia was always ⩾94%. Regarding the relative humidity (RH) of the incubation atmosphere, the average germination rates for low-quality conidia on Potato Dextrose Agar (PDA) in Petri plates was 49%, while germination of these conidia on PDA blocks kept under lower RH inside plastic boxes was ⩽23%. Use of lactophenol-staining and/or use of coverslips had a negative effect when germination assessment was performed for low-quality conidia, resulting in distorted counts or increased standard deviations compared to high-quality conidial batches. The occurrence of dislodged conidia (ungerminated conidia outside the inoculation zone due to hydraulic pressure exercised by addition of stains and/or coverslips added to the substrate by the time germination is assessed) was common place, whereas dislodged conidia were not seen in treatments with high-quality batches. This work underscores the importance of a number of parameters that anyone working with low-quality fungi needs to be cognizant of in their research.