Emulsifiable oils protect Metarhizium robertsii and Metarhizium pingshaense conidia from imbibitional damage

Abstract

Imbibitional damage (ID) may take place when dehydrated cells are immersed in water and, within certain limits, damage to the plasma membrane is inversely proportional to the immersion temperature. Dried Metarhizium anisopliae conidia formulated in pure oil are not severely impacted when rehydrated in a cold agar-based substrate, whereas dried, unformulated conidia experience reduced viability when immersion temperatures ⩽25°C. The primary objective of this study was to assess whether conidia of Metarhiziumrobertsii and Metarhizium pingshaense were sensitive to ID and, if so, determine whether oil dispersions (a formulation type in which fungal propagules are usually mixed with an emulsifiable oil prior to dilution in water) could be used to prevent this sensitivity. Unformulated Metarhizium conidia (isolates CG1091 and CG210) plunged into water equilibrated to different temperatures (0, 25, 31, 36, and 45°C) experienced significant reduction of initial viability at 0 and 25°C (and also 31°C for one of the isolates) following a 2-min exposure time or after exposure to 45°C for 60min. Dry, unformulated conidia plunged into cold water or water equilibrated to 25°C were physically harmed by ID, whereas viability of conidia following prolonged exposure to warm water was also harmed. Therefore, use of warm water in field operations was shown not to be an advisable strategy to alleviate ID, unless temperature and immersion time are critically monitored. On the other hand, mycopesticide formulation in an emulsifiable oil was shown to be an useful alternative to prevent ID on dehydrated M. robertsii and M. pingshaense conidia. Indeed, dried conidia formulated as an oil dispersion and plunged into water at either 0°C or 25°C showed significantly higher viabilities than unformulated counterparts.