Optimization of storage conditions for adequate shelf-life of ‘pesta’ formulation of<i>Fusarium oxysporum</i>‘foxy 2’, a potential mycoherbicide for<i>Striga</i>: Effects of temperature, granule size and water activity

Abstract

Abstract An adequate shelf-life of mycoherbicidial products is an essential requirement for their acceptance and commercialization. Therefore, attempts were made to study the effects of temperature, granule size, and water activity (R.H./100) on the viability of the encapsulated propagules of Fusarium oxysporum ‘Foxy 2’ in ‘Pesta’ granules during storage. ‘Pesta’ granules were made with different inocula of Foxy 2, including: microconidia; mixture of mycelia and microconidia; fresh and dried chlamydospore-rich biomass. Two sizes of each granular preparation (0.5–2 and 0.25–0.5 mm) were stored in the refrigerator at 4°C as well as at room temperature (21±3°C) for 1 year. Additional samples were also stored at water activities (aw) of 0.12 and 0.41 at 25°C. Regardless of the type of formulated propagules and the granule size, all samples stored at 4°C maintained a significantly higher viability compared to those kept under room temperature. At 4°C, the ‘Pesta’ preparations with the larger granule size (0.5–2 mm) maintained more viable propagules than those with the smaller one (0.25–0.5 mm) in case of microconidia, mycelia plus microconidia and fresh chlamydospore inoculum after 1 year of storage. Granule size did not affect the viability of the dried chlamydospores. At 25°C, shelf-life of all ‘Pesta’ granules was significantly prolonged when stored at a low water activity of 0.12 compared to the storage at 0.41 aw. The results of the combined effect of water activity and temperature also revealed clearly that all formulated propagules in ‘Pesta’ granules retained a significantly higher viability when stored at 0.62 aw and 4°C than at 0.12 aw and 25°C, indicating the most pronounced effect of storage temperature. Keywords: biological weed controlformulationFusarium oxysporumgranule sizemycoherbicideshelf-lifestorageStriga hermonthicatemperature, water activity The first author is grateful to the German Academic Exchange (DAAD) Service for the financial support. Special thanks are expressed to the Department of Agroecology in the Tropics and Subtropics, University of Hohenheim, Germany, for providing place and research facilities.