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

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 (a_w) 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 a_w. 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 a_w and 4°C than at 0.12 a_w and 25°C, indicating the most pronounced effect of storage temperature.