Water activity and other factors that affect the viability of Colletotrichum truncatum conidia in wheat flour-kaolin granules ('pesta')

Abstract

Optimization of shelf-life is critically important for biocontrol products containing living microorganisms. Conidia of Colletotrichum truncatum, a fungal pathogen of the weed, hemp sesbania (Sesbania exaltata), were produced in shake flasks (corn meal-soya flour medium) and on Emerson Yp Ss agar and formulated in wheat flour-kaolin granules ('Pesta'). The granules were conditioned at water activities of 0, 0.12, 0.33, 0.53 and 0.75 during storage at 25°C over desiccant or saturated salt solutions. The longest shelf-life (conidial inoculum viability) was found in samples in the water activity range 0-0.33, where the water was bound by the matrix and not readily available to the fungus. At a water activity of 0.12, granules were 100% viable (on water agar) for at least 24 weeks, and were 87% viable after 1 year. Sucrose (5% w/w) partially counteracted the detrimental effect of high water activity on the shelf-life of C. truncatum when incorporated in the granules.