Submitted to: Biocontrol Science and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/8/1996
Publication Date: N/A
Citation: Interpretive Summary: Whenever living, natural enemies of weeds such as fungi are used as a bioherbicide, inadequate shelf life can be a serious problem. We have found that a property of a material, called water activity, can influence shelf life of wheat flour and clay granules containing a fungus that kills the weed, hemp sesbania. Water activity is a measure of free water available to the fungus. If the granules are dried to a water activity of less that 0.33, shelf life at 25 C is much longer than if the water activity is higher. Adding sugar to the formulation prolonged shelf life at high humidities.
Technical 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 grown in shake flasks (corn meal-soy flour medium) and on Emerson Yp Ss agar and formulated in wheat flour-kaolin granules ("Pesta"). 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) resulted with samples in the water activity range of 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 one year. Sucrose (5% b wt.) Incorporated in the granules partially counteracted the detrimental effect of high water activity on storage life. Correlation of water activity with shelf life is important for biocontrol products.