Effect of water activity, temperature and substrate on interactions between field and storage fungi

Abstract

Field and storage fungi demonstrated a wide range of interactions on malt and wheat extract agars. A numerical Index of Dominance ({itI{in d }}) was derived for individual fungi. This was found to vary with water activity ({ita w }), temperature and substrate. Epicoccum nigrum and Fusarium culmorum were the only field fungi that competed successfully against other fungi on both substrates while Alternaria alternata, Cladosporium species and Verticilliutn lecanii were all uncompetitive, intermingling freely with many Aspergillus and Penicillium spp. Of the Penicillium spp., P. brevicompactum and P. hordei were most dominant although P. verrucosum var. cyclopium and P. roquefortii were favoured by malt and wheat extract, respectively. P. piceum competed well against both field and storage fungi but only at 30 °C. A. candidus and A. nidulans were the most competitive of the Aspergillus spp. but only at 25 and 30° while A. fumigatus was competitive at 30° only. A. repens and A. versicolor were uncompetitive regardless of {ita w }. Variations in temperatures between 15 and 30° were accompanied by changes in {itI{in d }} scores. Total {itI{in d }} scores of the most dominant fungi were usually lower on wheat extract than on the richer malt agar. Detailed microscopic examination of the interaction zones revealed granulation and vacuolation of hyphal cells and increased branching but no changes in permeability of the affected cells, loss of metabolic activity or cell death.