Effects of water activity and temperature on the growth of fungi interacting on barley grain

Abstract

Germination, growth and interactions of Aspergillus flavus, Penicillium verrucosum, Fusarium poae and Hyphopichia burtonii, either alone or in the presence of another species, were observed at water activities of 0·97, 0·95 or 0·90 aw and at 20 or 30 °C, both on the surface of individual barley grains by scanning electron microscopy and on thin layers of grain with a stereomicroscope. By both methods, each species exhibited characteristic linear growth rates which changed with aw and temperature. Spore germination was unaffected by the presence of another species, but distinct interaction patterns between competing species on the grain surface were identified, which were determined by rate of hyphal extension and branching, namely (a) faster growth of one species causing progressive inhibition of the slower-growing species; (b) faster growth initially of one species which is then inhibited by the slower-growing species; (c) one species grew faster than the other but with no adverse effects; (d) one species grew faster than the other initially, but growth rates of both declined later during interaction; (e) both species grew at similar rates initially but growth rate of one declined during competition; and (f) both species grew at similar rates initially but later reduced the growth of each other. On thin layers of barley grain, competing species generally showed mutual inhibition on contact, forming discrete colonies whose size was determined by the relative radial growth rates of the two species and positions of inocula. However, at 30°, A. flavus overgrew colonies of all other species.