Water and temperature relations and microconidial germination of Fusarium moniliforme and Fusarium proliferatum from maize

The effects of water activity (a(w), 0.994 - 0.85 = 0.4 - 21.0 (-)MPa water potential), temperature (5 - 42 degrees C), and their interactions on microconidial germination of three isolates each of Fusarium moniliforme and Fusarium proliferatum were determined in vitro on a maize meal extract medium. Temporal germination rates of microconidia of isolates of both species were significantly influenced by both a(w) and temperature. Germination was very rapid at > 0.94 a(w) with an almost linear increase with time. Germination rates of microconidia of F. moniliforme were slower than those of F. proliferatum isolates at marginal a(w) levels and 5 - 25 degrees C, while at higher temperature (30 - 37 degrees C), the former germinated more rapidly than the latter. The a(w) minima for germination of isolates of both species was 0.88, with none occurring at 0.85 a(w) over a 40-day incubation period. At 37 degrees C, isolates of F. moniliforme had slightly lower a(w) minima than those of F. proliferatum. The narrowest range of a(w) for germination was at 5 degrees C, and none occurred at 42 degrees C. The effect of a(w) x temperature interactions on the lag phases (h) prior to germination and the germination rates (h(-1)) were estimated using the Gompertz model and the Zwietering equation. This showed that lag phases were shorter at 25 - 30 degrees C and 0.994 - 0.98 a(w), and were increased to 10 - 500 h at marginal temperatures (5 - 10 degrees C) for F. proliferatum and longer for F. moniliforme. At marginal a(w) levels (0.92 - 0.90), lag times were increased to > 250 h. Germination rates (h(-1)) were different for the two species. Microconidia of F. moniliforme germinated optimally at 25 - 37 degrees C and 0.96 - 0.98 a(w), but this changed to 30 degrees C at 0.90 - 0.94 a(w), while germination of microconidia of F. proliferatum remained optimum at 30 degrees C, regardless of a(w). There were statistically significant (P < 0.01) effects of a(w), temperature, isolate, and two- and three way interactions for F. proliferatum, but there were no intraisolate effects for F. moniliforme. The ecological significance of these data for understanding colonization patterns of these important fumonisin-producing fungi are discussed.