The effects of water activity (aW, 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 aW and temperature. Germination was very rapid at > 0.94 aW with an almost linear increase with time. Germination rates of microconidia of F. moniliforme were slower than those of F. proliferatum isolates at marginal aW levels and 5-25 degrees C, while at higher temperature (30-37 degrees C), the former germinated more rapidly than the latter. The aW minima for germination of isolates of both species was 0.88, with none occurring at 0.85 aW over a 40-day incubation period. At 37 degrees C, isolates of F. moniliforme had slightly lower aW minima than those of F. proliferatum. The narrowest range of aW for germination was at 5 degrees C, and none occurred at 42 degrees C. The effect of aW 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 aW, and were increased to 10-500 h at marginal temperatures (5-10 degrees C) for F. proliferatum and longer for F. moniliforme. At marginal aW 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 aW, but this changed to 30 degrees C at 0.90-0.94 aW, while germination of microconidia of F. proliferatum remained optimum at 30 degrees C, regardless of aW. There were statistically significant (P < 0.01) effects of aW, 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.