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 interactio ns on microconidial germination of three isolates each of Fusarium mon iliforme and Fusarium proliferatum were determined in vitro on a maize meal extract medium. Temporal germination rates of microconidia of is olates 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. mon iliforme were slower than those of F. proliferatum isolates at margina l 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, wit h none occurring at 0.85 a(w) over a 40-day incubation period. At 37 d egrees C, isolates of F. moniliforme had slightly lower a(w) minima th an those of F. proliferatum. The narrowest range of a(w) for germinati on was at 5 degrees C, and none occurred at 42 degrees C. The effect o f a(w) x temperature interactions on the lag phases (h) prior to germi nation and the germination rates (h(-1)) were estimated using the Gomp ertz model and the Zwietering equation. This showed that lag phases we re shorter at 25 - 30 degrees C and 0.994 - 0.98 a(w), and were increa sed to 10 - 500 h at marginal temperatures (5 - 10 degrees C) for F. p roliferatum 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. moniliform e 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, reg ardless of a(w). There were statistically significant (P < 0.01) effec ts of a(w), temperature, isolate, and two- and three way interactions for F. proliferatum, but there were no intraisolate effects for F. mon iliforme. The ecological significance of these data for understanding colonization patterns of these important fumonisin-producing fungi are discussed.