Germination and survival of Fusarium graminearum macroconidia as affected by environmental factors

The effects of temperature (4-20degreesC), relative humidity (RH, 0-100%), pH (3-7), availability of nutrients (0-5 g/l sucrose) and artificial light (0-494 mumol/m(2)/s) on macroconidial germination of Fusarium graminearum were studied. Germ tubes emerged between 2 and 6 h after inoculation at 100% RH and 20degreesC. Incubation in light (205 +/- 14 mumol/m/s) retarded the germination for approximately 0.5 h in comparison with incubation in darkness. The times required for 50% of the macroconidia to germinate were 3.5 h at 20degreesC, 5.4 h at 14degreesC and 26.3 h at 4degreesC. No germination was observed after an incubation period of 18 h at 20degreesC in darkness at RH less than 80%. At RH greater than 80%, germination increased with humidity. Germination was observed when macroconidia were incubated in glucose (5 g/l) or sucrose (concentration range from 2.5 x 10(-4) to 5 g/l) whereas no germination was observed when macroconidia were incubated in sterile deionized water up to 22 h. Macroconidia germinated quantitatively within 18 h at pH 3-7. Repeated freezing (-15degreesC) and thawing (20degreesC) water agar plates with either germinated or non-germinated macroconidia for up to five times did not prevent fungal growth after thawing. However, the fungal growth rate of mycelium was negatively related to the number of freezing events the non-germinated macroconidia experienced. The fungal growth rate of mycelium was not significantly affected by the number of freezing events the germinated spores experienced. Incubation of macroconidia at low humidity (0-53% RH) suppressed germination and decreased the viability of the spores.