Germination of Gibberella zeae ascospores as affected by age of spores after discharge and environmental factors

The effects of age of ascospores (0-18 days after discharge), photon flux density (0-494 mu mol m(-2) s(-1) PAR), temperature (4-30 degrees C), frost (-15 degrees C for 30 min), relative humidity (RH; 0-100%), pH (2.5-6.5) and dryness (0 and 53% RH for up to 40 min) on the germination of the ascospores of the mycotoxin-producing fungus Gibberella zeae (anamorph Fusarium graminearum) were studied. Freshly discharged ascospores germinated within 4 h at 20 degrees C and 100% RH. The rate of germination and the percentage of viable ascospores decreased over time after the spores were discharged from perithecia. The time course of ascospore germination was not significantly affected by photon flux density. The period of time required to obtain 50% germinated ascospores at 100% RH was 26.90 h at 4 degrees C, 10.40 h at 14 degrees C, 3.44 h at 20 degrees C and 3.31 h at 30 degrees C. There was no significant effect of frost on the percentage of viable ascospores. A small percentage (6.6 +/- 3.8%) of the ascospores germinated at 53% RH. At RH >= 84% and 20 degrees C almost 100% of the freshly discharged ascospores germinated. The time course of ascospore germination was affected by pH. The maximum rate of ascospore germination was estimated to be at pH 3.76. Ascospores lost their ability to germinate following exposure to 0% RH almost instantaneously. No germinating spores were detected after an incubation period of 1 min at 0% RH. Incubating the ascospores at 53% RH decreased the percentage of viable spores from 93 to 6% within 10 min. The data demonstrate that age of spores, relative humidity, temperature and pH, but not photon flux density, are key factors in germination of G. zeae ascospores.