Diurnal release of ascospores by Gibberella zeae in inoculated wheat plots

The daily pattern of ascospore release by Gibberella zeae (= Fusarium graminearum), the causal agent of Fusarium head blight of wheat, was investigated in artificially inoculated wheat plots. Mature perithecia and ascospores appeared on corn colonized by G. zeae 2 to 3 weeks after being placed in the plots (mid June). Ascospores over the plots were sampled with a Burkard continuous 7-day spore sampler. Temperature, relative humidity (RH), leaf wetness, and rainfall were also recorded in the plots on an hourly basis. Ascospores were released during the first three weeks of July in 1992 and 1993, with hourly concentrations of 600 to 9,000 ascospores/m(3). Ascospore release typically showed a diurnal pattern. Release began around 1600 to 1800 hours, reached a peak usually before midnight, and declined to low levels by 0900 hours the following morning. The beginning of ascospore release was correlated with a rise in RH during early evening hours. Ascospore release occurred before leaf wetness was detected and was not correlated with rainfall or continuous high RH during the preceding daylight hours. Peak ascospore releases occurred 2 to 4 days after major rainfalls. Ascospore release was diminished on days with continuous RH >80% or rainfall >5 mm. Light rain during a spore release event temporarily washed ascospores from the air; however, heavy rain (>5 mm) stopped spore release. This data suggests rainfall may be needed for perithecial and ascospore formation and maturity on crop debris, but not to trigger the actual release of ascospores. Perithecial drying during the day, followed by sharp increases in RH, may provide the stimulus for release of ascospores.