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 i nvestigated in artificially inoculated wheat plots. Mature perithecia and ascospores appeared on corn colonized by G. zeae 2 to 3 weeks afte r being placed in the plots (mid June). Ascospores over the plots were sampled with a Burkard continuous 7-day spore sampler. Temperature, r elative humidity (RH), leaf wetness, and rainfall were also recorded i n the plots on an hourly basis. Ascospores were released during the fi rst three weeks of July in 1992 and 1993, with hourly concentrations o f 600 to 9,000 ascospores/m(3). Ascospore release typically showed a d iurnal pattern. Release began around 1600 to 1800 hours, reached a pea k usually before midnight, and declined to low levels by 0900 hours th e following morning. The beginning of ascospore release was correlated with a rise in RH during early evening hours. Ascospore release occur red before leaf wetness was detected and was not correlated with rainf all or continuous high RH during the preceding daylight hours. Peak as cospore 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 ascosp ores from the air; however, heavy rain (>5 mm) stopped spore release. This data suggests rainfall may be needed for perithecial and ascospor e 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 ascosp ores.