Factors affecting concentrations of airborne conidia of Oidium sp among poinsettias in a greenhouse

Atmospheric concentrations of Oidium sp. conidia in two research greenhouse s containing infected poinsettias were monitored to investigate the role of environment in prompting conidial release and dissemination. Hourly concen trations of conidia of Oidium sp. were estimated using a Burkard volumetric spore sampler. The influence of temperature on disease development was stu died by placing healthy poinsettias in each greenhouse for 7-day periods, r emoving them, and recording the days to the appearance of the first colony. When averaged over 5 December to 1 June, atmospheric conidial concentratio ns in greenhouse (GH) 2 were greatest during 1000 to 1800 hours with a peak (325 conidia/m(3)h) occurring at 1200 hours. In GH 11, peak concentrations occurred at 1300 hours (65 conidia/m(3)/h) and 1600 hours (75 conidia/m3/h ). Large numbers of conidia were sampled (greater than or equal to 100/m(3) ) within I-h periods, indicating conidial release events (CREs). Fluctuatio ns in relative humidity (RH) (either positive or negative) prompted CREs. I n both greenhouses, the highest number of CREs (up to 23) occurred followin g RH fluctuations of 5 to 15%. Watering resulted in an immediate increase ( less than or equal to 25%) followed by a rapid decrease in RH (less than or equal to 32%) beginning 1 to 2 h later. In GH 2 and GH 11, 89 and 48%, res pectively, of the CREs occurred within 3 h following greenhouse watering. W hen greenhouse temperatures exceeded 25 degrees C for 21 days in May (GH 2) and 19 days in March (GH 11), atmospheric conidial concentrations were red uced 80 and 75% from the previous months, respectively.