Major outbreaks of cucurbit downy mildew have been observed since 2005 in Michigan, the top state for pickling cucumber production in the United States. Airborne concentrations of Pseudoperonospora cubensis sporangia were monitored from 2006 to 2009 in a commercial cucurbit field in each of five Michigan counties. Temperature, relative humidity, leaf wetness, and rainfall were recorded on site in 2008 and 2009. Seasonal and daily trends for sporangial concentrations were examined using loess regression to determine when sporangia were present throughout the growing season. Airborne sporangia were detected from June through September, and concentrations increased during this period. Peak airborne sporangial concentrations occurred between 8:00 and 13:00 h. Airborne sporangial concentrations were positively correlated with temperature (r = 0.17 to 0.38, P < 0.0001) and negatively correlated with relative humidity (r = –0.16 to –0.45, P ≤ 0.0046) and leaf wetness (r = –0.14 to –0.39, P ≤ 0.0001) in 2008 and 2009 at all sites. In autoregressive moving average models, sporangial concentrations were negatively related to relative humidity (slope = –0.03 to –1.34, P < 0.0496) at three of five sites in 2008 and four of five sites in 2009. Hence, sporangia are likely to be airborne above the crop canopy during periods with high temperatures and low relative humidity and leaf wetness.
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