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Relating aerial concentration and escape of Pseudoperonospora cubensis sporangia from a cucumber canopy to disease severity during cucurbit downy mildew epidemics.
K. Neufeld (1), S. Isard (2), P. OJIAMBO (1). (1) North Carolina State University, Raleigh, NC, U.S.A.; (2) Pennsylvania State University, University Park, PA, U.S.A.

<i>Pseudoperonospora cubensis</i> causal agent of cucurbit downy mildew is disseminated aerially on a large spatial scale via asexual sporangia. A prerequisite to development of models to predict disease spread is the quantification of aerial concentration and escape of sporangia from infected fields. Sporangia concentrations, <i>C</i>, were monitored using Rotorod samplers deployed at 0.5 to 3.0 m above a infected cucumber canopy at two field sites with 1 to 40% disease severity. Mean sporangia concentration (<i>mC</i>) was higher at moderate than at low or high disease severity. Values of <i>mC</i> decreased rapidly with height above the canopy and values at 2.0 m were only 7% of those measured at 0.5 m when disease severity was moderate. No sporangia were collected at height of 3.0 m above the canopy. Escape of sporangia, <i>F</i>, varied with hour of the day and disease severity with a maximum value of 926.4 sporangia m<sup>-2</sup> s<sup>-1</sup>. Daily total flux, <i>DF</i>, was also dependent on disease severity and ranged from 5.9 to 2,242.3 sporangia m<sup>-2</sup>. The fraction of available sporangia that escaped the canopy increased from 0.028 to 0.171 as average wind speed above the canopy at high <i>C</i> increased from 1.7 to 3.6 m s<sup>-1</sup>. Variations of <i>mC</i> and <i>DF</i> with disease were well described (<i>P</i> < 0.0001) by a log-normal model with a severity threshold of 15%. Results will be incorporated in a regional-scale spore transport model currently used to predict the risk of spread of downy mildew among cucurbit fields in the eastern U.S.<p><p>Keywords: Oomycete, Vegetables

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