Comparing surface wetness inside and outside grape canopies for region-wide assessment of plant disease risk.

Wetness duration measured by fl at plate sensors inside and outside a grape canopy was recorded from December-March. Sensors outside the canopy generally recorded longer wetness duration than sensors inside the canopy. For days with rain, short wetness durations detected by outside sensors were not detected by inside sensors because of sheltering by the canopy. When wetness arose solely from dew, duration inside was much shorter than outside for prolonged wet periods. Wetness was used to calculate infection periods according to two botrytis bunch rot risk models. Agreement between sensors was worse inside the canopy than outside, although on occasions when rainfall exceeded 10 mm there was greater uniformity between sensors. For region-wide disease risk monitoring, wetness duration measured outside leaf canopies at standard meteorological sites would give a “worst-case” estimate of disease risk. Regression equations are presented that allow estimation of inside wetness duration from outside wetness duration for rainy and non-rainy days.