Theory 086/Food & Agriculture /Plant pathology
Disease Follows Leaf Wetness Duration
Disease forecasting counts rainfall. The pathogen counts hours of wet leaf.
Foliar disease epidemics are set by the duration of leaf wetness rather than by rainfall amount, so a season of many light dews can drive more disease than a season of fewer heavy rains.
01The evidence
Disease forecasting models based on leaf wetness duration outperform rainfall-based models in several pathosystems.
02The tension
Advisory services use rainfall because it is widely measured, which predicts infection from events and misses the long wet nights that drive epidemics.
03The mechanism
A spore needs free water and time to germinate and penetrate. Both are supplied by a wet leaf, and the duration is what determines whether the infection completes.
That makes duration the driver, and predicts that many short wettings with long dews should produce more infection than fewer heavy events that dry quickly.
04What this is not
Event models count storms. This counts hours wet, which explains epidemics in dry-looking seasons.
05Predictions
- P1Epidemics should track wetness duration.
- P2Dewy seasons should out-drive rainy ones.
- P3Duration should predict better than rainfall.
06Refutation
Epidemics tracking rainfall at controlled wetness duration would restore event models.
07What it changes
It would change disease forecasting and sensor deployment.