Release dynamics of Venturia inaequalis ascospores in Uruguay

Apple scab caused by Venturia inaequalis is the most destructive apple disease worldwide. To manage apple scab in Uruguay, protectant fungicides are applied in anticipation of each rain event during ascospore discharge period. To optimize fungicide application, we studied the beginning and the end of ascospore discharge period and its behavior during nighttime and successive rain days, in natural and artificial conditions. Results showed that V. inaequalis ascospore discharge occurs in a period of 2 months, starting in the green tip stage and ending in early November, around 40 days before the usual warning system communication. Mature ascospores are released immediately after a rain event begins, regardless whether it is day- or nighttime. During consecutive days or nights of rain, successive discharges occur during 3 and eventually 4 days, with the greatest discharge occurring in the first 2 days. This finding suggests that the fungicide applications could be suspended earlier than usual and that strategic irrigations previously to forecast could be implemented to manage apple scab. However, field trials should be conducted to further validate this finding.