Comparison of monitoring based indicators for initiating fungicide spray programs to control Botrytis leaf blight of onion

Botrytis leaf blight (BLB) of onion is a well-documented disease and several management tools are available. However, achieving good control with minimum use of fungicides is time consuming and requires dedication on the part of growers. In this study, three indicators for the initiation of fungicide spray programs were compared. The study was conducted in 2008, 2009 and 2010 at commercial onion farms located in the muck soil area southwest of Montreal. The indicators evaluated were: 1) first airborne Botrytis squamosa conidia detected; 2) cumulative airborne conidium concentration (ACC) of 15 conidia m−3; 3) first lesion caused by B. squamosa detected. The interval between subsequent fungicide applications was determined by taking into account sporulation potential, airborne conidium concentration and lesion density. Reliability of the indicators was evaluated on the basis of observed maximum disease level, area under the disease progress curve and rate of disease progress. Initiation of the fungicide spray program when the first airborne B. squamosa conidia were detected resulted in the lowest values for maximum disease level and area under the disease progress curve, followed by initiation when a cumulative concentration of 15 conidia m−3 was reached, and by initiation when the first lesion was detected. However, there was no significant difference in rate of disease progress, calculated using data from fields where spray programs were initiated based on the different indicators. The critical disease level for curative fungicide applications (ten lesions per leaf) was not reached when the spray program was initiated upon detection of the first conidia; was reached a few days after bulb initiation for a spray program initiated at a cumulative ACC of 15 conidia m−3; and was reached prior to bulb initiation for the spray program initiated when the first lesion was detected. These results suggest that to avoid yield reduction, BLB should be managed such that the disease does not reach the exponential phase. This was achieved by initiating the fungicide spray program based on first conidia detected.