Control strategies with reduced fungicide input for Botrytis leaf blight in lily--a simulation analysis

Leaf blight, caused by the pathogen Botrytis elliptica (Berk.) Cooke, is the most important foliar disease in ornamental lily cultivation. The conventional control strategy of Dutch bulb growers is to apply protective fungicides at weekly intervals from crop emergence until harvest. We applied simulation models to complement experimental studies in the search for control strategies with less fungicide input at minimum yield loss. Effects of cultural measures that reduce primary infection, were investigated across 5 years of historic weather assuming season-specific influx patterns of only primary inoculum. The results indicate that reduction of influx of external primary inoculum by cultural measures is a necessary basis of an effective strategy to control fire disease in lilies. A modest degree of spatial isolation, i.c. maintaining a distance of at least 10 m to fields where lilies were grown in the previous year, was much more effective than post-harvest measures aiming at reduced production or survival of sclerotia. Results showed that when cultural measures are combined with a relatively resistant cultivar, non-chemical control may well be a feasible option. Different chemical control strategies were evaluated across combinations of 5 years of historic weather and inoculum influx levels that had been derived in a previous study. Simulation results indicate that fungicide input can be strongly reduced, even with a highly susceptible cultivar and under a scenario of high disease pressure. Under such high-risk conditions, fungicides with a high initial and residual efficacy and weather forecasts of better-than-actual quality appear to be the prerequisites to make a warning system-based control strategy as reliable as the strategy with weekly applications of a low-cost fungicide. The conclusions concerning the strong beneficial effect of modest degrees of spatial isolation rely critically on our assumptions with regard to the dispersal distance of conidiospores. We therefore recommend that field studies on dispersal of B. elliptica spores be conducted, whereas system-level experiments are required to validate the conclusions.