30 years of polyetic development of the polycyclic onion disease Botrytis Leaf Blight
Hervé Van Der Heyden: McGill University
<div>Despite our deep understanding of cultural, genetic and chemical control, epidemics of plant diseases are still responsible for significant losses amongst several agrosystems. For diseases of annual crops the natural time scale for the study of an epidemic is often considered to be a single growing season. However, the temporal framework for their study should encompass several scales from within-field to farms and regions and between seasons. In Québec, onions are grown on more than 2 000 hectares of land. This production is threatened by several diseases including Botrytis Leaf Blight caused by <em>Botrytis squamosa,</em> a well-described endemic polycyclic disease. This disease was used to investigate the general hypothesis that final disease severity in a given field and a given season influences disease onset and disease progress rate in neighboring fields the following seasons. Disease severity, collected once a week over the past 30 years in about 120 georeferenced fields was used to characterize spatial and temporal heterogeneity at different hierarchical scales: from within field to regional level. In a first step, Gompertz, logistic and sigmoid models were fitted to the data to characterize individual disease progress curves. Secondly, seasonal spatio-temporal variations were characterized by adjusting Taylor’s power law on raw data as well as on the parameter estimates of the disease progress curves. Finally, the presence of autocorrelation between growing seasons was estimated using a Box-Jenkins approach. Data analysis suggests that at the regional scale, a reduction of disease intensity toward the end of a season influences disease onset the following year.</div>
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