Relationships among airborne Cercospora beticola conidia concentration, weather variables and cercospora leaf spot severity in sugar beet (Beta vulgaris L.)

Cercospora beticola is the most important pathogen of sugar beet in temperate climates and is the causal agent of Cercospora leaf spot (CLS). The relationships among weather variables, disease severity and the aerial concentration of C. beticola conidia were assessed in 2014 and 2015. Volumetric spore traps were deployed to quantify conidia concentration every 6h in Pain Court (PC) and Ridgetown (RT), Ontario. Mean daily air temperature (AT) and relative humidity (RH) were rarely at optimal levels for CLS development. Conidia were first detected in early May in RT both years and PC in 2015, and in mid-June in PC 2014. CLS symptoms were not observed until 75-91days after the first detection of conidia. Thus, the presence of conidia was not a limiting factor for disease development. Daily conidia concentration peaked (0.83 spores m(-3)) between 12 pm and 6 pm and was lowest (0.34 spores m(-3)) between 12 am and 6 am at all sites, providing evidence of a diurnal effect for airborne C. beticola conidia. A logistic curve explained the relationship between cumulative weekly conidia concentration and CLS severity (R(2)0.93, P<0.0001), but CLS severity associated with levels of conidia accumulation was variable among sites. Mean AT 0h before conidia capture was most influential on aerial conidia concentration compared with other weather variables (R-2=0.11, P<0.0001). Additional research is required to validate the relationship among mean daily AT, airborne C. beticola conidia and CLS epidemics.