Medium-term trends in dark leaf and pod spot epidemics in Brassica napus and Brassica rapa in Lithuania

Abstract

Alternaria brassicae causes dark leaf and pod spot in Cruci-ferae. Epidemics of the disease were studied in the medium-term field trials of winter and spring oilseed rape (Brassica napus; WOSR, SOSR) as well as in spring turnip rape (Brassica rapa, STR) in 2000–2006 and in 2000–2003, respectively. Final disease incidence (FDI), final disease severity (FDS) and area under disease progress curve (AUDPC) values on pods were significantly affected by the cropping season (p < 0.001) and by fungicide application (p < 0.001). Disease incidence and severity were significantly higher in wet seasons (2000 and 2001 for winter, spring oilseed rape and spring turnip rape, 2004 for spring oilseed rape) than in dry (2002 and 2003) or very dry (2006) cropping seasons. The correlation between final disease severity on the leaves (middle, upper) and final disease severity on pods was moderate and highly significant (p < 0.01) in all Brassica crops. The correlation between final dark pod spot incidence and cumulative rainfall during pod development and ripening growth stages in Brassica crops (pooled data) was moderate (rs = 0.568; p < 0.05), however final disease severity and AUDPC values on pods showed strong and highly significant correlation with cumulative rainfall (rs = 0.715 and rs = 0.742, respectively; p < 0.01) and with relative humidity (rs = 0.787 and rs = 0.781, respectively; p < 0.01) during pod development and ripening growth stages. The results of this study provide means for predicting dark spot disease progression on the pods based on disease parameters on the middle and upper leaves in relation to meteorological conditions.