Prevalence and Incidence of Sunflower Downy Mildew in North Dakota Between 2001 and 2011

Sunflower downy mildew, caused by Plasmopara halstedii, can be an economic problem in North Dakota, where approximately half of the United States sunflower crop is grown. Prevalence and incidence of downy mildew in North Dakota was assessed in mid-season and late-season surveys annually from 2001 to 2011. In aggregate, 2,772 fields were evaluated. Downy mildew levels fluctuated annually, and no clear connection between statewide rainfall or management tools available to growers could be made. Surveys at mid season enabled a two-fold higher detection in prevalence and incidence of downy mildew than in late-season surveys due to the disappearance of early infected plants by end of season. Both surveys identified years with relatively higher levels of downy mildew but were inconsistent when lower levels of the pathogen were recorded. The results of this study suggest that mid-season assessment of downy mildew more accurately estimates disease pressure than late-season assessment. Accepted for publication 3 February 2013. Published 22 May 2013.