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ARS Home » Plains Area » Fargo, North Dakota » Edward T. Schafer Agricultural Research Center » Sunflower and Plant Biology Research » Research » Publications at this Location » Publication #194682
Title: THE DOWNY MILDEW SITUATION ON THE 2005 NORTH DAKOTA SUNFLOWER CROP

Author
item Gulya Jr, Thomas

Submitted to: Proceedings Sunflower Research Workshop
Publication Type: Proceedings
Publication Acceptance Date: 3/24/2006
Publication Date: 3/24/2006
Citation: Gulya Jr, T.J. 2006. The downy mildew situation on the 2005 North Dakota sunflower crop. 28th Sunflower Research Workshop, January 11-12, 2006, Fargo, ND. http://www.sunflowernsa.com/research/research-workshop/documents/Gulya_Mildew_06.pdf

Interpretive Summary: The sunflower downy mildew fungus exists as many different races, and sunflower breeders use single genes to give sunflower hybrids resistance to individual races. Thus it is important for breeders to know which races exist in a region, and the relative proportion of the different races. Based on samples collected in 2005 from North Dakota sunflower fields, five races of the fungus were identified, and two of those races made up 75% of the total number of samples. Sunflower lines developed by the USDA, and released in 1985, continue to be resistant to all North American races, including the five identified in 2005. Thus, commercial hybrids using the genes in these USDA lines will be completely resistant to this disease. The incidence of sunflower downy mildew was extremely high in 2005, affecting nearly 5% of the ND crop and found in ~50% of all fields. This high incidence was a result of higher than normal rainfall, less than total disease control by seed fungicide treatments, and the fact that only a portion of sunflower hybrids marketed have downy mildew resistance.

Technical Abstract: Downy mildew of sunflower, caused by Plasmopara halstedii, was widespread and of economic impact on the 2005 sunflower crop in North Dakota (ND). Based on a mid-season survey by crop scouts from North Dakota State University, downy mildew was observed in 49% of 79 fields and affected 5.8% of the plants. During the annual NSA-sponsored fall survey in September, 2005, downy mildew was noted in 52% of 73 fields, with an average incidence of 4.9%. Late season surveys generally underestimate the impact of early season diseases like downy mildew, but this was not the case in 2005. Rainfall in June across North Dakota was 200 to 300% higher than normal and provided the waterlogged soils conducive for root infection which leads to systemic symptoms. While a portion of the seed was treated with azoxystrobin to combat the widespread metalaxyl-resistant strain of P. halstedii, the lower than optimal rate of fungicide used may have retarded infection rather than preventing it. With higher rates of azoxystrobin likely to be approved for 2006, more complete control of downy mildew is probable. Race identification of ND isolates of P. halstedii demonstrated that five races comprise the entire fungal population, with race 730 and 770 making up ~75% of the total. In contrast, the sunflower rust (Puccinia helianthi) population is currently comprised of 25 races. Commercial hybrids using USDA-ARS lines HA 335 and HA 336 (Pl6), HA 337, HA 338, and HA 339 (Pl7), or RHA 340 (Pl8) continue to be resistant against all P. halstedii races known to exist in North America.