Downy Mildew
Downy Mildew (Plasmopara halstedii)
Downy mildew is common and economically important in environments with cool and wet conditions at planting. The disease is particularly common in the northern Great Plains of North America, including North Dakota, northern Minnesota and Manitoba. The disease does occur in the US High Plains and Southern Great Plains, but is less frequent. Root infection of seedlings results in a stunted, systemically infected plants, many of which die. Plants which survive will have minimal seed set. However, total yield loss in a field depends on the number of plants infected and their distribution.

The pathogen is host specific and soil-borne, and can survive many years in the soil. The disease is favored by cool and wet/waterlogged soil conditions when the seeds are germinating and seedlings are emerging. Infection begins when motile zoospores, which swim in water, infect the roots of germinating seeds. Infected seedlings may die pre- or postemergence.
Surviving plants display systemic chlorosis on the upper side of emerging leaves, which radiates outward from the petioles (Figures 1 and 2). During periods of dew or rain, fluffy white growth will appear on the underside of the leaves, opposite the chlorosis (Figure 3). This is diagnostic and distinguishes the disease from powdery mildew (with white sporulation on upper leaf surface), herbicide damage and other abiotic or biotic ailments causing chlorotic leaves.
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Figure 1. Downy mildew in systemically infect plants
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Figure 2. Downy mildew on upper side of leaf
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Figure 3. Downy mildew on underside of leaf
If plants survive through the season, they remain extremely stunted (often 6 to 24 inches tall) and form a horizontal head with limited and nonviable seed (Figure 4). While infected plants have minimal or no seed yield, adjacent healthy plants do compensate and produce larger than normal heads due to the lack of competition.  Downy mildew is frequently concentrated in low/wet spots of the field, but sporadically infected plants throughout the field are not uncommon. Areas in the field with a high concentration of infected sunflower plants often appear as if they were not seeded and quickly fill with weeds.
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Figure 4. Downy mildew mature plant
Nonsystemic downy mildew infection can occur later in the growing season and appears as discrete irregular chlorotic spots on the upper side of leaves and occasionally white growth opposite them. However, this nonsystemic infection, termed ‘local lesions’, is not economically important as it does not lead to systemic infection (Figure 5).
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Figure 5. Downy mildew as a secondary infection
 
Selection of hybrids with genetic resistance and application of a fungicide seed treatment are the only viable management tools. However, the pathogen is highly genetically variable (over 30 races known worldwide) and has adapted to (and overcome) multiple resistance genes and fungicide mode of actions. All planting seed sold in the U.S. currently has one or more fungicide seed treatments aimed at DM control. Thus, consulting the most current recommendations before selecting resistant hybrids or fungicide seed treatments is critical.
Images
Figure 1. Downy mildew in systemically infect plants (Sam Markell, NDSU)
Figure 2. Downy mildew on upper side of leaf (Sam Markell, NDSU)
Figure 3. Downy mildew on underside of leaf (Sam Markell, NDSU)
Figure 4. Downy mildew mature plant (Sam Markell, NDSU)
Figure 5. Downy mildew as a secondary infection (Sam Markell, NDSU)
Additional Resources
Results of 2014-2018 virulence (race) survey of the downy mildew pathogen (Plasmopara halstedii) https://www.sunflowernsa.com/uploads/research/1340/2019NSAForumdownymildewofsunflowerupdate.pdf

Results of 2015 fungicide seed treatment trials comparing efficacy of oxathiapiprolin, azoxystrobin and acibenzolar-S-methyl against three races of the downy mildew pathogen (Plasmopara halstedii). https://www.sunflowernsa.com/uploads/research/1295/3Humann-2016NSASyngentaposter.pdf

Results of seed treatment trials conducted between 2011 and 2015 examining the efficacy of oxathiapiprolin (now marketed as Lumisena by Corteva and Plenaris by Syngenta). https://www.sunflowernsa.com/uploads/22/oxathiapiprolin_humann.2016.pdf
Other NSA resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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