Downy Mildew in Sunflower
The thing to keep in mind with downy mildew, as with most crop diseases, is that three conditions must be met for a disease outbreak to occur: 1) the presence of a crop host; 2) a pathogen strain or strains specific to the crop host; and 3) the right environment for infection to proliferate – if there is enough soil moisture for seeds to germinate, but not excessive rain, seedlings will escape infection even if the DM fungus is present.
Last year’s excessively wet and cool spring growing conditions were ideal for downy mildew in some parts of the Northern Plains. The occurrence and severity of systemic DM infection is dependent upon water-logged soils occurring soon after planting, which facilitates mildew spore movement through the soil and subsequent root infection. Rainfall totals for the month of June in parts of North Dakota and Minnesota were above average, and in some instances twice the normal monthly total.
Based upon 79 fields across the state of N.D., surveyed last summer by trained scouts under the supervision of North Dakota State University extension pathologist Marcia McMullen, downy mildew was observed in 58% of inspected fields. The average severity (% of plants showing systemic disease symptoms) was 5.8%, across all fields, or 9.9% in the affected fields. The incidence per field ranged from no plants observed to be infected (42% of fields) to a high of 49% infected plants. Yield losses will be expected to occur in fields with greater than 5% incidence, which from the survey data occurred in 25 of 79 fields, or roughly one-third of the fields.
The eastern third of N.D. had the highest incidence of DM, although affected fields were noted in all regions.
No one knows if weather conditions will be conducive for DM next spring. Growers concerned about the problem have two management choices: 1) plant hybrids resistant to DM; or 2) use a commercially-applied seed treatment which includes the fungicide Dynasty (azoxystrobin).
“If you were to ask me which is the more surefire way to control this disease, I would tell you to plant a genetically resistant hybrid,” says USDA-ARS plant pathologist Tom Gulya. He points out that about a half dozen seed companies have at least 17 sunflower hybrids resistant to DM on the market for growers in 2006 (see company summaries of hybrid products with reference to downy mildew resistant hybrids, online at www.sunflowernsa.com. Go to ‘Sunflower Magazine’ then ‘archives,’ and then ‘hybrid selection/planting.’ See the Nov 05 article, “The 2006 Sunflower Hybrids.”
Gulya says these hybrids originate from USDA-ARS germplasm, developed for resistance to all of the known races of DM. He says USDA analyzed 120 samples of DM infected sunflower plants last year, primarily from North Dakota, to identify prevalent DM races. A total of six races were present, with two races comprising more than three-quarters of all the samples. Gulya says further analysis is being conducted to assure that no new races have evolved which could overcome one or more of the resistance genes in today’s DM resistant hybrids.
Most seed companies are selling sunflower hybrids treated with Cruiser seed treatment and the option of bundled fungicide seed treatments (Dynasty, Apron XL, Maxim 4FS) to protect against early-season diseases including Pythium, Fusarium, Rhizoctonia, damping-off and seedling blight. Dynasty provides a measure of downy mildew suppression, but not total control, especially in weather conditions like last year that were perfect for downy mildew.
“I think we still got a fair amount of protection with Dynasty. Several years ago, I could go into sunflower fields and find 80% or more infection. I didn’t see that last year, and it was probably one of the worst years ever for downy mildew. So I think in general, the fungicide in many cases helped save us from a complete wreck,” says Mycogen Seeds agronomist Bruce Due.
Even suppression, estimated to be at 70% or more, would ultimately help protect sunflower stands, especially under more “normal” growing conditions. Generally, a 25% downy mildew infection level would be at the high end of disease incidence. With a population of 20,000 plants, an infection rate of 25% would impact 5,000 plants. The seed treatment at 70% control would result in 3,500 plants protected against the disease and 1,500 plants with the disease. That would be a plant stand loss of about 7%, which is could be viewed as tolerable given sunflower’s ability to compensate for early season plant loss.
Delayed or secondary infections
Last year, some plants displayed a secondary or delayed systemic infection, observed in later vegetative stages, resulting in partially stunted plants. It is possible that the delayed or secondary infections may be an anomaly, a function of the prolonged wet conditions conducive to a longer DM inoculation period that would not normally occur. NDSU extension plant pathologist Carl Bradley refers to past research to help explain what happened. Bradley writes:
“I refer to research conducted in Canada in the early 1970s (Cohen and Sackston, Canadian Journal of Botany 51:15-22 (1973)). In this research, the growing points of sunflower plants of different ages were inoculated with the downy mildew pathogen, and number of days to first symptoms, as well as % plants with systemic infections were measured. Results of this study indicated that plants inoculated at the 8-leaf stage could develop systemic downy mildew symptoms. In these cases (50% of the plants), the average time for symptoms to first appear was 3 weeks. These plants were in or near the flower bud stage when systemic infections became apparent, and only middle and upper leaves were affected. Why don’t we see this more often? My guess is that the amount of secondary inoculum produced by the early-infected systemic plants was very large due to the high levels of infection within fields this past year. With a large amount of secondary inoculum present, there would be a better chance for late-season systemic infections to occur.” – Tracy Sayler
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