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Review of Sunflower Rust (07/30/20)

We encourage growers to scout for sunflower rust before the sunflowers begin to bloom.

We encourage growers to scout for sunflower rust before the sunflowers begin to bloom.

Sunflower rust is favorable by long periods of leaf wetness (frequent dews) and moderate to warm temperatures, conditions we have recently had. The disease can cause yield loss, but can be managed with a foliar fungicide application. Confection-type sunflowers are particularly sensitive to rust, but oilseed hybrids can be susceptible to the disease as well. Yield and quality loss can be very high if an epidemic develops early in the season.

Signs and Symptoms.

Sunflower rust is specific to sunflower. Unlike the rusts on some crops (wheat, corn, etc.), the pathogen overwinters in North Dakota and does not need to blow up from the southern states. This makes its appearance a little less predictable, and is heavily dependent on weather and microclimates in the sunflower fields.  The disease is often observed first near shelter belts (where the plants have longer dew periods), near last year’s sunflower residue (where rust may overwinter) or near wild/volunteer sunflowers (also overwintering hosts). Rust pustules are cinnamon-brown and dusty, often first found on the leaves in the lower canopy (Figure 1). If rust is found in the lower canopy, it often will appear on the upper canopy later in the season. If an epidemic occurs, pustules may be found on stems, petioles and leaves.

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Management.

Timing. If rust reaches approximately 1% disease severity on the upper four fully expanded leaves at or before bloom (R5), a fungicide application should be considered (Figure 2-3, next page). At R6 or later (after bloom) fungicide applications have not had an impact on yield in our trials.

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Efficacy. The National Sunflower Association keeps a robust and searchable database of research reports and presentations on their website [www.sunflowernsa.com]. Last year, NDSU Ph.D. student Brandt Berghuis presented a three year summary of sunflower rust trial work, available with graphics and as a printed report. In short, he found that DMI fungicides [FRAC 3: Triazole] and QoI fungicides [Strobilurin: FRAC 11] and pre-mixes containing those modes of action tended to be the most effective in our trials; not all available products were tested. I encourage growers to use the NSA website when searching for data on numerous topics, and hope that is helpful if you manage rust.

 

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

 

Brandt Berghuis

Graduate Research Assistant and Ph.D. Student

NDSU Plant Pathology

 

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