NDSU Extension - Mercer County


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Conditions Are Favorable For Sunflower and Dry Bean Rust

sunflower rust, dry edible bean rust

Submitted by Craig Askim, Extension Agent, Agriculture and Natural Resources   

Heavy and/or frequent dews and warm temperatures provide a favorable environment for both sunflower and dry bean rust. We have recently received (and/or confirmed) reports of rust on sunflowers and dry beans. The two rust diseases are caused by two different pathogens (which are specific to their crop), but the ways they look, infect, spread and are managed are similar.

Signs and Symptoms. Rust is usually first observed as dusty cinnamon-brown pustules on leaf tissues. The spores (urediniospores) can be rubbed off the tissue easily, leaving a dusty brown streak. Pustules on the upper sides of the leaves will sometimes have a small yellow halo around them. Commonly, rust is easier to find on the undersides of the leaves because they have more rust pustules.

When a rust epidemic begins, symptoms usually are only found on the lower leaves and in ‘hot spots’, which are clusters of plants with relatively severe damage. Hot spots are often small and may be a few feet to several yards in diameter. Hot spots can occur anywhere in a field, but are more common near shelter belts (where plants have dew longer), near last year’s crop residue (the sunflower and dry edible bean rust pathogens overwinter in North Dakota), or near wild sunflowers (sunflower rust only).

Spread. Rust spores are easily and quickly dispersed by wind. Once rust is established and reaches the upper canopy, it will spread very quickly through a field and to nearby fields. If conditions remain favorable for infection, a rust spore can be dispersed, cause an infection and produce a new pustule with spores in as little as 7-10 days. A ‘hot spot’ can turn into an epidemic very quickly.

How do you manage sunflower rust? If rust reaches approximately 1% severity on the upper four leaves at or before bloom (R5) yield loss can occur. If that level of severity is reached at or before sunflower bloom, a fungicide application should be considered. Triazoles (Folicur, etc…) and Strobilurins (Headline, Quadris, Aproach) chemicals are among the most effective on rust. At R6 or later (after bloom) fungicide applications have not had an impact on yield in our trials. For severity guides and other information on sunflower rust, please consult the NDSU Extension publication on sunflower rust or visit the National Sunflower Association website [sunflowernsa.com] and search for rust.

How do you manage dry edible bean rust? Dry edible bean rust can cause significant yield loss when the disease occurs early in the growing season and conditions remain conducive for infection and spread. If bean rust occurs, a fungicide application should be considered. The best timing for a fungicide application to manage bean rust is shortly after it is first found. Strobilurin fungicides [FRAC 11] (Aproach, Headline, Quadris, etc.) and Triazole fungicides [FRAC 3] (Folicur and generics, Proline, etc.) have consistently been the most effective in research trials. Once pintos begin to stripe (or the equivalent growth stages in other beans) management is not necessary.

For more information on dry edible bean rust visit: http://www.ag.ndsu.edu/pubs/plantsci/pests/pp1601.pdf

Source: Sam Markell - NDSU Extension Plant Pathologist

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