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Rust on Dry Beans Being Found in North Dakota and Minnesota (07/27/17)

We have received multiple reports and photographs of common rust occuring on dry beans. In much of the dry bean production regions conditions have been very favorable for the development of rust (warm temperatures and mornings with dew) and will likely be favorable into the future.

Rust on Dry Beans Being Found in North Dakota and Minnesota

We have received multiple reports and photographs of common rust occuring on dry beans. In much of the dry bean production regions conditions have been very favorable for the development of rust (warm temperatures and mornings with dew) and will likely be favorable into the future. These conditions could allow the disease to emerge quickly. Rust can be very damaging when it occurs early if it is not managed. Fortunately, fungicides (specifically triazoles and strobilurins) can be very efficacious on rust if applied early in an epidemic. I strongly encourage dry edible beans growers to scout fields for rust and be prepared to manage rust if it is found. Below are questions and answers about rust that we hope will help you as the dry beans continue to grow.

What are the favorable conditions for rust? Free moisture on leaf tissue is critical for the development of rust. Heavy dew on plants (or fog) provide sufficient moisture for infection to occur; rain is not necessary. Moderate to warm temperatures are also favorable for rust infection and spread. Our current weather pattern in much of eastern ND and NW MN has been favorable for rust development.

In addition to favorable environmental conditions, two other factors may favor rust development this year.

 First, with the environment in some bean growing regions being relatively warm and dry, fewer fungicides for white mold might be applied. While some fungicides applied for white mold are not always the most efficacious products on rust, they help kept rust in check.

Secondly, rust occurred widely in the region last year, particularly on the North Dakota side of the river. Although it likely caused little yield loss last year, the pathogen likely overwintered in many locations.

What are the signs and symptoms? 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 (Figure 1). 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) or last year’s crop residue.

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 Rust is usually first observed as dusty cinnamon-brownmarkell.2 4 pustules on leaf tissue. The spores (urediniospores) can be rubbed off the tissue easily, leaving a dusty brown streak (Figure 2). Pustules on the upper sides of the leaves will sometimes have a small yellow halo around them (Figure 3). Commonly, rust is easier to find on the undersides of the leaves because pustules are more visible (Figure 4).

How does rust 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 fast.

 

Tip of the iceberg? When you first see rust in a field, you are only seeinga fraction of what’s actually infected; just the tip of the iceberg. Why? When a rust spore infects the leaf, there is a ‘latent’ period where the disease is actively growing in the leaf, but hasn’t yet produced a pustule. Commonly, this latent period may be 7 to 10 days long. For this reason, the rust you are actually able to see in the field is less than what is actually growing in the leaf tissue (i.e., the epidemic in your field may be worse than it looks). While this latent period phenomenon occurs in many diseases we see, it is particularly long in rusts.

 

When does a fungicide help protect yield? Critically, not every field gets rust. Even though rust was reasonably widespread last year and spores disperse long distances with wind, rust is still somewhat sporadic in nature. As such, scouting is critical. Scouting for hot spots in areas close to a previous years dry beans, in an area protected from wind and the sun (shelterbelt) or an area particularly prone to dew or fog may be the best areas to start. Although hot spots can be found anywhere in a field.

If rust is found and plants still have a long way to go before maturity, applying a fungicide is likely to help protect yield. The best timing for a fungicide application to manage rust is shortly after it is first found if you are actively socouting. Ideally, you want to apply a fungicide vary early in the epidemic, but not necessarily as a preventative. Once pintos begin to stripe (or the equivalent growth stages in other beans) management is not necessary and rust will simply help mature the crop.

What fungicides are effective? Strobilurin fungicides [FRAC 11] (Aproach, Headline, Quadris, etc.), Triazole fungicides [FRAC 3] (Tebuconazole generics, Proline, etc.) and fungicides containing one of those fungicide groups in a premix (Priaxor, Aprovia Top, ProPulse, etc.) have consistently been the most effective in our research trials. Other modes of action will offer some protection as well, but more variability exits among chemicals and among our trials.

What to expect if you apply a fungicide? As a consequence of the long latent period, once you apply a fungicide it may appear that rust continues to develop. Fungicides will not stop all the new pustules from forming if the disease is already well established in the leaves. It is more likely to limit many of the new infections that will occur in the next 14 days.

A thank you.

We appreciate those who contacted us about the appearance of rust in the region.

 

 

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

 

Julie Pasche

NDSU Dry Bean & Pulse Pathologist

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