USDA researcher lists challenge of soybean rust

Often called the “guru” of Asian soybean rust research in the United States, Monte Miles is one of only a handful of American researchers actively working on the disease. When reports of suspected rust in Louisiana surfaced in early November, Miles was one of the first responders.

“At this point, wherever the disease shows up in the United States, I'm likely to be there,” said the USDA-ARS research plant pathologist.

Although based in Urbana, Ill., Miles' research program encompasses sites in Asia, South Africa, and South America, requiring frequent travel abroad.

Today, though, he's driving down a graveled backwoods road on his way to check another Mississippi soybean field. Three days prior, Asian soybean rust was confirmed in a field south of Natchez, Miss. Additional checking, said Miles, is critical to help determine how far the fungal disease has spread throughout the South.

In between field stops, he spoke with Delta Farm Press about the recent “rust tours” in Louisiana, fungicide use and a proper perspective in dealing with the disease. Among his comments:

Q. Have you found anything suspicious in Mississippi beyond the field near Natchez?

A. I was in that field. Other than that one instance, though, I've seen nothing that jumps out and said it's soybean rust. That's not odd since it's very difficult to find. This late in the season we have a lot of other diseases and insects on the plants. This rust is tough to find under normal circumstances, but late in the season it's even harder.

Q. Can you describe how well the various entities involved in the Louisiana rust search worked together? Is there anything that needs to be improved?

A. Things have been run extremely well. The Louisiana Department of Agriculture, the Extension Service and LSU had one of the best run processes I could have asked for.

The survey teams were taken right to fields. The teams didn't waste any time searching for soybeans — the Extension people knew right where to take them. Four teams were able to cover a 10,000 square-mile area in one day. It was very efficient. The same can be said of Mississippi's setup.

Q. I assume you've traveled extensively in South America. Can you describe the situation with rust there and how it might relate to soybeans here?

A. Brazil has had the disease since 2001-02. It has spread rapidly. The first year it was found, there was a lot of denial. The farmers who didn't react lost significantly: 40 percent to 60 percent yield losses weren't uncommon. I've also heard that there have been 100 percent yield losses in unsprayed fields there.

This disease can cause pods to drop off plants. It's one of the most destructive foliar diseases of soybeans we know of. It will move through a field almost like wildfire. It has a very rapid cycle time: a week to two weeks from one rash of infection to the next. It defoliates the plant from the bottom up.

Q. Can you describe walking into some of these fields? What image does this disease produce?

A. The problem is you can't tell the disease is there until later in the season. From the roadside, the fields look perfectly green and healthy. But if you look at the plant bottoms, you can find rust in the lower canopy.

As the disease progresses, the plants turn yellow and then defoliate terribly. A 110-day bean may be completely defoliated at 90 days. It just depends on how fast the disease is capable of moving in your environment.

Q. What are the most logical scenarios that could play out with this rust in the United States?

A. The scenarios we have are extremes. First, we could have nothing happen next year because the disease can't overwinter here and no spores are blown in from the Caribbean. Or, on the other side of the coin, we could have a severe epidemic.

We've never seen what this disease will do in our environment. We just don't know what to expect. There are plenty of predictive models and lots of opinions, but no one really knows. You can take any scenario and run with it — and some are.

One thing to stress: the sky is not falling. This disease can be managed through scouting, predictive forecasts based on weather and the use of fungicides. As long as fungicides are available and producers use them well, this disease can be managed.

Q. What about the monitoring system to track this disease? How important is it?

A. Where producers are using early-planted/early-maturing soybeans, a monitoring system is critical — for all soybean producers.

In the Midwest, spores will come in on the wind. In Mid-South states, though, the level and time of infection will be critical in applying fungicides. The fungicides we have are most active if applied protectively before disease appears in the field.

Most of the chemicals in the arsenal are only effective if applied protectively. For instance, there are very few curative triazole products. If you wait until you've got disease in your field, you're limited to two or three products. And these few products aren't the best choices if you're also trying to manage diseases other than soybean rust.

Producers need to learn the differences between the fungicides available. They aren't the same. Farmers need to know which products are protective, which are curative, when to apply them and in what rotation.

(Editor's note: in an upcoming issue, Delta Farm Press will address fungicide use in detail).

Strobilurin and triazole fungicides both have single-site modes of action. If used alone, you're allowed one use per season. However, if you use a mixture of the chemistries, you can use them twice.

If you're going to have a spray program and have to plan for two applications, what will you use for your first product? What will you use for your second? These are critical issues that farmers must study to come up with decisions that best fit their fields.

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