Asian rust: drought, wind patterns, and alarming models

Back in January, rumors began circulating that Asian soybean rust (ASR) had been found in Mexico. While he found the (now-proven) rumors troubling, Rick Cartwright pointed out that if the rust had found a home downwind in the Delta’s prevailing wind pattern, it wouldn’t be a first.

Last November, Tom Isakeit, a Texas A&M plant pathologist, found ASR on a patch of Dayton-area kudzu in southeast Texas’ Liberty County. For Cartwright, an Arkansas Extension plant pathologist, that was worry enough.

“Obviously, if ASR overwinters to our southwest, our early soybeans will be under greater risk for seeing ASR spores. They’ll hit earlier and in higher numbers than if it overwinters only in Florida.”

Now, on the cusp of a new growing season, Cartwright is watching scouting reports closely. “Maps on the USDA Web site ( are very interesting. Go back and looked at where ASR was found in late March 2005. After it had overwintered for the first time it was found in just a few isolated sites in west Florida. This year, it’s overwintered in several states and multiple counties. The magnitude of ASR at the same date a year apart is markedly worse. For many folks, I think that raises a red flag.”

Wherever ASR has been found overwintering, it has been destroyed. However, “we’re probably just finding a fraction of what’s overwintering.”

The rust creeps

As a whole, 2005 was a bust for Arkansas soybean diseases. With the exception of several soil-borne maladies, it was too dry for most diseases to get a foothold.

“We did have some nematode trouble and lots of charcoal rot — things to be expected in drought years,” said Cartwright. “There was also some SDS (sudden death syndrome) in some irrigated areas.”

As for foliar diseases, having entered the country on the back of Hurricane Ivan in the fall of 2004, ASR was the newest, and most feared, concern. That concern, it turned out, was most justified in the Southeast. After overwintering in Florida, ASR began the year creeping slowly northward: first into the state’s panhandle area before spreading into southern Alabama and Georgia.

“The rust still hadn’t done a lot by the end of July,” said Cartwright. “That’s even in the face of some considerable rains in July.”

But the disease picked up steam. By the end of November, ASR had moved throughout Georgia (the hardest hit state), Alabama and the Carolinas.

“It got as far north as Kentucky and as far west as Texas. It was also found in a few Mississippi sentinel plots and just east of Baton Rouge, La.”

For Arkansas, “the big news is it moved further west than we’d seen in 2004.”

The drought barrier

During winter production meetings, Cartwright would flash an outline of the United States onto the screen. Overlaying the map was a geyser of colors representing the likely path of ASR spores during 2005. The South was heavily dosed with color, and the spore trails reached far into northern soybean-growing areas.

“This shows spores probably traveled much more widely than we were able to pick up on plants as disease symptoms. The model says they moved all around the Delta states.”

So why didn’t an ASR problem erupt in the Delta? Simple, say researchers: there was record, or near-record, drought from Texas through the Delta to the central Midwest and into the Ohio Valley. Cartwright believes that curve of drought hemmed ASR into the Southeast where rainfall was at a more normal level.

“That’s why it was only seen there. If we’d had a rainfall year like in 2004, given the mobility of the spores, there’s no telling how far north ASR would have reached — certainly, further than Kentucky.”

Now, following an “awfully mild” winter, conditions remain on the dry side. “That probably favors not having a huge ASR problem here. On the other hand, ASR has overwintered in so many more locations — including to our southwest — that it’s more threatening.”

During the first week of April, stripe rust began to fire up in Arkansas wheat. “That’s an indicator spores are moving around, perhaps with ASR spores mixed in. Who knows? With so many early soybeans planted — some up a good while — it will be interesting to see how the spring plays out.”

Not Brazil

In 2005, “we learned conditions in the United States aren’t as favorable for ASR as those in Brazil. We suspected that already but saw it in action. When it’s a drought year, especially, the rust doesn’t appear to be a major issue for our soybean crop. Brazil has more rain, and more rain consistently, during their growing seasons than we do.”

Fungicides will be the best option to combat ASR. However, even though fungicides proved capable of dealing with the disease, “after what was seen in the Southeast last year, there’s cause for both concern and confidence.”

As for concern, Cartwright points to yields in treated versus untreated plots. In several locations, “there wasn’t a significant yield enhancement from fungicide use even though ASR was present.”

ASR fizzled in the face of dry conditions, said Cartwright. That meant there wasn’t a lot of activity from the fungicides.

On the flipside, in several locations (one in Florida and one in Georgia) “there was absolutely significant yield help from fungicides. In the Georgia plot, where it stayed wet and the ASR kept progressing, there was as much as a 20-bushel yield increase from the fungicide use.”

The largest yield increases were in a two fungicide application program. The single application did a bit better than untreated “but not as much as was needed.

“Nevertheless, this information shows that with good research we can get to the point where we don’t have to have as much fungicide as we once thought. And in some cases with early varieties, we may be able to get away with single applications. We can’t count on that, though. The first application, timed before ASR gets cooking, is critical”

Compared to other treatments, tests conducted in the Southeast have shown “powerful triazole fungicides like tetraconazole (Domark) and tebuconazole (Folicur) to be the most effective against ASR. That isn’t to say other products won’t work, but the triazoles seem best.”

Researchers also learned that tebuconazole fungicides, “while effective on ASR, can be phytotoxic to soybean leaves. We don’t yet understand why that happens. We also don’t know the potential impact on yield. It probably won’t hurt the crop, but there are hints it could in certain circumstances. Late in the season, fungicides can also cause some leaf retention. Usually that isn’t a problem.”

Producers have also heard “a lot about adjuvants and crop oil. That aspect has been poorly researched in the United States. There’s already some contradictory information about the crop oil. So that will have to be resolved through our own research programs.”

Traps and more

Last growing season, lab work in the Arkansas spore trap project — done in conjunction with 13 other states and Syngenta — was overseen by John Rupe, a plant pathologist with the University of Arkansas in Fayetteville. Placed in strategic locations around the state, the traps use lubricant-smeared glass slides to catch wind-borne spores. Rupe studies the slides under a microscope. If he finds spores, producers are quickly informed.

“It allows more advanced warning than just finding rust in sentinel plots. The weakness of the traps is the spores can’t be identified positively. They may look like ASR spores, but there are several things that look like ASR spores. It’s a labor-intensive and costly undertaking.”

Based on 2005, it appears that the Southeast’s early-planted, early-maturing soybeans will carry ASR to the later crop, not kudzu. Prior to last season, many were concerned kudzu — an alternate host for the rust — would be the chief conduit to the later bean crop. But ASR doesn’t appear to sporulate well on kudzu, “which seems to be a survival plant for ASR. Soybeans, on the other hand are the ‘epidemic’ plant. For economic reasons, we plant soybeans early in the South. If they get infected and the weather is favorable, I think they’ll be the thing that drives a nationwide ASR epidemic.”

More soybeans

If early planting intentions for 2006 hold, there will be many more soybeans planted in the South. Couple that with the potential for ASR to ride in from the Southwest and both Delta and Midwest producers should be “nervous. In all the models I’ve seen, the spores-out-of-the-Southwest scenario is the most dangerous for us.”

It doesn’t comfort Cartwright much to know ASR reacts differently here than in Brazil. “In the United States, weather conditions will be critical for ASR. That will actually make tackling this disease more difficult, not less. Every year will be a different battle, a different war to fight.”

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