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Questions after first year of Asian soybean rust

Asian soybean rust might seem to have been just a flash in a pan after its first full season in the United States. But, before growers get too laid back, consider this: It took three years for Asian rust to become widespread in Brazil.

Soybean rust made only a few appearances in the Mid-South — in one county in Louisiana, two counties in Mississippi, one county in Texas and in kudzu in Kentucky — in 2005. The disease popped up more frequently in Alabama, Florida, Georgia and the Carolinas, but Mid-South growers mostly dodged a bullet last year.

Farmers may not be as fortunate in 2006 or 2007, says Randy Myers, fungicide product manager with Bayer Crop Science in Research Triangle Park, N.C., who has made numerous trips to Brazil to look at soybean rust.

“When we were trying to prepare for the disease in the United States last spring, I think we tended to overlook the fact that Brazil took three years for soybean rust to become widespread,” he said. “I think we were so focused on trying to make sure growers would avoid a disaster, we didn’t consider that it might be almost a non-event.”

“It turned out to be much less of a problem than we thought it might be,” says Sam Atwell, technical service specialist with BASF. “My concern is that growers will relax next year or the next, and we’ll have another hurricane blow in here, and farmers will get hurt.”

Everyone’s expectations for Asian soybean rust were high last spring, said Jamie Nielson, product manager for Valent’s Domark fungicide.

“When you go back to the Disease Triangle we’ve talked about, we had low disease pressure and dry conditions, but we did have the host. I believe had we had a normal environmental year, similar to what we saw in August and September, a number of states would have been affected earlier.”

“Based on Brazil’s experience with soybean rust, we went into 2005 knowing that anything could happen,” said Dave Ouimette, product development manager for Dow AgroSciences. “To a certain extent, we weren’t too surprised about the limited impact of rust, based on the timing of disease confirmation in November 2004, followed by unseasonably cold weather in the south, which we believe helped minimize over-wintering potential.”

The experience of farmers in Brazil and other countries indicates that growers must remain on alert for outbreaks of the disease even if it caused little, if any, damage to U.S. soybeans in 2005.

“Although conditions in the United States are different than those in Brazil, it might be helpful to understand what happened there,” said Michael Vanausdeln, who works for Syngenta Crop Protection. “Brazil suffered hundreds of millions of dollars in damage when the disease hit more strongly in the second and third seasons.

“However, since then, more than 90 percent of all soybean fields in Brazil are treated, and Brazil recently produced a record crop.”

Vanausdeln and others believe that U.S. soybean farmers helped their cause by spraying fungicides preventively where rust did appear in 2005. “They also were able to increase crop yields, although detailed numbers are still being analyzed.”

While growers’ fears of a serious soybean rust outbreak didn’t materialize, thankfully, the disease did show flashes of what it was capable of doing, according to Vanausdeln.

“Rust has appeared as far north as Kentucky and as far west as Texas, although, in most cases, the infection came too late in the season to affect the crop. The spread of rust is affected so much by weather that it is hard to predict where and when it will show up.”

But the localized spread of soybean rust was less aggressive than fungicide specialists expected. “We all anticipated more spore development, movement and activity from kudzu,” says Dow’s Ouimette. “We now know that kudzu is apparently not as susceptible to rust as soybeans are and, as such, probably contributed less to disease development than previously predicted.”

Some soybean pathologists also believe the spread of rust may be slower in areas of mixed crops such as in the South where soybeans may be growing near peanuts, cotton and other non-host crops, he said. Weather patterns also appear to have kept the disease from moving much beyond the Southeast.

Myers said he still believes farmers were fortunate that Asian soybean rust came in at the end of the 2004 season, giving fungicide manufacturers time to generate enough product to prepare for the disease.

And, although some criticized the industry early on for not responding with enough fungicide last spring, those fears proved to be groundless, he said. “A tremendous amount of product was made available to soybean growers.”

U.S. manufacturers had little choice in the matter, he says. “We were looking at a situation where farmers could have lost millions of acres of soybeans. It made it difficult for us to take a moderate approach.”

The low incidence of Asian soybean rust in the first year may not have helped specialists learn much about how the disease will perform over a wide area of the U.S. Soybean Belt once it becomes widely established.

“One big question concerns the issue of rust in determinant vs. indeterminant soybean varieties,” says Myers. “That is, if you have the bottom portion of an indeterminant variety protected with a fungicide, will you need to treat it again as it continues to put on pods and new leaves at the top of the plant?”

Most of the soybeans grown in Brazil and northern Argentina are determinant varieties, while the majority of U.S. soybeans are indeterminant; i.e., they continue to set pods until environmental conditions are no longer favorable.

“I thought that next year we would be a lot smarter about some of these issues, but it may be the following year before we get more answers.”

Another issue: the impact of ultraviolet light on rust spores. And there’s the impact of wind currents in the United States on rust spores that are relatively small compared to something like wheat rust spores.

The fungicide manufacturers see little need to change their companies’ recommendations for dealing with the disease in 2006.

“Our experience both in the United States and Latin America suggests that the recommendations we’ve made and the product line we offer is still the most effective way to protect against rust,” says Syngenta’s Vanausdeln. “We should remember, however, that recommendations on spraying are determined by individual situations, considering potential for infection, climate conditions, the time of year and any residual control of the fungicide.”

“In regards to rust, I think the industry and Valent had an accurate plan in place,” said Nielson. “I do not feel that we need to make any changes to the fungicide recommendation for Domark. It should still be applied prior to rust infestation and at the R-1 stage for maximum protection. The one key learning was that Domark helps protect new growth, which is important when soybeans continue to add the additional trifoliate.”

“We know that the triazole fungicides have curative activity, but are actually the most efficacious when used early as a preventive application,” said Ouimette. “We still recommend that the first application be with a triazole-based product at R-1 or the appropriate growth stage if the disease is present or an imminent threat.

“The second application should be primarily aimed at traditional late season diseases but also to help manage any continuing rust conditions,” he noted.

Myers said he and other fungicide experts were impressed with the “excellent cooperation” between USDA, the Extension Services and land grant universities on sentinel plots and other early warning strategies.

“We’ve never seen that level of cooperation before,” he said. “The specialists put in a tremendous number of hours to put together programs to educate growers about Asian soybean rust and to make sure they knew about rust before it got to their fields.

“And growers are looking at their fields much closer than they were before soybean rust arrived in this country. That has to pay off in better production practices and higher yields.”

“The general consensus is that 2006 could be more active for rust since we know it took at least a year to start causing major problems once introduced into South America,” said Ouimette. “USDA is renewing and expanding its sentinel plot program, and university specialists will continue to intensely monitor conditions for disease development in their respective states and provide the appropriate recommendations to growers.

“The tools are certainly in place to help ensure growers make the appropriate decision for their crop.”

“If you look back at Brazil as the example, many growers became complacent and then had significant losses in yields,” said Nielson. “With nitrogen prices being very high and affecting corn margins, growers will need to make sure they are protecting their soybeans to maximize profits.”


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