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Asian soybean rust: better all around

Better chance of infection, but better prepared.

Far from being complacent when it comes to Asian soybean rust, the Ontario soybean sector is said to be more prepared for the entrance of this disease than any other pest or condition, ever. Granted, growers, agronomists, researchers and extension personnel from the Gulf States to Ridgetown have had more than three years to take stock and ready themselves for its arrival. But its detection in Ontario in late October 2007 confirmed its appearance was never a matter of ‘if’ but ‘when’.

Soybean fields are likely to be a haven for a growing number of diseases and pests in 2008, meaning scouting will be more important.

And now that it has arrived, the key question is ‘How bad will Asian soybean rust turn out to be?’ For Albert Tenuta, there is no time like the present, literally. The 2007 detection of soybean rust in Ontario was certainly a wake-up call for many in the soybean sector. However, it came at a time when there was almost no risk of any damage to the provincial soybean crop. But such a near miss may be a portent of things to come in 2008. Which means, the time is now to study and determine the best course of action before the disease has a chance to inflict greater damage on the Ontario soybean crop.

“Determining Ontario’s risk in 2008 depends on a number of factors and this process has already begun. The winter and early spring/summer conditions in the southern US and Mexico are critical,” explains Tenuta, field crops pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs. On a positive note, a cold snap in Florida and Louisiana in early January 2008 caused defoliation in kudzu plants, the over-wintering host for soybean rust. “The kudzu is very important in determining risk factors because if it maintains its leaves through winter with rust infection, this potentially increases the inoculum load going into the spring.”

Steady progression north
The 2007 growing season actually marked the third year that Asian soybean rust has advanced farther north than the previous year’s infection. In 2005, rust infections were limited primarily to the Gulf States, only to migrate north through Mississippi and Missouri into Kentucky and west-central Indiana in 2006. Although the disease’s progress has been slowed early each year by a continued drought in the southeastern US, Tenuta confirms the number of states and locations within each state have increased year by year.

“The number of locations is probably quite a bit higher than what was determined because once a region has soybean rust, then the levels or intensity of scouting may decrease,” he says. “In 2007, we saw more of a western expansion into Kansas, Nebraska, northern Iowa almost up to the Minnesota border, and of course, into Ontario.”

Tenuta and his Agriculture and Agri-Food Canada colleagues in Ottawa and Harrow expect to be using sentinel plots, spore traps and computer models to pinpoint and fine-tune the scouting and detection methods in 2008, especially since unseasonable cold in the southern US is no guarantee against earlier infection. “They had those cold temperatures during the first week of January where the citrus crop in the southern US was at risk, and there were a number of leaves on the kudzu that had defoliated,” he reports. “But during the past few weeks, we’ve been getting e-mails from people in Texas and other locations where there’s kudzu that survived the frost and have soybean rust on them, as well.”

That becomes worrisome as winter turns to spring, particularly if weather conditions return to what is considered normal, and rust spores that have remained viable move northward with early spring rains. In that case, it could push the potential for infection in Ontario to July and August instead of October. “In 2007, it was an early April frost that took a lot of kudzu away which delayed the disease’s development,” says Tenuta. “At that point, we didn’t find soybean rust spores in Ontario until June 28. But in 2008, if kudzu is defoliating with a January frost and we’re starting to see the rust re-establish itself, that provides us with two or three months of potential infection.”

On the other hand, another late winter frost would provide some control, as well.

More control agents coming
When Asian soybean rust first emerged as a potential problem for Ontario, Tenuta calmed fears of widespread damage, noting that with diligent scouting and the original four chemistries provided, rust was a controllable disease. Folicur (tebuconazole), Tilt (propiconazole), Headline (pyraclostrobin) and Quadris (azoxystrobin) were the first formulations registered and promoted as suitable products to control soybean rust in 2005. Folicur and Tilt were the curative and protectants, while Headline and Quadris possessed primarily protective qualities. Now, Quilt (azoxystrobin + propiconazole) is another of the pro-tectant/curative agents preventing cell membrane formation and inhibiting sporulation and germination with a wider window of control.

“Folicur is a potential for emergency use and we have other products in the pipeline that have been or will be submitted to the Pest Management Regulatory Agency (PMRA),” says Tenuta. “The PMRA has played a key role in making sure Canadian soybean producers have access to soybean rust products. In fact, many products were available, if needed, soon after the disease was detected in North America in the fall of 2004. Availability of fungicides is important since all Ontario commercial soybean varieties are susceptible to the disease and the PMRA’s involvement has been greatly appreciated by soybean stakeholders.”

“The other thing we’ve seen demonstrated during the past two or three years is the monitoring programs, the computer models, the spore tracking, have been shown to be very effective, not just in Ontario but across the North American soybean producing areas,” adds Tenuta. “We’re trying to be as proactive as possible, and take a cautionary approach to soybean rust, to reduce the risk and if we’ve done everything right, the impact should be minimal.”

For more information on Asian soybean rust, visit the Ontario Soybean Growers web site at: www.soybean.on.ca or the United States Department of Agriculture (USDA) web site dedicated to the latest on soybean rust at: www.sbrusa.net