Soybean rust alert system fine-tuned

The unified system of monitoring Asian soybean rust sometimes sounds like a part of the Star Wars Defense System, with spore traps, sentinel plots and daily tracking updates.

The end result is a safety net for soybean rust protection that, if carefully followed, offers valuable protection for soybeans in the Southeast John Mueller, who is a plant pathologist at Clemson University's Edisto Research and Education Center, has been in the frontline battle against rust for the past three years. Though confident the monitoring system will provide ample warning for South Carolina growers, Mueller contends there is still a lot to learn about how rust spreads and exactly when and how it infects soybean plants.

With the reports of rust innoculum over-wintering in kudzu in Alabama, Georgia and Florida, most experts expected rust would appear earlier and in more severe cases than in 2005.

Mueller contends there is a lot to learn about the role of kudzu in the spread of soybean rust. “We will learn more this year about how rust moves from kudzu to soybeans and how much it takes to generate significant levels of the disease.

While there is some concern that more rust inoculum over-wintered further north than in 2005, that is not necessarily an indication that 2006 will be a bad rust year, according to Mueller.

“In 2005, we found rust in sentinel plots and expected to find rust nearby, but in most cases we didn't. However, the use of spore traps to predict the presence or spread of rust is less reliable. In many cases when spores have been found in a trap rust is never found in adjacent plots. Therefore, in South Carolina we will not recommend sprays based on spore trap results.

The expense of spraying is too high and the profit margin too small on soybeans to make mistakes spraying for rust, or any pest problems. Mueller says spray recommendations for South Carolina will be based on the presence of rust in South Carolina or adjacent areas of Georgia.

South Carolina researchers monitor 15-20 sentinel plots each week during the soybean growing season. In addition, fields of maturity Group III and Group IV soybeans in Lee County are sampled weekly by Randy Cubbage, the local county ag agent.

He sends these leaves along with leaves from his sentinel plots to Mueller on a weekly basis. Sentinel plots keep researchers 2-3 weeks ahead of growers. Last year rust did not appear in a field until soybean plants began to flower. In most cases the disease did not occur until soybean plants in these plots were in the R3 to R4, and in some cases to the R5 stage.

The system of sentinel plots is part of a national effort, coordinated by the U.S. Department of Agriculture and monitored by researchers at research and on-farm sites across the lower Southeast, from south Florida to south Texas, and in tiers from south to north on to Canada.

An immense amount of work goes into the USDA Web site (www.usda/ Researchers from throughout the Southeast monitor the movement of rust, even the detection of rust spores. Within 12 hours at the latest, and usually within minutes of finding meaningful results, information about rust is posted on the USDA website.

“This Web site is extremely accurate and timely. I post to that site first, then to my county agents and then to my growers, and I think that's typical of most of my counterparts who work with soybean diseases,” Mueller says.

Sentinel plots in South Carolina are concentrated along the Georgia border and also along the coast to prevent rust from looping in from storms coming off the ocean.

Fortunately, growers have a good tool chest of fungicides to use against rust.

Strobulurin fungicides protect soybeans against infection and triazoles are used to combat the disease after it infects soybean plants. Both should be used judiciously both for cost and possible resistance problems associated with over-use.

In South Carolina, Group VII maturing varieties are more the norm than early maturing soybeans. In sentinel plots, researchers plant maturity Groups IV and V soybeans the last two weeks in April and the first week in May. These varieties bloom earlier than the more popular, later-maturing varieties.

When Mueller's team detects rust at 3-5 percent of leaves infected, with 1 or 2 pustules per leaf, they begin the process of warning growers about possible disease outbreaks. To do this requires bringing samples in from the field and looking at them under a dissecting microscope. Growers don't have the time nor the technical expertise, to document these low levels of disease.

“The monitoring plots allow us to detect rust at extremely low levels. When we compile information across our entire network, we can make some observations as to the severity of rust — will it be light year with one spray, a heavy year with two or more sprays, or a year in which there is not need to spray. Either way, our recommendations are made on empirical data and not on guesses,” Mueller emphasizes.

Based on this information, growers can make sound business decisions whether to spray or not. If growers have a good crop of beans with a realistic yield potential of 40-50 bushels per acre, the plants are flowering, rust is reported within 25 miles of these fields, and weather forecasts called for thunderstorms daily over the next two to three weeks, a logical decision would be to spray with a strobilurin fungicide to protect these beans from rust and from other soybean diseases.

If a grower has rust in a field of soybeans, the decision is simpler, spray with a triazole fungicide or a combination fungicide containing a triazole. Triazoles can eradicate the rust present, but are not as effective as strobilurins in preventing new infections.

If the same scenario described above occurs, except weather forecasts call for no rain, Mueller contends the wise decision would be not to spray, because there is good evidence to indicate soybean rust doesn't move much in dry conditions.

In 2005 and 2006, South Carolina growers who planted Zone 4 and 5 beans in April and early May missed out on soybean rust. If that is a practical production system, it may prove to be the best way to manage the disease.

However, for most growers, soybeans are a second or third crop — often planted late, when weather or other conditions aren't conducive to plant other crops.

In 2005, a number of South Carolina growers sprayed for rust because to wait would have put them in conflict with defoliating cotton and digging peanuts.

In 2005, for the majority of the growing season in the Carolinas, rust barely moved 50 miles a week from south Florida northward. Later in the season, when it got to South Carolina, there were large acreages of Zone 7 beans in the R5 growing stage. The disease moved much quicker, apparently because there was more inoculum. From mid-September to late October, the disease spread almost to the Virginia line.

With nearly 500,000 acres of soybeans in South Carolina and over a million acres in North Carolina, the disease has a more dense population of beans on which to spread than it encounters in Florida, Alabama and Georgia.

“Our model in the Southeast may not be a good comparison for soybean rust in Brazil, nor for the Midwest,” Mueller contends. There are still plenty of things we don't know about rust that will be added to management programs each year, he adds.

With the effort being put forth from researchers nationwide to monitor soybean rust, growers should have adequate warning as to when to spray. And, with good fungicides available to provide control or prevention of the disease, growers should be able to make informed decisions on a field to field basis as to whether to spray and what to spray to manage the decision.

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