Early planting slows soybean rust

The continued interest in early planting of Maturity Group IV beans in the upper Southeast may have a side benefit in managing soybean rust.

The indeterminate growth habit for most of these varieties generally produces a less bushy canopy that appears to be less susceptible to rust. These beans are planted and managed much like corn in the Southeast.

With corn acreage expected to jump dramatically in 2007 in the upper Southeast, there are some concerns about management of soybean rust. Though some conflicts with equipment and labor needed to produce corn, and other crops is likely to occur, these should not interfere with management of soybean rust, if growers follow a simple rule of thumb — don’t plant more soybeans than you can spray in a 10-14 day period. That’s the window of opportunity growers typically have to apply fungicides, from the time soybean rust shows up in sentinel plots.

John Mueller, a pathologist at Clemson University’s Edisto Research and Education Center in Blackville, S.C., says there is a definite correlation between these indeterminant varieties of soybeans and reduced susceptibility to rust infection.

“In our plots (in Blackville, S.C.) we had rust infections on soybeans all around the Maturity Group IV beans, but never got an infection in these plants. We never did get canopy closure on the Group IV beans, and I think that is the main reason, but there was definitely less rust infection in the Maturity Group IV beans,” Mueller says.

Pawel Wiatrak, an agronomist at Clemson, working with Mueller on soybean rust, warns that from an agronomic perspective planting Group IV soybeans comes with some risks. These varieties have to be harvested in a timely manner and can be more susceptible to drought conditions than later planted beans with determinant growth habits. Harvest of early planted Maturity Group IV beans may conflict with corn harvest, and the early maturing beans will shed pods, significantly reducing yield and quality, if left to wait in the field.

With a large increase in corn acreage expected throughout the Southeast in 2007, planting and harvesting Maturity Group IV beans may be problem for growers with large acreages of both crops. Mueller contends that availability of equipment and labor are already more frequent problems when it comes to spraying for soybean rust. If more acres of corn are planted and more acres of early-planted Group IV beans are also planted in 2007, there could be some management problems.

When it comes to rust, timely spraying is critical. Soybeans are most vulnerable to rust during the bloom stage By the time seed development begins in growth stage R5-6, rust is no longer a threat. While planting Maturity Group IV early gives growers a double shot at missing rust damage, competition for time and resources with corn or other crops is often a liability for these beans.

While there is more uncertainty than fact concerning how many acres of corn and early-planted, early maturing Group IV beans will be grown in the Carolinas in 2007, there is no uncertainty as to growers’ ability to manage soybean rust.

Managing Asian soybean rust in the Southeast has been a big success story in 2005 and 2006. With a new cropping season on the near horizon, Mueller and Wiatrak agree the future looks good for the 2007 soybean crop when it comes to managing rust.

Predictions of destruction of soybeans in the U.S. due to the arrival soybean rust, were dire in 2004. However, actual losses to the disease caused by the Asian soybean rust fungus have been minimal. In 2006, the cost of managing rust was significantly lower, thanks to a network of information that got to farmers quickly and accurately, and in most cases required only one application of fungicides to prevent or control rust.

In large part the management of rust can be attributed to a small group of plant pathologists in the Southeast who banded together to develop a system of sentinel plots that have worked to near perfection. Information from these plots was quickly and accurately delivered to farmers, and by 2006 most farmers ignored rumors and stuck with official updates.

Mueller, who is among the recognized experts on soybean rust in the Southeast, says serendipity played a big role in the development of the unofficial rust team. “All of us worked on other crops, but knew each other through our work on these various crops. When rust became an issue, we did what we were already doing — we talked to each other often and developed some plans to cope with the fungus,” he says.

In 2006, Wiatrak joined the Clemson staff and is headquartered, like Mueller, at the Edisto Research and Education Center in Blackville, S.C. Prior to coming to Clemson, he worked as post-doctoral scholar at the University of Florida’s North Florida Research and Education Center near Quincy in the northwest Florida Panhandle. One of the major areas of his work at Florida was soybean rust.

The history of successful application of multi-state crop protection programs in the Southeast has been shaky at best in the past. Yet, the soybean rust program has been a resounding success, despite a lack of any formal organization or leadership.

Mueller says information about soybean rust is posted almost immediately and is easily accessible by farmers, Extension agents, pesticide representatives and anyone who wants to know. “I can imagine, Mueller says, that some farmers with little knowledge of computers, leaned on their more computer-savvy teenage children to download and print updates on rust movement from our Web site.”

The biggest battle has been to convince farmers to avoid rumors and stick to spray guidelines developed from a series of sentinel plots that stretch from Florida to Canada. Once rust is identified in sentinel plots, it is immediately posted, in most cases giving growers a 10-14 day window of opportunity for spraying.

When soybean rust was first detected in the U.S., there were dire expectations that growers would routinely have to spray up to five times a season to control the disease. In virtually all cases, the sentinel system has restricted the number of sprays to two, and in most cases only one fungicide application is needed.

When rust is detected, growers have a number of choices for treatment. Three different families of fungicides have Section 3 labels for use against rust on soybeans.

These products include:

• Nitrile products, Bravos, Echo and Equus.

• Quadris, a strobilurin.

• Headline, a similar, but slightly different strobilurin fungicide.

In addition, seven more products have a Section 18 label. These include four triazole-based fungicides:

• Tilt, PropiMax and Bumper, which are propconazoles.

• Folicur, Uppercut and Orius, which are tebuconazoles.

• Laredo EC, which is a myclobutanil.

• Domark 230 ME, which is a tetraconazole.

In addition seven more fungicides from these same families of products have labels pending. There are a number of choices for either controlling the disease once it occurs or for preventing it from occurring.

The number of fungicides available, the options for timing of application and the advanced warning system provided by the sentinel plots give growers many options for managing rust in the Southeast.

“I don’t know of any case in which these sentinel plots didn’t work. If a grower in South Carolina says he or she has rust in their soybeans, and the sentinel plots don’t show it, rust is not there,” he contends. Mueller also stresses that it is virtually impossible for anyone, even a trained pathologist, to document rust infections without a microscope.

From an agronomic standpoint, Wiatrak says that soybean rust may cause significant yield losses of up to 80 percent if the infection starts at early bloom. Rust rapidly defoliates soybean plants, robbing the plant of any chance of normal pod set and development.

Though scientists have methodically tracked rust movement in 2005 and 2006, what occurred in those years, likely will provide little insight as to what it will do in 2007. Rust came to the Carolinas later in 2006 than in 2005. However, the disease organism over-wintered farther north in 2006 than in 2005. “So far, we don’t have a reliable model for predicting when the disease will occur,” Wiatrak says.

“In 2006, rust came later than 2005. It was somewhat surprising that it skipped most of Georgia, probably because of the extreme drought in the southern part of the state. When it got to South Carolina, it took about two months to spread throughout the state,” Wiatrak says.

The expected drop in cotton acreage and the continued interest in biodiesel production indicate an increase in soybean acreage throughout the upper Southeast. For growers new to soybeans, Wiatrak says soybeans with a yield potential of 35-40 bushels per acre is too valuable to not spray for soybean rust, when sentinel plots indicate it is needed.

“Last year growers told me they saw few bushels per acre gain in yield in fields sprayed for rust. Probably this was due to controlling other diseases than soybean rust,” he says. With fungicides costing in the $7-$8 per acre range and soybeans still selling for over $7 per bushel, one or two sprays, when such recommendations are made, should be easy for growers to justify.

Regardless of how far north rust over-winters in 2006-2007, optimism is high that the disease can be effectively and economically managed. The tools for rust management are in place, and if farmers use these tools wisely, rust should not be limiting factor in soybean production in the Southeast in 2007.

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