Soybean South

 - Disease Control -

Rust Update:
Spores Survive The Winter

University personnel report that rust spores did overwinter in Florida and Mexico.
What are the implications for southern soybean production,
and how can producers prepare for it?


By Carroll Smith

Mississippi soybean producers dodged a bullet last year, according to state soybean specialist Alan Blaine. The only rust found in Mississippi was in two sentinel plots, which were destroyed, and later in several growers’ fields in extreme southern areas of the state.

“Those growers did apply fungicide for rust, and that was the extent of our ‘05 rust problems,” Blaine says.

This past winter, the Mississippi “rust watchers” also kept a close eye on the legume species that act as host for the spores, but, as of this writing, had not found anything. However, because rust is being found in other states, Blaine believes the potential exists for growers to have more of a problem this year than last year.

“I think the majority of them (rust spores) overwintered in Florida and Mexico,” he says. “Based on our past experience with diseases, the Mexico population is the one the Mid-South needs to watch. A lot of our diseases, such as wheat rust and corn rust, come through here out of the Mexico and south Texas area on a prevailing northeasterly wind pattern.”

If the disease does show up in 2006, Blaine says the good news is that an effective arsenal of compounds exists to treat it.

“We like what we saw last year,” he says. “We think fungicide use at the proper time will add a degree of management and potential yield increases that we may not have seen before on a large scale, particularly if a lot of acres are sprayed.”

Blaine says from 45 to 50 percent of Mississippi’s soybean crop was sprayed with fungicides last year – some for rust and some for just a higher level of management.

In a statement released in the middle of March, Clayton Hollier, a plant pathologist with the LSU AgCenter, also expressed concern about prevailing winds from the southwest bringing spores from Mexico and Texas into Louisiana.

Like the other southern states, the LSU AgCenter has been planting sentinel plots, which are carefully monitored for signs of the disease. In 2004, the first discovery of soybean rust in the United States was made in Louisiana.

“We’re putting the plots out early – not just where soybeans are grown but where there may be spore showers coming through,” Hollier says. “Once it (rust) is in the area, since all varieties are susceptible to some degree, fungicide applications will be called for.”

In Arkansas, Extension soybean specialist Chris Tingle says they began planting their sentinel plots in southeast and southwest Arkansas and are marching their way north.

“Our pathology group has been doing a lot of update training with first detectors – scouts, consultants and county agents,” Tingle says. “We know we have a bigger spore load this year than we had last year because they are already picking it (rust) up in Georgia and Alabama.”

To concur with other Mid-South specialists, Tingle says Arkansas is concerned about the Texas and Mexico finds.

“That is why we have changed our defensive front to the southwest area of the state, which we plan to monitor very heavily for rust,” he says.

No doubt, all eyes will be on the southern crop this year, considering the increased potential for rust to appear. And thanks to the efforts of many university “soybean rust teams,” most producers are knowledgeable and will be prepared to take action if the disease does, in fact, show up.

2006 Rust Pointers For Georgia Soybean Producers

According to soybean experts with the University of Georgia Cooperative Extension Service, “Soybean rust will very likely affect all of our soybean producers in 2006. Given the present location of the disease on kudzu in northern Florida, Alabama and Georgia, we predict that the disease will affect the soybean crop in Georgia earlier in 2006 than in 2005.

“We also predict that there will be larger numbers of spores infecting the crop this season; thus, the disease will likely arrive earlier and with greater severity,” they add.

Based on 2005 experiences and the results of scouting for rust in 2006, they ask that Georgia producers consider the following points for the upcoming season:

• Consult your county agents for the latest soybean rust finds in the 2006 Sentinel Plot Monitoring Program.

• Budget the cost of two fungicide applications in your pre-season planning at a total cost of $25 to $35 per acre. You may not need a second application, but it is best to assume that both will be needed.

• Early reproductive growth, e.g. R1/first bloom, may turn out to be the appropriate time to make the first fungicide application. However, we are recommending that you wait to apply a fungicide for soybean rust management until the crop is at or very near reproductive growth and rust has been found in appropriate sentinel plots in the state.

First shot counts the most
Your first fungicide application is critical to managing rust. If you are late, you will lose yield. Beginning a fungicide program when you can see rust in the field is too late. Because of this, we recommend the following:

• Be on time with the first spray.

• Use an effective fungicide. If you are confident rust is not yet in the field, you can use a strobilurin, triazole or strobilurin-triazole mix. If you suspect or recognize that rust is already in the field, your best options are a triazole or triazole-strobilurin mix.

• It is our belief that the better triazole fungicides include tebuconazole, tetraconazole and myclobutanil. We believe that propiconazole is considerably less effective.

Source: Phil Jost, UGA Extension soybean specialist; Bob Kamerait, Extension plant pathologist; and Layla Sconyers, research post-doc, plant pathology.

Soybean Rust Symptoms: What To Look Out For

• The most common symptom of soybean rust is a leaf lesion. On the upper leaf surface, initial symptoms may be small, yellow flecks or specks in the leaf tissue. These lesions darken and may range from dark brown or reddish brown to tan or gray-green.

• The lesions tend to be angular to somewhat circular in shape and may be concentrated near leaf veins. Initially, lesions are small, barely larger than a pin point. Mature lesions may be somewhat larger, and lesions may merge or run together.

• Pustules form on the lower leaf surface and may appear to be small raised blisters or calloused bumps. As the pustules mature, they produce large numbers of light-colored, powdery spores that emerge through a hole in the cone-shaped pustule. Masses of spores may mound up out of the opening in the pustule.

• Rust pustules are most common on the underside of leaves but may also develop on petioles, pods and stems. Leaves may turn yellow and drop prematurely.

Source: Alabama Cooperative Extension System