"Ag Today" Newspaper Column

 

Michael R. Milam

Agronomy Specialist and County Program Director

For Daily Dunklin Democrat-Ag Today

October 24, 2006

Asian soybean rust increased late in the season

While soybean researchers and extension specialists have suspected that Asian Soybean Rust had the potential to become a problem in the United State, most were caught off guard when the rust finally arrived. In early November 2004, USDA announced that this rust that had been devastating in Brazil had been confirmed in soybean plots at one of the Louisiana State University research farms. Within weeks, numerous fields in the Mid-South were confirmed. The disease was found in Alabama, Arkansas, Florida, Georgia, Mississippi, Missouri, South Carolina, and Tennessee.

Asian soybean rust has been found in the Missouri Bootheel for the first time since November 2004. Dr. Allen Wrather, Plant Pathologist at the UMC Delta Center, found rust pustules and spores near the McDonalds in Portageville. This was the location where rust was first discovered in Missouri. In case you might be wondering, I will remind everyone that this location has a security light and the plants at the edge of the field did not drop their leaves and were not harvested because the beans were still green.

After finding the rust pustules in 2006, Dr. Wrather found the disease in late plots on the Lee Farm near Portageville. Since then rust has been confirmed in Scott and Stoddard Counties making four counties in Missouri. Even though Mississippi County was part of the screening network, to date no disease has been found in that county. I suspect that the disease is present in Dunklin and many other counties in Missouri, but this should have no consequences for Missouri producers this year.

According to the Missouri Crop Condition and progress report of October 22, 93 percent of the Missouri soybean crop is mature with 64 percent harvested. Once the soybean crop gets to the growth stage, R6, the disease will not reduce yield since the leaves are falling off of the plants due to the physiology of the plant. The R6 stage is when the pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed trifoliate leaf node.

As I had mentioned earlier this year, the protocol for scouting had changed and that much more effort was required to insure that the disease did not get into the area without detection. The USDA and North Central Soybean Research Program Protocol for 2006 Sentinel Plots recommend that the collection of 100 leaflets from the lower canopy be taken. These leaflets were to be sent to campus so that microscopic examination could take place. The plots were to be examined every two weeks until bloom or until rust was detected in neighboring areas. Also, if the environmental conditions were favorable for rust development, then plots were to be sampled more frequently. 

On Thursday of last week, I met Dr. Wrather at the Delta Center to look at soybean leaves from the affected area. We looked at the leaves under magnification using a bi-nocular stereoscope. We were using much greater magnification that the 20x hand lens. I had no difficulty seeing the pustules. Soybean rust is the only disease with raised pustules on the leaves. I could see both the pustule and spores on the leaves.

With small pustules, it is very difficult to see them with a 20x lens. However, a hand lens could be used to indicate an area that needed to be examined under higher magnification.

As has been pointed out on many occasions, Asian soybean rust is a tropical disease that has taken root in Asia, Southern Africa, and South America. Although is has been found during the growing season in the Southeast U.S., it has not developed into a serious problem. Including reports on kudzu, there is a total of 191 counties in fifteen states with rust this year including 13 in Alabama; 18 in Florida; 21 in South Carolina; 24 in Louisiana; 15 in Georgia; 4 in Texas and Mississippi; 26 in North Carolina; 16 in Kentucky, 7 in Illinois; 4 in Missouri and Indiana; 10 in Tennessee; 23 in Arkansas; and 2 in Virginia.

However, since the disease came in so late in most locations, it did not cause any problems. The disease can’t over winter on kudzu in Missouri so the spores are spread by the wind.

Information about how to treat soybean rust can be found at the university of Missouri Soybean Rust site at http://agebb.missouri.edu/mgt/soyrust/index.htm. We also have a publication, “Using Foliar Fungicides to Manage Soybean Rust” in my office that is available to producers or dealers.  This multi-paged booklet contains many of the scenarios that could face Missouri producers. This provides the nuts and bolts of how to protect the soybean crop from yield loss. It provides the when, where, and how of protection measures.

 

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