Scientists from around the United States recently met in New Orleans to review progress of checkoff-funded research being conducted on Asian soybean rust (ASR). New Orleans was an appropriate venue for the meeting, because the disease was first reported in university field trials just a few miles from New Orleans.

It is clear from the progress reports that a large amount of knowledge has been gained on this potentially devastating disease since it was first reported in the United States in 2004.

Two years ago all that was known was that this new soybean disease could quickly infect and destroy a soybean field and that U.S. soybean farmers had no genetic resistance to protect their crop. Today, we know how to manage the disease with fungicides and we have found some material that is showing some tolerance to the pathogen.

Three hundred and thirty soybean germplasm accessions were tested for susceptibility to Asian soybean rust at 22 locations in 2006. Forty-seven accessions showing partial resistance were selected for further testing in 2007. USDA researchers are also testing germplasm materials in Uruguay and Vietnam.

Genetic resistance is the key to maintaining profitability. Researchers are mapping the four genes known to provide resistance for ASR. Rpp1 obtained from a cross of Williams and PI200492 has been mapped to linkage group G. USDA-ARS researchers at Ft. Detrick and Beltsville are mapping the other Rpp genes. Researchers are also cloning the Rpp genes and developing molecular markers for the various resistance genes for use by soybean breeders.

Researchers at Iowa State are investigating the mechanism by which Rpp2 provides resistance to ASR. They are looking at the genes activated in resistant and susceptible soybean germplasm when infected with P. pachyrhizi.

Monte Miles, USDA-ARS scientist, and co-workers in several states have completed a four-year fungicide application study. They found that mid-sized spray droplets, applied at high pressure and with ample water, provide maximum canopy coverage. Studies are now available to provide fungicide application recommendations for optimum plant coverage and ASR control.

The University of Florida group continues to search for ASR spores in kudzu to determine over-wintering of the pathogen in the Southern U.S., conducting field plot studies in ASR-infested fields; managing 640 plots of fungicide studies; and providing research support and facilities for researchers in other states. In 2006, they found that row spacing (7 1/2, 15 or 30 inch rows) did not affect ARS infection levels; ASR spread and soybean yields were not correlated; and ASR reduced plant photosynthesis, soybean seed weight and yields. The Florida group also provided several educational activities for groups interested in learning more about ASR. Twenty-five scientists from around the U.S. utilized the Quincy Research Station in 2006.

Researchers at Louisiana State University reported dry weather during the summer of 2006 delayed onset of ASR in Louisiana; spore numbers seem related to environmental temperatures (70-75 degrees F ideal); and lack of ASR spread in July and August may be due to the temperature being too high in southern states for spore growth and development. They concluded that ASR is encouraged by cool and humid environmental conditions.

ASR spores have been collected using several spore traps in studies led by Les Szabo. The survey collected 1,869 rain samples in 2006, of which 319 were positive for ASR spores compared to 85 positives of 1,646 spores collected in 2005. The locations of the spores were widely distributed in the 2006 survey. Szabo reported that 25-50 spores were needed to produce an ASR pustule. He concluded that the spore trapping studies demonstrated that P. pachyrhizi spores could be detected throughout the U.S.

Three research groups (IA, PA, NC) are developing and evaluating computer models for ASR disease movement. Summer storms continue to be important in spreading ASR spores; predicting spore spread has value in alerting growers to the ASR threat.

Soybean growers through their soybean checkoff program have invested several million dollars in ASR studies. A recent USB-funded study identified that on October 1, 2006, 43 projects with a total investment of $2.505 million were being funded with soybean checkoff funds. This indicates soybean grower concern about this increasing threat to soybean production.

SOURCE: North Central Soybean Research Program Feb. 2007 newsletter.