A basic concept of plant pathology is the "disease triangle". Three elements, susceptible plants, a suitable environment, and the pathogen must come together in the "disease triangle" for a plant disease to develop. Within the 7.2 million acres of soybeans in Minnesota, some area will likely be suitable for soybean rust development in 2006. All commercial soybean varieties of soybean are susceptible to soybean rust, and the environment will likely be favorable in some places for soybean rust to develop. In Minnesota the missing element will probably be the pathogen. Since the soybean rust (SBR) fungal pathogen does not survive over the winter here or in any states near Minnesota, spores must be introduced each year - just as they are for rusts of wheat and corn.

The definitive way of knowing if the SBR pathogen has entered Minnesota from overwintering areas to the south is to see disease developing on plants in the field. Soybean rust sentinel plots have been established throughout MN to provide this information. However, the fact that soybean rust pustules are small and initially develop on leaf undersides within the plant canopy make field scouting difficult - although not impossible. Another problem with this approach is that by the time soybean rust pustules are detected on plants in the field, the disease may be well established and perhaps may have progressed beyond the optimal time for management with fungicides. For this reason, it may be more effective to forecast the risk of SBR on the basis of weather variables, and inoculum (spore) availability to enable timely implementation of disease management practices.

Soybean Rust spores (or the DNA from SBR spores) were detected in rainwater samples collected in Lamberton, MN as well as other locations in the region in July 2005. It is important to understand that soybean rust did not develop; only the spores were detected. Disease may not have developed because it was either too dry when the spores were deposited or because there was an insufficient number of spores to initiate infection. Although one spore is theoretically enough to initiate disease, realistically many more spores of a pathogen are needed to initiate and sustain an epidemic.

The spores detected in Lamberton were captured in a rainwater sampler established and run by the National Atmospheric Deposition Program (NADP). These samplers are placed in hundreds of locations throughout the U.S. Experimental use of the rainwater samples that are collected weekly from them was initiated by Dr. Les Szabo and colleagues with the USDA and University of Minnesota within the last 3 years to monitor plant pathogens. Funds from the United Soybean Board, USDA, and Minnesota Soybean Research and Promotion Council have supported this work. This is still a very new, experimental system that is focused on researching long-range spore dispersal. However, the system has potential to be part of a soybean rust forecasting system.

A critical aspect of the analyses of these samples is determining the origin of spores that fell with rainwater at Lamberton and other locations. Using a modeling technique called retrospective analysis, Dr. Sagar Krupa of the Dept of Plant Pathology, U of MN, and colleagues from the Illinois Water Survey determined the origin of the spores with support from the Minnesota Rapid Agricultural Response Fund (RARF). Their results were very interesting. The analysis pointed to areas in eastern and southern Texas, and to the Yucatan Peninsula and other areas in Mexico as sources for the spores found in Minnesota. Most previous efforts to predict SBR spore movement focused on the known source regions in the southeastern U.S. These new results demonstrated another potential source region that could be more important for Minnesota. Subsequently, SBR was detected on soybeans in Texas and Mexico on a track that agreed with the back trajectory, retrospective analysis. This dispersal track also generally agrees with known tracks of dispersal of the wheat rusts from Texas to Minnesota.

The NADP samplers can only collect spores deposited in rain water, because the collectors are closed when rain is not falling. Other types of spore collectors have different abilities. In 2005 another type of spore collector, which can collect spores deposited by either wet or dry deposition, was deployed in association with sentinel plots throughout Minnesota. These collectors which filter all particulate matter, spores and debris, deposited in rainfall were also analyzed using technique to detect DNA from soybean rust spores. Analysis of samples from this system has been technically challenging, but preliminary results indicate that soybean rust spores were transported to Minnesota during the summer of 2005.

In 2006, both of these types of spore "traps" will be used at multiple locations throughout the state to help us understand when and where soybean rust spores are being transported to Minnesota. In addition, another type of spore detection system that focuses more on local spore transport is being tested in Minnesota on an experimental basis during the summer of 2006. This system relies on less precise microscopic identification of soybean rust spores and does not use the more accurate DNA-based technique for spore identification. These spore traps, which are part of a multi-state network sponsored by Syngenta, will be placed at two locations in Minnesota.

The results obtained from the spore collection efforts in 2005 have yielded insights into the behavior of SBR that are relevant specifically to Minnesota soybean growers. In particular they point to potentially unrecognized sources of SBR spores in Texas and Mexico. The fact that SBR did not occur even thought spores were detected indicates that we still a considerable amount to learn about the behavior of SBR. In order to build on the results obtained in 2005 we are making a major effort again in 2006 to monitor and track potential movement of soybean rust spores into Minnesota. Results from spore trapping efforts in combination with weather data and observations from sentinel plots will be used to test and validate the soybean rust forecasting system developed in 2005, which was supported by the Minnesota RARF. This forecasting system has tremendous potential to guide timely, effective management practices for the benefit of all involved with soybean production.

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