LEXINGTON, Ky. – Researchers are still trying to understand how the fungus of Phakopsora pachyrhizi, which causes Asian Soybean Rust (ASR), cuts grain yield in soybean plants. One such study should eventually result in a yield loss prediction tool useful to educators and growers.</p><p>
In 2005, Saratha Kumudini, a crop physiologist with the University of Kentucky College of Agriculture, and other researchers from Louisiana State University and Brazil received a USDA grant, as well as funding from the Southern Soybean Research Program and Kentucky Soybean Board, to develop the online tool. But to predict yield loss, she explained the researchers first needed to understand how ASR causes it in the first place.</p><p>
“What we understood about soybean rust was this tremendous leaf loss,” she said, adding their working hypothesis was that because of this, the soybean plant cannot absorb as much radiation – sunlight – through photosynthesis.</p><p>
To that end, she and other researchers began with experiments on soybean plants in Brazil. They inoculated plants with ASR at the R1 and R5 stages to track their natural destruction; at the same time, to mimic leaf loss, they manually stripped leaves from healthy R1 and R5 plants in another plot as infected counterparts lost those same leaves. They also had a healthy control plot they left alone.
Kumudini said they weren’t surprised to find the infected R1 plants suffered more yield reduction than infected R5 , since they were in earlier development and more susceptible to damage. Still, each had severely-reduced yield compared to the control plot. All things being equal, the researchers expected similar yield results from the manually-stripped plots.</p><p>
“If this was all about radiation absorption, we’ve have lost yield as we lost leaf area,” Kumudini pointed out.</p><p>
To their surprise, yields for the stripped plots were nearly similar to the control group, larger than their infected counterparts – twice as high for R5-infected plants and nearly three times as much as R1-infected soybeans.</p><p>
Their hypothesis confounded, the researchers searched for other factors. Kumudini changed the formula by which she figured radiation absorption to include leaves with ASR pustules at a reduced “green leaf” surface area, compared to a non-infected leaf – for example, a leaf covered in 30 percent pustules would be figured at 70 percent effectiveness.</p><p>
While this did better explain yield loss, it still wasn’t telling the whole story. The researchers finally accounted for the extra loss by figuring that ASR lesions on a leaf not only reduces its sunlight-catching surface area, it reduces the efficiency of what green is left. By the end of the experiment, not only had the researchers learned leaf loss alone wouldn’t cut grain yield, they aren’t sure it’s even the most important factor.</p><p>
According to Palle Pedersen, extension agronomist for Iowa State University, there’s no fixed number of seeds in a soybean pod. That is decided early in a plant’s reproductive stages, and size is determined later, such as when growers in the U.S. are praying for rain in August.</p><p>
Kumudini agreed, adding seed number is determined by dry matter accumulation in the plant between the R1 and R6 stages, whereas weight comes from accumulation between the R4 and R7 stages.</p><p>
At the National Soybean Rust Symposium in Louisville, Ky., on Dec. 13, she asked fellow researchers with ASR data from their states to contact her so she might use that information to test the preliminary yield loss prediction model. More details can be found at www.uky.edu/Ag/Agronomy/Department/sbr