A New Theory
Last year, Asian soybean rust spores moved a great distance, but the disease did not. The fungus itself may have something to do with it. Rust spores were detected many times in each state of the northern soybean-growing region. Why then, was the disease not found there?
A University of Illinois study found that rust spores cannot survive more than two consecutive days of sunlight. The theory is that if spores were exposed to more than two days of a cloudless sky, they would be too weakened to cause disease, reports X.B. Yang, Iowa State University professor of plant pathology. A preliminary study suggests that rust spores may be sensitive to light, a factor previously unnoticed. Shaded rust spores resulted in a 95% rust infection, compared to only a 25% infection rate in unshaded spores.
This might explain why in South America the disease has always been found in lower portions of the canopy, despite the upper leaves being more vulnerable due to more dew and greater spore deposition.
Perhaps the cloud cover of rainy weather is what creates the connection between rust outbreaks and rainy weather. “If future research confirms these findings, the Midwest's past two summers' dry weather may have prevented the broad dispersal of spores from turning into a dangerous outbreak.