Whatever happened to soybean rust?

Ten to fifteen years ago, fear of soybean rust was all the rage. Growers from North Dakota to Maine to Florida had something new to worry about and chemical companies found an issue that would help them drive sales. But for growers in the upper Midwest, it never became the headache that matched the hype. As combines roll through the fields over the next few weeks, we may take a minute to review the facts and put what we know about soybean rust in proper perspective.

Asian Soybean Rust (Phakopsora pachyrhizi) is a potentially devastating disease that has caused yield reductions of more than 60% in fields where it is left untreated. The spores are spread by winds from any of the more than 60 different host species that it infects. It was first reported in Puerto Rico in 1976 and on the continental United States in 2004. The question we face every year is whether the rust will penetrate into the upper Midwest from the Gulf Coast states where it currently where it has its toehold.

There is certainly little doubt in the minds of most researchers that the vast soybean fields of the Midwest would be an excellent place for the disease to develop and expand. Moderate temperatures and the persistence of leaf wetness are ideal for rust development. However, there are three conditions which have heretofore limited its expansion. First, its spores are not particularly robust when it comes to cold temps. In fact, the spores are quite fragile in that respect and do not survive freezing temps at all. Second, the spores appear to be very susceptible to ultra-violet light. The winds which carry the spores aloft also subject the spores to the destructive power of sunlight. So spores are unlikely to move very far before being inactivated by light. Finally, the area of the United States where residual populations of the disease does overwinter (primarily Florida) is an area where prevailing winds are unlikely to be of sufficient strength and duration to carry spores very far towards the Midwest.

Some folks have wondered if soybean rust might evolve or adapt to cooler weather, creating a variant that is capable of overwintering in colder climates. While this is not impossible, it is unlikely to be something that would develop easily. Some have suggested that if the disease did evolve to be more resistant to cold temps and sunlight that its virulence would be similarly affected, and it would be much less likely to be a serious disease for soybeans, perhaps even losing its ability to infect the plant under normal weather conditions.

All this doesn’t mean it couldn’t be a problem for the upper Midwest. However, environmental conditions which would permit the disease to become a serious problem for Corn Belt growers haven’t yet developed early enough in the growing season for the disease to become epidemic. So the risk is actually quite small that we might ever see it as a serious disease for regions north of the Mason-Dixon line.