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Threat of soybean rust in Md. still lingers

By Bryan R. Butler, Ag Today

Monday, September 10, 2007

Thanks to this year's drought, there hasn't been much of a threat of soybean rust. But the situation has been changing and we are not out of the woods yet. The following is an update from Dr. Arv Grybauskas, associate professor and extension plant pathologist at the University of Maryland.

The onset of soybean rust in commercial soybeans in the Gulf states was delayed somewhat by two factors. The first was the Easter-period frost that knocked back kudzu growth. Kudzu is the primary alternative host for soybean rust in the U.S. and is the live host on which the soybean rust pathogen survives winters. Frost knocked back the population of the fungus, so that reinfection of kudzu and introduction into soybean fields was delayed.

The second factor has been the drought, which has been very significant throughout the soybean production areas of the U.S. The soybean rust pathogen needs moisture to survive transport, cause infection on new hosts and reproduce. The drought also changed the planting of soybeans so that many full-season soybean acres in the South were planted late and there was also a significant increase in double-crop soybeans after wheat.

Once soybeans reached the peak susceptible period, the overriding factor has been rainfall. The center of activity this season has been Texas, Oklahoma and Louisiana. This region experienced several periods of extended and heavy rainfall when the Florida and Georgia region only received a few scattered showers. With the center of soybean rust activity being farther west, the primary concern has been of a direct and relatively early hit in states with large soybean acreage just due north.

We are now in a situation where all soybeans that are still green in the U.S. are susceptible. Soybean rust has increased to sufficiently high levels especially in the Texas, Oklahoma and Louisiana region, so that there is a greater chance of longer-distance transport with survival in greater numbers. And to complete the disease triangle, we are also heading into a period when cooler and wetter conditions are to be expected. We are basically looking at the exponential phase of the epidemic to get started.

What does this mean for Maryland?

Up until last week, we had zero risk of soybean rust due to the drought. Even if spores landed here, conditions were not suitable for infection. But there also wasn't a large enough source of spores close to us to really produce a threat to our area. The source is now large enough, but without a hurricane or some other major storm system, it is unlikely that we would see any significant volume of spores moving our way in the immediate future. A second avenue for infection could come from the Southeastern states.

Soybean rust is increasing in Florida, Georgia and Alabama, and once that gets into enough commercial acreage the coastal route that brought soybean rust to our doorstep last year will be a potential avenue this season as well. I estimate that most full-season soybeans will hit R7 in a month, if not less.

In any case, the window for infection and for any fungicide response is only about a month. Without a hurricane, if soybeans in the central Midwest become infected in the next two weeks then we could still see soybean rust in full-season beans within this window. With a hurricane, the chances go up, but for now, nothing is developing in the Atlantic.