September 9, 2005 | By: Laura Skillman

The majority of Kentucky’s soybean crop looks to have escaped Asian soybean rust damage this year. 

Soybean rust continues to be confined to the deep southern United States, primarily in southern Alabama, Georgia and Florida, said Don Hershman, plant pathologist with the University of Kentucky College of Agriculture.

It is very likely that Hurricane Katrina moved spores of the soybean rust fungus into the state as it passed though this past week. But, even though spores are likely to be in Kentucky, the overall concentration of spores will be low and dilute, he said.

“There simply is not enough acreage of soybean or kudzu affected by rust in the Deep South to result in large numbers of spores blowing into any field in Kentucky,” Hershman said. “And, for the spores that do blow in, it will take four to six weeks before the disease will be to the point where significant damage is possible, resulting from defoliation.”

That means if Katrina brought in a reasonable number of spores, it will be at least the end of September or early October before any damage will be possible. By that time, most soybean fields in the state will be close to or at physiological maturity.

“In other words, soybeans that will be near, at or beyond physiological maturity by the end of September are home free for 2005,” he said.

Fields that will not reach physiological maturity until mid-October or later, primarily soybeans planted following the wheat harvest, still may be at risk. However, even in those cases, fields will not go from being clean to having a lot of rust overnight, Hershman said. There will be plenty of time to react, should spraying be required, and there will be indications that the soybean rust risk is high, based on the disease being found close to or even in Kentucky. 

Spraying would only be advised in later soybean crops if the risk is high, and there is still more than a month to go before the crop will be at R6, the full seed growth stage. Fungicides should not be applied once a crop is at the R6 stage. In fact, most fungicide labels prohibit spraying soybeans that have reached the R6 stage.

“In my opinion, it is very unlikely that any more than a handful of fields may need to be sprayed for rust this year,” Hershman said. “It is highly possible that no fields will need to be sprayed. Overall, in my opinion, the prognosis is quite good that soybean rust will cause little or no damage to the 2005 Kentucky soybean crop.”

From a scouting perspective when one pod on the main stem has lost its green color, the entire plant is at or very close to physiological maturity, said Chad Lee, UK Extension plant and soil scientist. This means that all dry weight (yield) has been accumulated and soybean rust will no longer hurt yields.

Hershman and Lee noted that by mid-October frost may play a bigger role in the development of late planted soybeans. Frost will stop the development of soybeans along with rust, Lee noted. So, if in mid-October rusts hits the state but there’s also a good chance of frost within a couple of weeks, farmers will have to decide if spraying for rust will be justified.

County Extension offices have charts available that show the likelihood of frost within the state. Frost dates vary somewhat by location.


Writer: Laura Skillman 270-365-7541 ext. 278

Contact: Don Hershman, 270-365-7541 ext. 215
Chad Lee, 859-257-3203