December 2, 2005 | By: Laura Skillman
PRINCETON, Ky.

Few areas saw much devastation from Asian Soybean Rust in the first growing season since it was discovered in the United States, but the disease created substantial concern. In Kentucky, it was discovered in early November after most soybeans had been harvested.

Soybean rust was not discovered on any of the state’s soybean crop but was found Nov. 11 on a kudzu leaf in Caldwell County by Don Hershman, plant pathologist with the University of Kentucky College of Agriculture. Hershman’s preliminary diagnosis was later confirmed by the U.S. Department of Agriculture. 

“I found one leaf with lesions and pustules, and after several trips back to the same kudzu patch, I could not find another infected leaf,” Hershman said. 

Freezing temperatures have since killed back the kudzu patch, so Hershman said he presumes there is no longer soybean rust in Kentucky. The rust find after the growing season is somewhat of a non-event, he said, although this is the farthest north soybean rust has been confirmed.

The kudzu patch is about a mile away from a spore trap at the UK Research and Education Center which picked up “rust-like” spores five times this year. Hershman said one or more of these spore detections may be related to the recent kudzu find, but that can’t be confirmed.

Before the cold weather set in, Plant Disease Diagnostician Paul Bachi observed kudzu leaves collected from Ballard, Fulton and Hickman counties in the far western part of the and from Perry County in the southeast corner of the state. He found no evidence of soybean rust.

Seven of 10 spore trap sites in Kentucky had multiple hits during the summer and fall, while three trap sites had none. Despite finding what appeared to be spores of the soybean rust fungus, rust was not detected at any site at any time in Kentucky during the 2005 growing season, Hershman noted. 

Detecting spores for the fungus while not seeing the disease in fields confirms the importance each part of a disease triangle plays in infection. Disease can only exist when three highly specific conditions are met: the disease organism must be present and capable of infecting the host; the plant must be present and susceptible to infection; and the environmental conditions must support both infection of the host and subsequent disease development.

“If we assume that we were picking up spores of the rust pathogen, and we know that soybean is pretty much uniformly susceptible to the fungus, that leaves the weather as the probable reason why we did not pick up infections in 2005,” Hershman said.

The weather could have significantly influenced any number of different aspects of the disease triangle, he said. For example, much of Kentucky was under drought or near-drought conditions for much of the season. The hot, dry weather could have killed spores outright, or they may have died while they were waiting for the occurrence of moisture and temperature conditions that would support infection.

Another possibility is that infection did occur, but the numbers were so low that no amount of scouting soybean fields would have detected it. Detection would be dependent on additional infections taking place, and these would have almost certainly been hindered by the hot, dry weather, he said.

The message as growers prepare for 2006 is that spore trap data, even if it were possible to identify spores with 100 percent certainty, will never provide more than one corner of the disease triangle, Hershman said. However, spore trapping can be useful for letting growers and scientists know where and when scouting should be increased for soybeans.

As more experience is gained with the traps, the data may be very useful and even essential in disease prediction models, he said.

Contact: 

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

Contact: Don Hershman, 270-365-7541 ext. 215