When Asian soybean rust was detected in Kentucky and other Mid-South states in mid-October, University of Tennessee Extension plant pathologist Melvin Newman gathered up four truckloads of Extension workers who drove out into the west Tennessee countryside to see if the disease had found a home in the state.
By that time, most of west Tennessee's sentinel plots had matured out and many of the spore traps had been taken up. A lot the state's commercial fields had been gathered as well. There were soybean leaves to be found, however, and by the end of the day, the Tennessee team rust team had collected suspicious samples in almost half of the 40 counties they visited. When the results came in later, their suspicions were confirmed.
“There must have been a spore shower two to three weeks ago or so, we don't know when exactly,” Newman said. “Kentucky has a bunch of counties that have turned red on the soybean rust Web site and we've found a bunch in west Tennessee now.”
On Oct. 24, the www.sbrusa.net monitoring system issued 17 soybean rust alerts for counties in the Mid-South states and Kentucky. Nationwide, the monitoring program indicated the presence of rust in 161 counties in 15 states ranging from Texas to Indiana to Virginia.
By Oct. 26, the total had climbed to 203 counties in 15 states, 227 if you count findings on kudzu. There were 16 detections in Alabama; 18 in Florida; 21 in South Carolina; 24 in Louisiana; 15 in Georgia; four in Texas and Missouri; six in Mississippi; 30 in North Carolina; 16 in Kentucky, eight in Illinois; six in Indiana; 18 in Tennessee; 27 in Arkansas; and 13 in Virginia.
While the explosion of rust comes too late to have any impact on U.S. soybean yields, it underscores how quickly the disease can explode, and according to Newman, gives pause to what might have happened had there not been droughty conditions in the south Delta this growing season, which likely slowed development of the disease.
“This little rust dry run shows that it can spread far and wide and quickly. If it can do this with a drought down south early in the season, when the rust needed moisture to get going, think about what would have happened with some wet weather coming in at the prime time for rust, then have some wind currents blow it this way.
“If this had happened in July, we would have been spraying a lot of soybeans in west Tennessee and there would be a lot of concern,” Newman said.
Newman said that leaves that tested positive for rust “weren't plastered with it. There were as few as one and as many as 50 pustules per leaf, and you may have to look through four or five leaves before you find another. So it was sporadic. It looked to be either the first or second generation of rust.”
Newman says the late-season explosion of rust “tells that we can't go to sleep on this. We have good mechanisms in place with conference calls between state Extension plant pathologists every week and sentinel plots and spore traps in all the soybean growing areas. Let's stay alert.”
Newman says he is still concerned that despite wide-reaching efforts to keep growers informed of the spread of the disease and methods to combat it, “I'm still not convinced that our farmers, especially our larger farmers, are set up cover a large amount of acreage in a short time.”
Since the disease entered the United States in 2003, it has overwintered in the deep South regions of Texas, Louisiana and Florida. “All that needs to happen for the disease to spread is wetter weather in the spring in those areas, and weather patterns that bring it north, when a lot of our beans are in the pod fill stage or before,” Newman said.