For those keeping track of Asian soybean rust, the week of Aug. 14 was a busy one. The week began with an ASR find in Louisiana's Natchitoches Parish and, mid-week, moved to discoveries in Avoyelles and Tensas parishes. Several days later, Iberia and St. Mary parishes were added to the ASR list.
Will the week of Aug. 21 be similar? It certainly started off the same. “More rust has been found in Iberia Parish,” said David Lanclos, LSU AgCenter soybean specialist on Aug. 21. “Last week, it was found in an Iberia sentinel plot. Over the weekend, it was found by a consultant in a production field at R-7.”
Mid-South producers also need to keep a wary eye turned towards the southwest. On Aug. 20, it was announced that ASR was detected at low levels in kudzu near Dayton, Texas, in Liberty County.
Regarding the Texas find, the USDA ASR Web site (http://www.sbrusa.net/ ) reported “no rust was seen in nearby soybeans or in kudzu to the north. There have been scattered thundershowers during the past week but hot, dry weather will not support development of an epidemic in the next few weeks.”
ASR is “definitely floating around,” said Lanclos. “But it remains very difficult to detect and it isn't running rampant. The weather is keeping it in check.”
The Natchitoches and Avoyelles ASR sites were in commercial fields. The Tensas Parish site was a sentinel plot on the experiment station at St. Joe's.
“The Tensas plot beans are at R-3, the Natchitoches field is at R-5 and the Avoyelles field is at R-7,” said Clayton Hollier, LSU AgCenter plant pathologist. “The Tensas site has the youngest ASR.”
As of Aug. 15, “the pustules were just forming. Not all were sporulating…The other sites had ASR that was quite a bit older.”
How is the rust moving in such dry, hot weather? “I think what's happening is there's just enough moisture staying within the canopy. We're late in the season and the canopy has closed. It's holding a lot more moisture within and currently humidity is extreme. There have also been a few showers. Those factors are allowing the dew-leaf period to maintain much further into the day. This fungus needs that moisture. It has a long dew-length requirement.”
Spore showers have apparently been occurring throughout the season. “Now, the spores are finding conditions to their liking. But on several of the sites the original infection probably occurred as much as three weeks ago. On those, what we've found is probably secondary infections.”
Hollier says growers need to be cognizant of how ASR is spreading. “Spore showers aren't selective. They won't hit a single field and not the ones surrounding it. Under the conditions we're currently seeing, I expect we'll find more rust.”
Hollier's words proved prophetic when more ASR — this time in Iberia and St. Mary parishes — was announced two days later. The infection level was “very, very low,” said Lanclos. The finds “aren't situations where huge areas are eaten up with rust. Actually, it's a point of pride that we have people able to find this disease at such early stages.”
With 50 percent-plus already out of the field, the state's soybean harvest is moving rapidly.