ASR is moving slowly through Louisiana. On the heels of an ASR find in Natchitoches Parish over the weekend, two more discoveries have been confirmed in Avoyelles and Tensas parishes.
“The Natchitoches and Avoyelles sites are in commercial fields,” says Clayton Hollier, LSU AgCenter plant pathologist. “The Tensas Parish site is 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.”
The Tensas site has the youngest ASR.
“As of yesterday (Aug. 15), the pustules were just forming. Not all were sporulating. They’re in a bag and I bet they’re producing spores now. The other sites had ASR that was quite a bit older.”
Fungicide recommendations still aren’t one-size-fits-all.
“If the crop is in the critical stages — particularly R-3 to R-5 — we’re suggesting producers in the areas ASR has been found should seriously consider using a rust fungicide. If they’re looking at just dealing with rust and aren’t seeing cercospera or aerial blight, check out rust materials. If there are other problems in the field, they should look at materials that can handle all the diseases.”
How is the rust moving in such dry, hot weather?
“I think what’s happening is there’s just enough moisture that’s 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.”
Farther north of infected sites where it’s been drier, “there’s much less of a chance for the disease to develop. If producers have been in an extended dry period, I wouldn’t make an application without further information. But they need to be scouting.”
All commercial fields up to R-6 should be scouted carefully in the lower canopy, along tree lines first.
“From there, scout the rest of the field. Make fungicide applications as situations warrant.”