Asian soybean rust continues to pop up in both Louisiana and Texas. On June 21, three new finds were reported: sentinel plot infections in adjacent central Louisiana parishes and a sole find in extreme southern Texas commercial soybeans.
With the new reports, odds have improved that Louisiana soybean producers will have to use a fungicide at least once.
“We’ve had two new ASR finds in Avoyelles Parish and Rapides Parish,” says Boyd Padgett, LSU AgCenter plant pathologist. “Now, if you’re in the northeast part of the state and have good beans, you should consider protecting them with a fungicide. That doesn’t necessarily mean adopting a full-blown ASR program but using a fungicide that will take care of multiple diseases.”
But don’t spray all soybeans, warns Padgett. “Just help the good-looking fields. All fungicides do is protect a percentage of the yield. So if the yield potential is rubbish to start with, you’ll end up with a rubbish yield even after a fungicide application. You won’t double yields with a fungicide.”
“We’re talking about protecting $8.50 beans with (a fungicide product) that barely costs a bushel,” says a north Louisiana consultant. “When looking at it in those terms, I think my clients need to spray. Too much could be lost otherwise.”
David Lanclos doesn’t disagree. “If your beans are close to R-3, the recommendation hasn’t changed: use a mixture of a triazole and a strobilurin,” says the LSU AgCenter soybean specialist. “I’m often asked, ‘If my field is three days from R-3 should I wait to spray a fungicide?’ If you’re three days, five days, even seven days, from R-3, you don’t need to spray prior. Just wait those few days and then use the mixture.”
With the continuing early ASR finds (the Rapides Parish location was a sentinel plot at the Dean Lee Research Station in Alexandria) Lanclos says there is “serious potential” for two shots of fungicide on late-planted double-crop soybeans. “In those situations, if someone wants to go with two shots, I’d suggest a triazole at R-2 followed by a mixture at R-3.”
What should Louisiana producers who have sprayed a mix or strobilurin at R-3 do now? “I say do nothing, just wait. You’ve bought yourself protection and yield bump from the strobilurin at R-3. If we do spray again, it’ll be a straight triazole plus, or minus, Topsin M in the tank at R-5. Sit tight and reevaluate things at R-5.”
According to Clayton Hollier, LSU AgCenter plant pathologist, about 50 percent of the plants at the Rapides Parish sentinel plot were infected with ASR. “But the infections weren’t more than 1 percent,” says Padgett. “So there were many plants infected, but those infected had few pustules.”
At the Avoyelles Parish site, the infection rate was even less. “About 1 percent of the plants were infected, and of those the severity was only at 2 percent, or less.”
Lanclos noted the two sentinel plots where ASR has been found were the first planted in state. “Both were planted around March 5 and are the first where ASR has shown up. To me, that’s very interesting.”
Tom Isakeit suspects the latest Texas ASR discovery (near Brownsville in Cameron County) is on commercial soybeans that are probably the southern-most produced in the continental United States.
“The field is practically on the Mexican border,” says the Texas A&M plant pathologist. “I’ve been checking that particular field for the past two years and never found anything even though on two occasions ASR has been located 50 or 60 miles away in neighboring Hidalgo County.”
There are only some 15 soybean fields in the two Texas counties with ASR. “I suspect all told, there’s 400 or 500 acres total. That’s it. So far, there’s one field in the Brownsville area with ASR. Plus, there are three fields north of Weslaco in Hidalgo County with the disease. Those three fields are within 2 miles of each other.”
Isakeit found the sporulating Cameron County ASR in R-5/R-6 soybeans at a field’s edge. “There’s a mixture of soybean maturities in the area — seedlings to R-6. The grower had asked for some training on what to look for with ASR. And he definitely got it. It’s uncanny how it works sometimes, but I just got out of truck, walked into the field, pulled up a leaf and there the ASR was.”
The infection rate at the Cameron County site was around 5 percent with severity at about 0.5 percent. The infected leaves are easily identifiable with many pustules. “But the whole leaf isn’t covered.”
Isakeit has been on “a whirlwind tour” of almost all the soybeans in southern Texas counties. The possible consequences of recent rains with regard to ASR have him a bit worried. “Some areas got an inch of rain just last night (June 21). That means the amount of ASR could be picking up.
“In that southern area, temperatures typically hit 95 with lows in the upper 70s. This latest front cooled things down and leads me to suspect there are new infections occurring.”
Isakeit says Texans aren’t keen to start spraying fungicides. “I’m told they’ll be scouting hard for ASR before making any spraying decisions. Part of that is the belief temperatures will rise again and be less conducive for ASR development.
“I’ll be watching the situation closely. If any south Texas producer is concerned, according to current recommendations it’s advisable to spray. It’s somewhat unusual with the rain down there. If that continues, I expect more farmers will be considering a fungicide before ASR hits mid-canopy, at least.”