In 2006, the Mid-South's first case of Asian soybean rust in soybeans is a central Louisiana sentinel plot. Announced on July 26, the ASR was discovered on the Dean Lee Research Station south of Alexandria in a Group 5 variety. The plants were infected at R-7, fairly late in the growth cycle.
After looking at plot leaf samples under a microscope, Clayton Hollier estimates 75 percent have rust. The LSU AgCenter plant pathologist says there are between eight and 10 pustules per leaf.
“The pustules were fairly new and they're sporulating vigorously. Many spores are being produced that can spread to adjacent plants and anywhere the wind will take them. And, lately, conditions in much of Louisiana have been absolutely perfect for rust development.”
Those favorable weather conditions — humid, overcast and frequent showers — are continuing and have allowed ASR “to explode.”
“I was just speaking with people in both central and northeastern parts of the state and they're being rained on, right now,” said Hollier on July 27. “So, our concerns haven't let up.”
Since the find, scouting for the disease has intensified. A 3-mile to 4-mile radius around the infected sentinel plot is currently being combed.
The LSU AgCenter is “scrambling to get the word out on ASR,” says David Lanclos, the state's soybean specialist. “I've been on the phone since early yesterday morning until late last night (July 26). Then, the calls started back up at 6 a.m. But that's great. It means the word is getting out. I hope producers continue to try and stay in touch with AgCenter personnel. Keep calling us, keep e-mailing, keep visiting.”
Fungicide recommendations should be made on a field-by-field basis, says Lanclos. There are several factors that should go into the spraying decisions:
- First, obviously, is the growth stage.
- Second, what is the yield potential for the crop?
- Third, have the fields already been sprayed?
“In many situations, crops are at R-4 but weren't sprayed with anything at R-3. That kind of situation means the recommendations, by product, can change very rapidly. It's very difficult to make blanket recommendations — spray Product A or Product B — across all your fields.”
Soybean fields are at varying growth stages and have been grown using a variety of cultural practices.
“All those factors must be considered when making a fungicide decision. But farmers should be very positive about this — the fungicides will work!
“We just need to make sure the right compound is put out at the proper rate, at the proper time. Everyone needs to make educated decisions.”
Lanclos estimates some 240,000 to 260,000 Louisiana soybean acres are in an ASR-susceptible growth stage. Those acres are mostly in south-central and southwest parishes. There are also a handful of vulnerable fields in northwest and northeast Louisiana planted behind wheat.
Prior to the Alexandria find, ASR had already been found on kudzu in southern Louisiana.
Hollier says following those sites indicates how the rust has spread.
“There were two kudzu sites south of Lafayette in Iberia Parish.
“I visited those on July 24 with a colleague who's also working on ASR. We were down there to find some fresh inoculum for his lab tests.”
A few weeks ago, Hollier estimated the first site at 5 feet to 6 feet in diameter. Now, it's at least 100 yards long.
In the same 5-acre block of kudzu, another ASR patch was only 2 feet or 3 feet in diameter originally. Now, it's much larger. “I couldn't find the exact edges of the outbreak simply because the kudzu was too high and thick. But I had no trouble finding the disease.”
Another ASR site on kudzu is in northern Iberia Parish.
“There are two patches separated by a paved road. One patch is near a sugarcane field along a ditch bank. The other is also on a ditch bank growing up into trees. Originally, the rust was found on only one side of the road and was scattered. Now, it's easily found on both sides.”
The third site is in the south end of New Iberia. “This spot had some rust in a relatively confined area — perhaps a stretch of 30 feet. Now, it's all over.
“That's happened over the last three or four weeks. Now, finding ASR in the middle of the state, logic says there should be more of it in between and, probably, beyond.”
The northern tier of the state has remained fairly dry. Hollier says the chances of ASR having made it there are “fairly slim. But in the southern half of the state, where daily showers have hit regularly, the probability of rust means a fungicide application is a good idea.”
Many producers are asking if they'll have to spray a fungicide twice. “I wish I knew,” Lanclos says from a truckbed where he just finished speaking to farmers. “But at this point, any answer would be pure speculation.”