The Mid-South wheat crop is large, and through late March it appears headed towards a bountiful conclusion. But leaf rust is building in Louisiana fields and will likely soon necessitate a region-wide fungicide application on susceptible varieties.
Already, though, fungicides are being — or have been — applied in some fields. Extension specialists and university researchers in Louisiana and Arkansas are worried many of those early applications were unnecessary.
“Any kind of blanket advice on early fungicides in wheat should be questioned,” says Boyd Padgett, LSU AgCenter plant pathologist. “That doesn't mean there aren't rare situations where it's necessary. But farmers should know early applications aren't even close to automatic.”
Padgett has heard from farmers being advised “to apply fungicides for a plant-health effect. I've gotten enough reports from farmers to be concerned about the message being put out there.
“In the absence of disease it's particularly (questionable advice). We certainly don't recommend a fungicide without disease. If disease is present, there may be a cause to spray a fungicide early. But that's strictly on a field-by-field basis.”
In Arkansas, Scott Monfort has heard “a lot of talk” about farmers with wheat around growth stage 6 — “beginning of stem extension with first node of stem visible” — applying fungicides at reduced rates. “We don't recommend a fungicide go out until growth stage 8,” says the Arkansas Extension plant pathologist. “Between growth stage 8 and growth stage 10 is where we see the most benefit from a fungicide.”
Typically, the Arkansas wheat crop will be at that stage around mid-April. But some very early-planted wheat may be there already.
“Regardless, spraying fungicides early, or at cut rates, isn't a wise practice. Some folks are telling farmers to put out a small rate very early at tillering and then come back with a later shot. That just doubles the input cost for little proven benefit.”
And it isn't as though the wheat crop won't have to be sprayed with a fungicide soon. However, so far, there's been only one confirmed finding of stripe rust east of the Rocky Mountains. “That's in some Chicot County (in extreme southeast Arkansas) research plots planted extremely early,” says Gene Milus, University of Arkansas professor and wheat pathologist. “That isn't to say there isn't more stripe rust around. But that's the only confirmed incident.”
Most Mid-South wheat varieties have some resistance to leaf and stripe rust. In most of those, “you'll be able to push the spray date into growth stage 9 or growth stage 10,” says Monfort. “That's true unless you have big hotspots of stripe rust or very severe cases of leaf rust.”
All interviewed are especially concerned with reduced-rate fungicide applications.
“Farmers need to use these fungicides judiciously,” says Padgett. “Use them only when disease is present. Making an application when disease isn't present — especially in a resistant variety — makes no sense.
“I don't like reduced rates of fungicides. There are some isolated situations where that might be an option. But even in those I try to avoid it because of resistance issues.
“When we begin using less than the labeled rates it encourages the development of resistance. I wouldn't put on a reduced rate unless I just had to.
“If possible, we're trying to encourage growers to wait for flag leaf emergence before applying a fungicide. And some Louisiana wheat is already at flag.”
Back in Arkansas, Monfort is on the same page. “Unless you planted a really susceptible variety that is having major issues with leaf rust, our stance is to wait until growth stage 8, or later. I haven't seen very many fields that needed to be sprayed.”
Outside stripe rust, there's no compelling reason to spray a fungicide early, says Milus. “If you think you need to spray for other diseases, the earliest stage I'd recommend is flag leaf emergence. And the optimum time is around early boot stage — and outside the Chicot County plots, I doubt much wheat in the state is at boot.”
If a farmer planted a variety susceptible to stripe rust, “it wouldn't hurt to start scouting it now. If stripe rust is found, the fields need to be sprayed. The most at-risk fields are susceptible varieties planted early.”
Later-planted fields largely escaped fall leaf rust infection, says Milus. “That's very clear with how the leaf rust has acted this year. There are adjacent fields with the same variety planted about a month apart — the early field has a lot of leaf rust and the later field has none.”