Sunday, July 25, 2021

The Race Against Leaf Spot

Aggressive protection is needed as defoliation happens quickly.

• By Amanda Huber •

leaf spotHave you ever watched an advanced level track event, such as the Olympic 100-meter dash? The athletes spend a lot of time warming up on the track, stretching, fixing the blocks, getting into their starting stance, and even at times standing up and starting the set-up process all over again.

Why spend that much time setting up for the race? Obviously, the runner must get off to a good start to have chance to win. The same can be said for the race against leaf spot. Without a good start, yield will be lost.

Three Weeks To Defoliation

Why is a good start needed? Because leaf spot can quickly defoliate a peanut plant. North Carolina State University peanut agronomist David Jordan took pictures of the defoliation of an untreated check plot to demonstrate how fast yield can be lost.

“The untreated control plot shows how quickly peanuts can go from good canopy coverage to defoliated. On Aug. 23, the crop shows a few spots coming in, and by Sept. 1, those spots are moving up the canopy.

“One week later, it is amazing to see the level of defoliation on the plant. By Sept. 16, it is nearly completely defoliated, all in a three-week time period. Things can go downhill fast.”

Leaf Spot By The Numbers

Jordan says if 20% of the leaves have visible lesions, then fungicide sprays are no longer as effective. At 40% leaf defoliation, there will be measurable yield loss.

“Our fungicides are more of a protective than a curative. If you get to 20% lesions in the bottom of that canopy, you are in a battle with how quickly defoliation occurs.

“You can go from 20% lesions to 80% within two weeks, and that’s why fungicide applications early in the season, maintaining those spray intervals and being intensive with the spray schedule as much as possible is important to protect yield.

Jordan says one reason for a solid spray program is that it increases the flexibility with digging. “We need to make sure we have the crop protected to maturity. We can’t dig everything at the same time, even though they often are ready at the same time. Vines need to be protected so that there is more flexibility for how long we can leave peanuts in the field to reach optimum maturity.”

The Key Is Early Protection

University of Georgia Extension plant pathologist Bob Kemerait says producers should know the susceptibility to leaf spot of the variety they are planting. “Some newer varieties have more tolerance to leaf spot.” He also says the in-furrow product Velum gives a boost to leaf spot control.

“If you had problems with leaf spot in 2020, go back and look at your spray dates. What was the interval between sprays?

What products were you using?

“We have quite a few premium leaf spot fungicides out there. However, the backbone of a leaf spot program remains chlorothalonil. It is not the best product by itself, but if we tankmix it with another product, you can expect continued protection against this pathogen.

“Use the premium products earlier in the season and then the chlorothaonil tankmixes. Used effectively, you probably won’t get into a problem with leaf spot later in the season.”

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