Cotton fungicides — tweaking

Growers are still learning to use fungicides on cotton, and the system will require some tweaking, says Jack Royal, a southwest Georgia cotton consultant.

“We’re continuing to look at fungicides to see if they work,” said Royal at the recent Beltwide Cotton Conferences held in San Antonio. “If we can make our growers $1 per acre more after all of his expenses, then it would be worth the trip. We’re all in the same boat — if the growers can’t stay in business, we can’t stay in business.”

Royal, who has been in the crop consulting business for 31 years, says he has seen cotton make a comeback in his area. “Luckily, we can grow a variety of crops. We’re about 70-percent irrigated. We still plant corn and we have a good many peanuts in our area. We don’t plant a lot of back-to-back cotton — we might plant two years of cotton, then a year of peanuts, or we put a year of corn in there with the peanuts. That’s basically our rotation,” he says.

Royal says there weren’t many cotton disease problems in his area of Georgia last year, but that wasn’t the case throughout the state.

“In the southeast region of the state, there were problems, and there’s no doubt foliar diseases can be a problem. At times, they can be isolated. And like in southeast Georgia in 2008, some years are worse than others and some fields seem more susceptible than others,” he says.

It’s most likely, he adds, that weather patterns drive disease development. “On the east side of the state, conditions were extremely dry, and growers were irrigating a lot. They also got a hurricane earlier in the season. Overall, diseases seemed much worse in that part of the state,” he says.

The major foliar diseases of cotton are cercospora leaf spot, alternaria, stemphylium and aerolate mildew, he says.

The severity of the problems with these diseases varies, says Royal. “In my area, 95 percent of the cotton is planted in DP 555. A few spots can develop into a real problem,” he says.

The big question, he says, is do fungicides work on these diseases? “I don’t think there’s any doubt about that. When we compare untreated fields with fields treated with Headline at 6 ounces — where diseases are a problem — it’s obvious fungicides do work. We see a yield difference in the treated fields of about 370 pounds per acre,” he says.

It’s pretty well documented, says Royal, that a potassium deficiency can drive disease development. Potassium deficiencies cause weak cells in the plant that encourage the spread of disease.

In soil and petiole samples taken after severe disease development had occurred, there was significant difference between potassium levels in the soil between the treated and untreated fields, says Royal. However, there was a significant difference in the potassium levels in the petiole samples.

“This raises a lot of questions,” says Royal. “Did the treatment of Headline help with the uptake of potash from the soil? Did diseased foliage affect potash uptake? Why was the potassium level higher in the treated side than in the untreated? You have to start off with enough fertilizer potash in cotton, and if you cut fertilizer, you’ll cut yields. In my area, we run from a Greenville soil all the way down to an Americus soil. Since we soil test each year, and we put out our soil amounts, we always trim about 20 to 30 pounds off.

“If it calls for 100 pounds of potassium, we’ll put out about 70 to 80, and when we come back with our nitrogen at first bloom, it’s become a habit with our growers — and it seems to help especially on our deep sandy soils — to side-dress all of our cotton with 20 to 30 pounds of potassium in with our nitrogen before first bloom, and that usually carries us.”

Royal says he used to pull petiole samples on every field, using 200 or 300 petiole kits each year. “Over the years, we probably had less than five that came back with low potassium, and usually it was with heavy rain events and in very sandy soils. Some growers might be trying to cut back, and that’s the reason for the difference in the petiole uptake.”

In trials conducted in east Alabama last year, there was a slight increase in yield in fields treated with Headline, but it wasn’t enough to pay for the application, says Royal.

Farmers need to ask themselves if there is a value in using fungicides on cotton, he says. “There definitely is if you need it — if you know you’re going to have a problem. I don’t know which conditions need to occur to make the application cost effective. We do know that fungicides must be used preventatively. It’s like buying car insurance — you’ve got to have it because you don’t know whether or not you’ll need it. The trick is knowing when.”

Growers are still learning to use fungicides on cotton, says Royal.

“We’ve learned to use fungicides on grain and on corn. Eight years ago, I sprayed corn for the first time, and no one else was spraying corn. But we knew rust was coming in from the South. It made us look like a hero with our growers, and for several years after that, we didn’t need to spray corn.”

As far as the growers in his area, Royal says they’ll be planting DP 555 for two more years. “We know we can get the seed this year, and if we can get it in 2010, we’ll plant it. We don’t know if the new varieties will be different — there are a lot of unanswered questions. Right now, I can’t go to my growers and suggest they spray every acre with a fungicide, based on the data I’ve seen. We’ll continue to look at it, and see if we can find a niche.”

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TAGS: Cotton
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