Target spot (Corynespora leaf spot), he says, starts out as a grayish, water-soaked lesion on the leaves in the lower mid-canopy. “Usually, you’ll start to see it in the second, third or fourth week in July in Alabama. It progresses to a number of leaf spots on the leaves, then a little coalescence of the lesions, and finally the leaves fall off. It’s not uncommon on some varieties to see 75-percent defoliation by Sept. 15. It can really knock the leaves off of cotton,” he says.
This past year, researchers started looking for disease in cotton around the third or fifth leaf stage in early to mid-June, says Hagan.
“We saw some leaf-spotting out there in central and north Alabama and down in Fairhope, and we thought target spot might already be in the cotton. It turned out it was actually Aschochyta leaf spot which is known to be an early season leaf spot disease on cotton and usually doesn’t cause a problem. But, we had a lot of rain this (past year) in May and June. If you stand there and look at it without a microscope, you’d have a hard time telling a difference between the two of them.
“And down in Fairhope, there was another leaf spot – Myrothecium – which is identical in appearance target spot. This never has really developed into a problem. It’s just an occasional spot out there. This was in early cotton, sometime in June. Target spot took off at mid to late summer, so we know it’s a mid to late-season disease in cotton. It’s not showing up – so far – on seedling cotton.”
In 2012, target spot symptoms started showing up the last week in July on Phytogen 499 and DPL 1050, and the disease progressed until the middle of September, with 80-percent defoliation on the cotton, says Hagan. This past year, the progression was pretty much the same on Phytogen 499 and DPL 1252. Growers probably won’t really notice the disease until sometime around the first part of August, he adds
“The other big question with target spot is does it cause a yield loss? To some degree, if you look at target spot in the middle of September and you’ve lost 50 to 75 percent of the canopy, it actually helps you defoliate the cotton. So if it’s late enough, it might not be much of an issue.”
In the past, growers have had leaf spot diseases in cotton and no one has been concerned about it because they haven’t been associated with yield loss, he says.
“Well, this one has been association with a loss of yields. We’ve seen significant yield losses in 2012 and in 2013 as the disease intensified.”
So what is the yield loss? “On a variety like DPL 1050, you’re probably talking about 50 to 100 pounds per acre. It would be hard to recover that yield with a fungicide. But with Phytogen 499 and to some extent DPL 1252, you’re talking about 300 to 350 pounds per acre. Some of the folks in south Georgia are saying that it’s taking up to 600 pounds on Phytogen 499. At the current price of cotton, that’s a loss in value of about $480 per acre.”
The disease appears to be worse along Alabama’s Gulf Coast, says Hagan. “From what we saw in the trials that were conducted this past year, not much happened in the Tennessee Valley. There was target spot in some of the cotton up there but not much. At least from the first year of observation, it may or may not be an issue in that region. It’ll probably show up there, but it probably won’t have an effect on yields.”
Target spot is a moisture-driven and probably a heat-driven disease, says Hagan. “We had less rain in 2012 than we did this past year. I would have thought we would have seen more target spot in 2013. I think the reason we didn’t see as much is because it was cooler this past year than in 2012. The fungus that causes the disease is a subtropical pest, so it might be more active when it’s hotter.”
Target spot develops quickly after canopy closure, he says. It needs the moist environment in a cotton canopy before it can develop. The tighter the canopy, the more likely it is that the disease will develop, so it is a disease of rank cotton, says Hagan.
“So far, our trials really haven’t shown a rotation effect. It’s not worse in continuous cotton than it is behind cotton planted behind corn or peanuts. I’m not sure about tillage. There’s some question about innoculum carryover. I would guess that if you had continuous cotton in strip-till or no-till production, it would probably be a situation where we could see the highest risk, but I’m not sure whether or not we can show that.”
Late-planted cotton is less vulnerable to the disease, he continues.
“We had a fair amount of double-cropped cotton, particularly in the southern part of the state, and when I looked at it in late September or early October, there wasn’t very much target spot. It’s probably not a good option from a management standpoint to plant so late, but that’s what we saw. The losses decline when you go from the southern part of Alabama up to the Tennessee Valley.”
Management options limited
Management options currently are limited because cotton varieties tend to differ in their susceptibility to target spot, says Hagan. Phytogen 499 tended to have the greatest defoliation and the highest risk for a sizeable yield loss, he says.
“The flip side of that is that Phytogen 499 tends to be at the top of the yield list anyway, despite target spot sometimes. It may be a pretty good strategy to go ahead and spray if you’re planting this variety.”
Most DPL, Stoneville and Fibermax varieties are moderately susceptible and/or tolerant but can suffer sizable yield loss under ideal conditions, he says.
“We really don’t know what the yield loss might be on those varieties from target spot. In general, none of the varieties available today have a high level of resistance. They differ some in their sensitivity, but that’s about it.”