Target spot is by no means a new disease of cotton, but it’s certainly attracting the attention of more producers as it spreads throughout the Southeast, says Austin Hagan, Auburn University Extension plant pathologist, speaking at the recent Central Alabama Row Crops Workshop in Autaugaville.
Target spot, which is caused by the fungus Corynespora cassiicola, was first noted on irrigated cotton in southwest Georgia in 2003, says Hagan, and lint losses due to the fungus have been estimated at 200 pounds per acre.
“In some instances, defoliation may occur so late in the production cycle that the disease served more of a harvest aid and had minimal impact on yield. But starting in 2011, target spot outbreaks were noted statewide in Alabama cotton,” he says.
While target spot is found in dryland cotton, heaviest leaf spotting and defoliation has been seen in irrigated cotton, particularly when strip- or conservation-tilled, notes Hagan.
In 2012, disease outbreaks were observed in cotton in the Florida Panhandle, South Carolina, North Carolina and Virginia.
The disease was observed beginning in 1959 on cotton in Mississippi and may have been present in the Mississippi Delta for 25 years prior to that observation, says Hagan.
“While the mechanism for the rapid spread of target spot in cotton has not been determined, the fungus is transmitted through seed of soybean and sesame. While seed transmission may account for the rapid spread of target spot into Alabama and up the eastern seaboard, the fungus may have moved from soybean or vegetables into cotton.
The spotting, says Hagan, follows a different pattern than that seen in potash-related leafspot.
“We do have issues with potash-related leafspot in cotton, but those are not target spot. There are several fungi associated with these leafspots, but they’re more associated with a potash deficiency or a combination of drought, rapid boll set, or a lack of uptake of potash or other minerals by the plant.”
Potash-related leafspot is mostly an issue on high-yielding, early maturing varieties, particularly on sandy soils where the minerals tend to move quickly through the soil profile, says Hagan.
“It seems to be triggered by moisture stress and rapid boll set. If the plants aren’t fruiting heavily, they don’t show the symptoms. But when they set a lot of bolls, you start to see the yellowing in the canopy and the spotting, and then the leaves fall off.”
There are fungicides registered for the control of these types of leafspot, he says, but research from the University of Georgia has never shown significant yield response. But when potash is applied earlier in the season, a positive response is seen.
With target spot, says Hagan, the plant starts shedding the older leaves and it moves upward through the canopy.
“It follows a classic leafspot pattern like we see in peanuts, starting at the bottom and moving up, towards the shoot tips. These spots are large, discreet and usually circular. The color can vary, but usually they have a tan center and a darker outer ring.
“If you look closer, you see a concentric ring that’s consistent with this leafspot disease. Occasionally, if it’s really wet outside, you might see some type of leaf blighting.”
In the last seven years, says Hagan, the epicenter of the target spot problem has been in southwest Georgia on irrigated cotton.
“It was probably in Alabama before 2011, we just didn’t look very hard prior to that time. We’re not sure why it has moved so quickly over such a short period of time. The type and size of this fungus won’t move in thunderstorms or in air currents over long distances. It also goes to tomatoes and cucumbers, so it could have moved from vegetable crops.”
Looking at time frame
Researchers continue to look at the time frame that the leafspot and defoliation develop, says Hagan.
“I had two trials where I rated cotton over an extended period of time. In the central Alabama study, the spotting probably started about mid-July, and the cotton was about 55 days old at that time.
“On the Gulf Coast, the leafspotting got started at the end of July, and this cotton was planted in early May, making it 75 to 80 days old, so we really haven’t pinned down the time frame of when the leafspotting begins.”
Studies conducted this past year were intended to help determine the type of yield loss that might occur due to target spot in cotton, says Hagan.
“We used Phytogen 499 and Deltapine 1050, which we identified in 2011 as being susceptible and maybe a little less susceptible, respectively, to this disease in a variety trial. At the end of the season, just before the cotton was sprayed with a defoliant, we had about 80 percent leaf shed in the untreated Phytogen 499 versus about 50 percent in the Deltapine.”
It’s very hard to define how much yield loss is occurring, and it’s hard to set up a trial in order to get the high level of disease control needed to show that, he says.
The estimate was made of 250 pounds of lint per acre, on average. Some consultants in Georgia suggested that the yield loss in some cases was up to 600 pounds per acre. These, however, are rough estimates, says Hagan.
“We do know that target spot does reduce the yield of cotton,” he says.
“On Phytogen 499, as the disease intensified, yield went down. We need to more specifically determine what the yield loss might be. On the Deltapine variety, as the disease intensified, yield did not go down.
“As we increased the number of Headline applications on Phytogen 499, we got a 100-pound increase in yield with each successive Headline application. By the time we got done with five Headline applications, we came up with a 600-pound increase in seed cotton yield.
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“With DPL 1050, we also got a significant yield gain as the number of Headline applications increased. With that variety, it was an increase of about 60 pounds of seed cotton per application.”
There are risk factors to be considered in target spot incidence, says Hagan.
“There is some difference in sensitivity among cotton varieties to this disease. You’d think that crop rotation would have a major impact with this pathogen, but that’s still up in the air.
“When we’ve gone out and looked at problems, it seems like the worst incidence of the disease has come in fields where cotton followed cotton in strip- or no-till production. We also need to look at the influence of tillage on this disease.
“The other factor is canopy moisture. You can’t look at higher defoliation and say that will be the higher or lower yielding variety.”
In fungicide trials, the only material that significantly increased yield over the non-treated control was Headline at 9 fluid ounces, says Hagan. Cotton was sprayed at first bloom and then 14 days later, and that’s a standard preventative treatment, he says.
Did get defoliation reduction
“We did get a reduction in defoliation with Headline and Twinline. But we need something else in the mix other than strobilurin fungicides and we don’t have it, at least not a really good one.
This year, we’re going to try a five or seven-spray program on cotton with Bravo, just to see if it works. We may at some point need a broad-spectrum fungicide for this disease.”
Based on research thus far, the recoverable yield or what you can protect in cotton probably will be in the range of 100 to 200 pounds of lint per acre if you use a fungicide, says Hagan.
“You’re limited to two applications of whatever is on the market, because there isn’t an option with the chemistry. If we overuse them, they may not work at all.
“We may already have tolerance or resistance to strobilurin fungicides, and there’s nothing in the pipeline for the next couple of years. Work is being done on a cotton registration package for Fontelis, but those take awhile.”
In 2012, Alabama researchers had five rotation patterns at the Wiregrass Substation that included cotton for 10 or 15 continuous years, says Hagan, and there was no cotton cropping frequency impact on the incidence of target spot, at least in small plots.
“In our variety trials on the Gulf Coast, we were planting cotton for the third consecutive year in one plot, and the other plot was planted in cotton for the first time in several years. The amount of disease was the same in both areas.”
Target spot is moisture-driven — the wetter it is, the worst it will be, particularly when you start at flowering or at pinhead square and go through the end of the season with such conditions, says Hagan.
It may attack seedling cotton, but it won’t make much difference, he adds.
“What are the management options? At least in dryland, you might plant one of the varieties that is a little less sensitive to target spot. You might sacrifice on the yield side, but you could have less disease. If it’s irrigated cotton, where you’re talking about a high yield potential, that’s where I might use a fungicide.”
Looking at fungicide use on dryland cotton, Hagan says he would scout at first bloom and apply at the first sign of disease. If there’s heavy defoliation, there’s probably not a reason to treat late, he says.
“It might help to have a very dense wheat or rye cover, roll it down, and then strip-till into that cover. The heavy leaf cover from a small grain probably will interfere with the splash dispersal of the fungus.”
The question remains, says Hagan, as to whether the use of strobilurin in soybeans has contributed to the poor fungicide performance in cotton. Or, whether or not it’s a coverage problem.
“Do we need to put drop nozzles in the middle of the row and spray into the lower canopy to get Headline down into the bottom of the plants? We were spraying over-the-top this past year. We’ll continue to look at this.”
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