Conditions in west Tennessee are setting up as conducive for target spot infestations in cotton. Or maybe not.
Heather Kelley, Extension pathologist at the University of Tennessee Research and Extension Center in Jackson, says several factors need to coincide for target spot to pose a threat to cotton.
“Temperature plays a role,” Kelly said during a break from a recent cotton scout school at the nearby Research and Education Center at Milan, Tenn. “But we have had favorable temperatures in the past without target spot. Temperature is not as big a factor as moisture.”
Moisture in the canopy, she explains, is a big factor, before and after the canopy closes. “The pathogen can infect and develop very quickly” with the right combination of temperature and moisture, she says.
She says last year started off “really hot, like this year is turning out to be, and seemed to set up for target spot. But the continuous canopy wetness we had in 2016 that caused target spot to explode, didn’t occur. In 2017, fields received about the same amount of rain, but they dried out between showers.”
Kelley says cotton producers should be aware of risk factors for target spot, including:
• A history of the disease
• Cotton following cotton
• Narrow row spacing
• Vigorous growth
• High plant populations
• Hot, humid weather
• High nitrogen rates, and
• Irrigation/rain events
She warns against limiting irrigation below what the plant needs to produce yield goals as a means of managing for target spot. “Do not reduce water and limit yield potential,” she says.
Growers should “watch for it. We usually do not see target spot before the canopy closes, around first bloom. Don’t spray until you see lesions. And remember, only 20 percent or less of the time do we see target spot taking away yield. So, 80 percent of the time we see no effect on yield from target spot.”
Fungicide applications may be necessary, she adds, depending on the risk factors, including the weather forecast. If lesions are present with a forecast for more rain, growers may be wise to consider fungicide application.
“Consider spray as soon as you see lesions, but, if the weather looks good, with potential for clear days between rains events, or if no rain is in the forecast, wait.”
The most likely time to need a fungicide is from the first week to the sixth week of bloom, Kelly says. “That will vary from field to field and usually runs from the second week in July to the first or second week of August. In Tennessee, we have seen target spot in late July the last few years. The earlier we see lesions, the higher the potential for yield loss.”
Good fungicides are available to treat target spot on cotton, Kelly says. “Headline and Priaxor are at the top of our trials.”
She recommends only one application. “If we time it right, we should need only one fungicide application. If we spray too early, we might need a second. That’s why I say ‘wait until you see lesions.’”
Target spot symptoms may resemble other disease organisms. Kelly offers this suggestion on identification in a blog (https://bit.ly/2LFjA1z) from last summer: “Look for lesions with irregular concentric rings. More concerning than the lesions of target spot is the premature defoliation it causes; both lesions and defoliation start in the lower canopy.”
In the blog she lists these fungicides and rates: Elatus (5-7.3 fl oz/a), Headline (6-12 fl oz/a), Priaxor (4-8 fl oz/a), Quadris — and generic azoxystrobin products — (6-9 fl oz/a), Stratego YLD (5 fl oz/a), and Twinline (7-8.5 fl oz/a).
Additional information/images of target spot and other foliar diseases of cotton can be found on the mobile friend guide at http://guide.utcrops.com/.