"When is a disease a pest? When is it considered to be an epidemic causing yield loss?" asked Heather Kelly, at the National Conservation Systems Cotton and Rice Conference in late January in Memphis, Tenn.
Kelly, Extension researcher in plant pathology and IPM coordinator for the University of Tennessee, used the acronym P.E.S.T., pathogen, environment, susceptible host, and timing, the four factors necessary for yield loss to occur. Using these factors, target spot can be determined if it is truly a pest in cotton on a case by case perspective.
"P stands for pathogen, which will be influenced by your field history and location," she said. "The right environment, E, is also a factor, so most pathogens like it wet, humid, and hot. There are a couple that like it dry. A susceptible host, S, is another factor, so know your variety. Lastly, T is for time. You might have those first three factors, but if they happen late enough in the season, with target spot, it's not necessarily going to affect yield."
Late in the season, target spot might knock off the leaves of cotton that needs to be defoliated anyway.
"You have to make sure you're taking into consideration these four factors to know whether you are going to have yield loss and need to take action to avoid yield loss," Kelly said.
Target spot risk factors
Target spot is caused by a fungal pathogen. It survives in debris, so if it occurred in a field one year, there's a good chance it is going to come back in the same field the following year.
"It likes a hot, humid, and wet environment," she said. "It's going to start in the lower canopy after the canopy closes. Rarely, do you see target spot unless your canopy has already closed. All varieties are susceptible hosts, but there are varying levels of susceptibility.
"The earlier you see target spot in the season, the greater potential it has for yield loss, but again, it also is still going to depend upon the other factors: environment and variety, if yield loss will occur."
Factors that increase your risk for target spot include use of no-till or strip-till, because the pathogen can overwinter on debris.
"Target spot is going to develop quickly if you have a bunch of overcast, rainy days where the canopy never dries out," Kelly said. "High nitrogen fertility and narrow row spacing all build that right microclimate in the canopy for the disease to really explode. Your field history and variety also play a role in target spot."
The first action step, according to Kelly, is to make sure target spot is the problem. One way to check leaf spots is with a resource provided by the University of Tennessee Institute of Agriculture that helps identify diseases and pathogens. The guide is at www.guide.utcrops.com.
Data, product recommendations
In Tennessee, target spot was first reported in the fall of 2013 on cotton.
"Starting in 2014, I started timing trials," Kelly said. "We knew target spot could respond to fungicides, but we wanted to know the best time to put out those fungicides. We looked at just Priaxor, 4 fluid ounces per acre."
All the trials looked at multiple varieties, and they looked at either the first week of bloom, third week of bloom, or fifth week of bloom, using either single or double applications.
"In 2015 and 2016, I added what is called IPM timings," Kelly said. "Once we saw the lesions in the field, we would put out our first application, and then the double IPM would be following the first application up two weeks later with a second application."
2014 through 2019, they conducted additional trial work looking at product efficacy, concentrating on products being put out as a single or a double application.
"Most recently, in a regional trial from 2017 through 2019, we incorporated canopy management into our understanding to reduce target spot disease and early defoliation, and to protect our yield," she said. "From Tennessee's data, while we did significantly impact the height of our plants based on variety and canopy management, canopy management did not impact target spot development or yield loss."
Factors that influenced the percent defoliation, which is what's been correlated to yield loss with target spot, include the crop year and variety as well as fungicide, timing, and product.
"The later applications, in Tennessee from 2014 to 2016, showed better protection of our yields compared to some of the earlier timings, but the timing did somewhat vary by year," Kelly said. "The later applications tend to do better at protecting yield, though."
In single-shot versus double-shot applications of a fungicide, the data showed less of a difference between single and double application, and that a well-timed single application can protect yield.
"Statistically, our double is not different from our single application, and if you put economics behind this, you might only be breaking even on that second application versus putting out only the single," she said. "Both statistically reduce defoliation, but the bottom line is protecting your yield."
Kelly recommended combination products for good fungicide resistance management.
"Essentially, of the products I've tested, the ones with the most consistent protection in Tennessee have been Headline, Priaxor, Miravis Top, and TopGuard EQ. Interestingly, TopGuard EQ has not done as well in some of our other states as in Tennessee," she said.
Based on regional trial data from 2014 to 2016, 4% to 6% yield protection was observed, but it was dependent upon disease pressure, environment, the timing of fungicide, and variety if you needed a fungicide application to get yield protection.
"It always goes back to that disease pyramid and understanding when you need to take action and put out a fungicide," Kelly said. "Waiting to see lesions before making an application and understanding your weather going forward provides the most consistent yield protection.
"Know your pathogens, be aware of your weather, and know your variety. Then take into consideration the growth stage you're at as well as the input costs and your end price to know if you can really invest in a fungicide."