Paul Price | 10/10/2016 6:53:33 PM
Paul P. "Trey" Price III, Myra Purvis, Hunter Pruitt, Jerry Bartleson, Dan Fromme and Boyd Padgett
Over the years in Louisiana, target spot, a foliar disease caused by the fungus Corynespora cassiicola has merely been noted by producers, consultants and specialists in cotton-producing parishes and has not been a major issue. Since 2011, the disease has been a significant, sporadic issue in Arkansas and Mississippi, and in 2014, a moderate to severe epidemic occurred in central and northeast Louisiana with the most affected areas in Franklin, Madison, Rapides, Richland and Tensas parishes (Figure 1). These severe epidemics left many stakeholders with questions concerning management of the disease: Where did it come from? Why was it so severe? Was there something that could have been done to prevent it? How much yield was lost? What if this happens again? What do people need to look for?
The following information should shed some light on target spot management so that producers will be better prepared for a future outbreak and answer the question is target spot a lion or a lamb?
Generally, symptoms of target spot will not appear in cotton until around flowering or canopy closure. The pathogen causes brown to tan, pencil eraser to dime-sized (or larger) lesions (Figure 2), beginning on the lowest leaves. As the disease continues to develop, defoliation begins in the lower canopy and progresses upward (Figure 3). This is important because the disease can easily go unnoticed if cotton is not being scouted properly and often. Mature lesions will exhibit a bull’s-eye-like appearance because of the concentric growth of the fungus (Figure 4). It is unknown if the defoliation is caused by a toxin produced by the fungus, a defensive reaction by the plant or a combination of both. The disease will rarely completely defoliate cotton plants. In 2014, however, defoliation of up to 75 percent was observed in producer fields (Figure 5). The fields looked completely normal from the field edge, but upon closer inspection, the upper canopy was merely a shell.
Rainfall, wind and overhead irrigation fuel epidemics, and the pathogen can overwinter in cotton and soybean plant debris. Soybean is an alternative host for the pathogen. No cotton varieties are completely resistant to target spot; however, disease severity may vary among varieties, presumably because of differing plant structure. Any cotton variety that is subject to rank growth is susceptible to target spot. In Alabama and Georgia, where target spot has been an annual problem since 2005, yield losses have been estimated from 60 to 280 pounds of lint per acre in severe cases. In these areas, the disease usually occurs midseason (the first few weeks of bloom), and weather conditions consistently favor disease development. Fungicide applications have been shown to slow disease development and preserve yield in the southeastern United States.
During 2014 and 2015, LSU AgCenter scientists researched fungicide efficacy on target spot in Louisiana. Over the two-year project, 13 foliar fungicide trials were conducted at three locations – the Dean Lee Research and Extension Center in Alexandria, the Macon Ridge Research Station in Winnsboro and the Northeast Research Station in St. Joseph – in an effort to provide cotton producers with key management strategies for the disease. The experimental designs of the trials were relatively simple, using fungicides with different modes of action and two different timings. Fungicides used in the studies included Headline, Priaxor, Quadris, Quilt and Topguard applied during the first and/or third weeks of bloom. Three varieties were evaluated, and disease development in the trials varied by location and variety. Disease severity, as measured by defoliation, ranged from zero to 60 percent. Disease incidence, determined by estimating the percentage of foliage with at least one lesion, ranged from 5 to 85 percent. Because of low disease pressure, some trials were not successful. In an effort to avoid redundancy and to be concise, results from two representative trials are presented here.
At Dean Lee during 2014, Headline (6 fluid ounces per acre) or Priaxor (4 fluid ounces per acre) was applied during the third week of blooming and significantly slowed target spot development compared to the nontreated controls (Figure 6). Disease incidence ranged from 35 to 85 percent while disease severity ranged from 19 to 60 percent on the final rating date, August 26. There were no statistically significant differences in yield; however, there were trends of preserved yield in plots treated with Headline or Priaxor during the third week of blooming.
At the Macon Ridge Station during 2014, in a trial with a slightly different design, Headline (6 fluid ounces per acre) or Priaxor (4 fluid ounces per acre) applied twice (first and third week of blooming) resulted in significantly less target spot incidence and severity (Figure 7). Headline applied during the first week of bloom followed by Priaxor during the third week, or vice versa, also resulted in significantly less target spot incidence and severity. Disease incidence ranged from 34 to 80 percent while disease severity ranged from 17 to 53 percent on the final rating date, Sept. 18. As with the other trial, there were indications of yield preservation at this level of disease pressure.
Weather conditions were ideal for target spot development at all three locations with much cooler and rainier conditions than normal for Louisiana during the cotton blooming period in 2014. In 2015, weather conditions during the growing season were hotter and drier than in the previous year. For instance, at Chase, Louisiana, near Macon Ridge, average temperatures from July 1 to Aug. 15 were 79.4 degrees in 2014 and 84.8 degrees in 2015 while average rainfall was 10 inches in 2014 and 2.5 inches in 2015. As a result, target spot only developed to trace amounts in the state, indicating the disease is strongly driven by environmental conditions.
Based on this research, the AgCenter has developed these management strategies for target spot:
Scouting is key. Because the disease starts low in the canopy, efforts must be made to identify the presence of target spot early. This can be done while scouting for tarnished plant bugs and other insect pests.
Watch the weather. If cool and rainy conditions prevail, target spot epidemics are likely. If weather is hot and dry, damage from this disease is unlikely.
Watch neighboring soybeans. If target spot epidemics are occurring in soybeans, conditions are also favorable for development in cotton.
Canopy management. Timely applications of plant growth regulators to cotton can reduce plant height and girth, possibly reducing the risk of developing target spot.
Timely application of fungicides. Fungicide applications during the first several weeks of blooming have been shown to slow disease development and preserve yield in other states. Results from AgCenter research indicate slowed disease development and trends toward yield preservation; however, more research is needed to confirm this in Louisiana. If applications are deemed necessary, ground applications with high water volume will ensure maximum coverage.
Don’t panic. Just because target spot has been identified in a cotton field does not mean a fungicide application is necessary. Make application decisions field by field, taking into account the stage of the crop, prevailing weather conditions, the likelihood of a return on the investment and previous experience.
Don’t seek revenge. If a target spot epidemic catches you off guard and significant defoliation has occurred, a “rescue” application will not likely deliver enough material or preserve enough foliage to make a difference.
Given observations over the past few years, target spot can be either a lion or a lamb, and, unfortunately, outbreaks are just as easy to predict as the weather.
Author’s Note: During the 2016 growing season, target spot was widespread, and moderate to severe outbreaks occurred. Frequent rainfall events during July and August fueled epidemics. Unfortunately, data from the current growing season were not available at publication time.
Acknowledgments: The Louisiana Cotton Incorporated State Support Committee and the National Institute of Food and Agriculture for their generous funding of this project.
Paul P. “Trey” Price III is an assistant professor and Myra Purvis, Hunter Pruitt and Jerry Bartleson are research associates at the Macon Ridge Research Station in Winnsboro. Dan Fromme is the Tom & Martha Burch/Delta & Pine Land Professor in Cotton Production/Genetics, and Boyd Padgett is director of the LSU AgCenter Central Region.
Figure 1. Louisiana target spot distribution in 2014 and 2015. Darker color indicates higher disease incidence and severity.
Figure 2. Initial symptoms of target spot.
Figure 3. Defoliation begins in the lower canopy and progresses upward.
Figure 4. Mature target spot lesions.
Figure 5. Severe defoliation due to target spot.
Figure 6. Effect of fungicide and application timing on target spot incidence and severity, 2014, Alexandria.
Figure 7. Effect of fungicide and application timing on target spot incidence and severity, 2014, Winnsboro.