By Travis Faske, Extension Plant Pathologist
There have been widespread reports of target spot (TS) on cotton across multiple cotton cultivars along the MS delta region in Arkansas. Target spots causes the lower leaves in the canopy to defoliate and the percentage of defoliation in some field range from 10 to 75%. Although there were a few isolated reports in late July, the recent prolonged rainfall has promoted disease development causing statewide concern of TS. Based on university trials, yield loss has been reported when the disease develops early in the season (at bloom) causing significant amount of defoliation (50% +) well before cut-out with yield losses decreasing as the disease develops later in the growing season as the cotton crop nears cut-out. Though fungicides are available and registered for use on cotton, fungicides do not always contribute to a yield benefit and late-season “revenge-applications” are not recommended as they provide little or no yield benefit. Producers should be aware that this disease has been in Arkansas since 2013, but predicting its impact on yield has proven to be difficult.
Target spot is caused the fungus Corynespora cassiicola, which causes leaf spots on soybean, cucumber, sweet potato, tomato, and cotton. The characteristic symptom on cotton is the formation of relatively large (1/4 to 1in. in diam.) circular to irregular shaped spots with concentric rings or a “target pattern” (Fig. 1). Conditions that favor disease development are extended periods of rainfall with frequent irrigation when conditions are warm (e.g. July through August). Infection typically begins in the lower canopy (Fig. 2 & 3) and continues to move upward as long as conditions favor disease development, but often slows down when reaching the upper canopy where leaf wetness does not persist. Some defoliation in the lower canopy can be beneficial at reducing boll rots.
Leaf spots in the upper canopy are often those caused by other fungi, which include Cercospora spp. Stemphyllium spp. or Alternaria spp. (Fig. 4). These commonly produce smaller lesions with reddish to purplish margin (Fig. 4). These fungi are often observed on plants that are weak due to a nutrient deficiency. In some fields where there is excellent boll set (as is the case for this season), the nutrient deficiencies are more pronounced and these leaf spots are more common.
Managing target spot on cotton is a multi-prong approach by utilizing resistant cultivars, crop rotation, and fungicides. Cotton cultivars like Phytogen 499 WRF are more susceptible to target spot than Deltapine 1050 B2RF. In 2015, Stoneville 4946 GLB2 had more target spot than Stoneville 4747 GLB2 in some fields. At this time of the season, TS can be found somewhere in nearly all fields. Rotation with a non-host crop is helpful to reduce field inoculum for the subsequent crop. There are a few fungicides registered for use on cotton and though defoliation may be reduced they do not guarantee a yield benefit.
Most of the information we have about fungicide use to control target spot comes from those high risk areas that have consistent disease pressure like Alabama and Georgia. In these areas, two applications of fungicides like Headline (pyraclastrobin), Quadris (azoxystrobin), Twinline (pyraclastrobin + metconazole) Priaxor (pyraclostrobin + fluxapyroxad), and Elatus (azoxystrobin + benzovindiflupyr) have been effective at suppressing target spot development, but do not consistently result in
a significant benefit to yield. The most effective fungicide timing has been at the 1st and 3rd week of bloom when leaf spot was observed at bloom. Fungicides applied at 5th and 7th wk of bloom or on-demand (when leaf spot was first observed then 2 wk. later) were similar in effectiveness to the early fungicide application. Fungicide penetration into the dense canopy will likely be of concern with these late season applications thus, adequate water is needed to penetrate the canopy. For comparison of application timings, in another university trial, 76% defoliation occurring on the non-treated plots, two fungicide applications (1st and 3rd wk bloom) reduced defoliation by 23%, while one application (3rd wk bloom) reduced it by 20%; HOWEVER, there was NO significant difference in yield from the non-treated control. So, producers should be aware that even though a fungicide may limit some defoliation in mid-canopy,it will not consistently protect the yield potential in all cultivars. Growers and consultants that apply a fungicide are strongly encouraged to leave a strip to determine if the fungicide application was useful.