I recently followed a thread on social media where the need for applying fungicides to cotton for control of target spot was debated. As you might imagine, opinions were varied from “yes we are going to spray” to “spraying fungicides on cotton doesn’t make us any money.”
As the contributors to the conversation farmed cotton across the United States, it was not surprising that opinions differed and that farmers most likely to use fungicides were in the Southeast and those less-likely were in the West.
My stance for management of target spot (and areolate mildew) continues to be that not every cotton grower in Georgia needs to treat every season, but that every grower needs to assess their situation carefully and that use of fungicides can protect yield and improve profits.
Losses to Target Spot.
From our research here in Georgia, it is documented that target spot can easily reduce yields by 150-200 lb lint/A. In some situations, the amount of lint lost to this disease may be higher, in other situations less lint may be lost.
The factors that determine how much yield loss will occur is tied primarily to
- time at which infections occurs (earlier infection results in greater yield loss),
- environmental conditions (wetter years increase opportunities for development of disease and also lead to better crop growth that favors prolonged leaf-wetness periods for infection),
- yield potential (fields with higher yield potential tend to be at greater risk to target spot),
- rotation (cotton-cotton rotations likely increase risk to target spot), and
- location (cotton in southern Georgia has significantly greater risk to target spot than does cotton in northern Georgia).
Varieties may also differ in susceptibility to target spot, though all cotton varieties are susceptible to one degree or another and may benefit from use of a fungicide.
Management of Target Spot.
The most effective way to manage target spot is with the use of well- timed fungicide applications. I recommend that growers begin to scout for the classic marble-sized, target-like lesions in the LOWER CANOPY of the crop beginning at the first week of bloom.
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Note: Stemphylium leaf spot, which results from deficiencies of potassium in the cotton plants, can be mistakenly identified as target spot by the untrained eye. While spots associated with target spot are initially found in the lower canopy of “good” cotton, Stemphylium leaf spot is found across the entire canopy of “poorer-growth” cotton that often has red and yellow coloration. This is important because use of fungicides DOES NOT protect a cotton crop from Stemphylium leaf spot.
If target spot is not found in the lower canopy at the first week of bloom, growers are encouraged to continue scouting for the disease on a weekly basis. If the disease has not been found by the 6th week of bloom, then it is likely that the crop will not need a fungicide application, even if target spot appears later.
However, if target spot is observed within the first six weeks of bloom, and where there is favorable weather for disease development and good yield potential, then use of a fungicide is likely warranted. The “best” time to apply a fungicide occurs when the initial symptoms of target spot appear in the field.
Growers who are not able to scout should consider a fungicide application at the 3rd week of bloom and possibly a second application 2-3 weeks later is conditions remain favorable for disease development and spread.
Note: Target spot can be an explosive disease when conditions are favorable (i.e., prolonged periods of leaf wetness). In a worst-case scenario, 80% defoliation can occur within two weeks after initial symptoms. Therefore, if fungicides are to be beneficial, they MUST be timely.
If a field has already lost 50% of its foliage when the disease is observed, then there is little reason to treat with a fungicide.
Fungicides for controlling target spot.
Below is a chart of fungicide labeled for use on cotton. The most effective fungicide for management of target spot is Priaxor; Miravis Top has also been a very good fungicide. Headline is likely our next best option for control of target spot; followed by azoxystrobin products such as Quadris and AzoxyStar. Elatus, Proline, and TopGuard are also effective, but I have less data on them.
Regardless of choice of fungicides, timing of application is absolutely critical.