I think many are over-reacting to target leaf spot on cotton. First, every field in North Carolina does not have Corynespera (target leaf spot). Thankfully it does not appear to be quite as wide-spread as we had initially feared. Based on calls, emails and pictures it appears to be more prevalent in coastal areas and northeast part of state. If it is not moving up, it is probably not doing anything to yield and it may aid in defoliation and reduce boll rot, rather than hurt yield.
Below is a picture of leaf picture sent to me. You will noticed circled in yellow a spot that looks suspicious. This may or may not be Corynespera (target spot). The only way to know for sure is to send in samples to the plant and disease clinic. Other leaf spots can cause spots that look like this. The other blemishes on the leaf look more like normal cut-out leaf spots we normally observe.
The biggest key is are the symptoms showing up on the bottom or the top of the plant? If they are starting on the bottom it is more likely to be Corynespera.
In this particular field it was described as being up and down the plant in areas about the size of a vehicle. This could be cut-out leaf spots that started from the top and worked their way down the plant due to low K or root restriction resulting in low K. It could also be due to Corynespera that has advanced to work its way up the plant.
In this case I would survey decent areas around the “bad” area and try to find if the disease seems to be coming from the top of the plant or the bottom.
Although experience with Corynespera is limited in cotton, we probably don’t need to spray every field. I would concentrate any spray efforts on fields where the disease is moving up from the bottom portion the plant and is not limited to only the bottom leaves, especially if the cotton is later maturing. If the disease is limited to the bottom portion of the plant I would just closely monitor the field. Dryer weather will hopefully stop the spread up the plant.