Kemerait – My Wheelhouse: disease and nematodes after bloom

2025 Georgia Cotton Commission Newsletter July

Kemerait

My Wheelhouse: disease and nematodes after bloom

Twenty-five years ago, I spoke to growers about managing plant-parasitic nematodes and seedling diseases. As tactics to reduce damage from both are applied at-planting, or very early in the growing season, there wasn’t much for a cotton pathologist/nematologist to do for the rest of the season. Back then, I would help county agents diagnose damage from Stemphylium leaf spot, Fusarium wilt, and nematodes. I would offer advice on how these problems could be avoided “next time”. We worried about boll rot then. We worry about boll rot today. Though we have more effective fungicides, this disease-complex is still a major problem. It is simply very difficult to get the fungicide deep in the canopy to the bolls with enough product and coverage to protect against rot.

Today, cotton growers have additional opportunities to protect their crop from diseases that appear later in the season. Within the past 20 years, two diseases, target spot (Corynespora cassicola) and areolate mildew (Ramulariopsis gosypii and Ramulariopsis pseudoglycines), have become of concern in the Southeast. Target spot was largely unknown until it was reported in Georgia in 2005. Areolate mildew was frequently observed in southeast Georgia late in the season but was not thought to cause much yield loss. This changed in 2017 when the disease began to spread across the Cotton Belt and yield losses were observed.

When considering management of target spot and areolate mildew, there are 10 points that I share with growers. These include:

  1. To date, no cotton varieties are available which are resistant to either target spot or areolate mildew, though some varieties may be more susceptible than others. Cotton grown in Georgia is susceptible to both diseases.
  2. Both target spot and areolate mildew can result in significant damage, and fungicides can protect against subsequent yield loss. The potential for yield loss depends upon a) when in the disease appears (earlier appearance leads to greater threat), b) the environment (these diseases are more severe in wetter years), and c) the timely use of fungicides. When either target spot or areolate mildew occur early enough in a season, losses in untreated research plots typically range from 100 to 250 lb lint/A versus those plots treated with fungicides.
  3. Neither areolate mildew nor target spot are likely to occur prior to first bloom. In fact, target spot is most likely to occur somewhere around the 3rd week of bloom while areolate mildew is most likely to occur near the end of July or in early August.
  4. Target spot first develops deep in the canopy where leaves stay wetter longer and the ambient humidity is high. Areolate mildew seems to be less dependent on humidity and leaf wetness and occurs higher on the plant.
  5. Target spot is characterized by numerous marble-sized lesions which contain concentric rings. Areolate mildew is characterized by gray, fuzzy sporulation on the underside of the cotton leaves.
  6. Target spot that appears in a crop after the 6th week of bloom and areolate mildew that appears within 4 weeks of anticipated defoliation are unlikely to affect yield.
  7. Fungicides commonly applied for management of target spot and areolate mildew include azoxystrobin, pyraclostrobin, Priaxor, and Miravis Neo. These fungicides are appropriate for management of target spot; azoxystrobin is less appropriate for management of areolate mildew as it seems resistance to this fungicide has developed in some locations in Georgia.
  8. Fungicide should be applied at sufficient pressure and sufficient spray volume to penetrate the leaf canopy to ensure good coverage.
  9. Fungicides are unlikely to be of benefit if disease has progressed to the point that 20% of the leaves have prematurely defoliated.
  10. Where crop scouts are used, fungicide applications can be delayed until one of these diseases is observed. Where scouts are not used, as a general rule of thumb, growers can apply a fungicide to manage target spot at the 3rd or 4th week of bloom. Management of areolate mildews typically occurs at the 4th or 5th week of bloom.

It would be a mistake to believe that every field of cotton in Georgia should be treated with a fungicide to protect against target spot and areolate mildew. It would be a mistake to believe that cotton growers, especially in southeast and southwest Georgia, do not need to worry about these diseases. As you consider your cotton and my wheelhouse, if you could protect 100 to 250 lb/A lint with a single, judicious fungicide application, would you do it? If nothing else, you should be aware of the opportunity now to protect your cotton from losses to these two diseases. Many thanks to the Georgia Cotton Commission for this opportunity.