Kemerait: In the World of Diseases and Nematodes, as One Season Ends, the Next Season Begins

As September dwindles and October looms ahead, Georgia’s cotton fields are being defoliated and picking has begun.  For most growers there is relief that they have neared the end, the final laps, the short-rows, and the finale of the 2022 season.  But in truth, when it comes to diseases and nematodes, as the 2022 season ends, planning for the 2023 season should already begin.

There are eight primary disease and nematode issues affecting Georgia’s cotton fields now, though each is not likely present in every field.   There is not much to be done about these problems now, unless you are more than a month away from defoliation in which case management of areolate mildew still matters. However, growers should remain vigilant as harvest approaches and to note where symptoms of diseases and nematodes occur in their fields.  Important symptoms include significant stunting and poor growth and widespread pre-mature defoliation.  Recognizing and understanding the causes behind problem areas in the field at the end of 2022 make it possible to improve management practices in 2023.  Recognizing these problems as they affect yield now will allow the grower to better protect yield in 2023.  Listed here are issues in cotton fields at harvest for which planning for 2023 should begin now.

  1.  Stemphylium leaf spot is present in most, if not all, fields and is identified by small-to-moderate sized lesions, often encircled by a dark, purple ring, most prevalent on leaves showing signs of nutrient (potassium) deficiency.  Stemphylium only occurs in conjunction with a potassium deficiency in the plant and this disease can lead to rapid defoliation and significant yield loss.  Stemphylium leaf spot is a very important problem in Georgia’s cotton fields and may be overlooked as growers have either become too familiar with it or do not think that there is much that can be done.  Stemphylium leaf spot typically occurs in the same areas of a field year after year- sandier areas, sometimes infested with nematodes.  Grower should take special steps to manage soil fertility (and nematodes) to reduce losses to this disease.  Fungicides are NOT effective in the management of Stemphylium leaf spot.
  2. Target spot often leads to rapid and significant premature defoliation in a cotton crop.  This disease has been less problematic this season than I had expected, despite extended periods of wet weather.  This may be because cotton growers in areas most affected by target spot and areolate mildew are now more likely to use preventative fungicide applications than they have in the past.  Use of fungicides is not always profitable if the level of target spot is low because of hot and dry conditions.  However, I believe most growers who protect their cotton crop with fungicides in a wetter season like this has been will see economic benefit in doing do.
  3. Areolate mildew has been problematic again in the cotton production region of Georgia, though slower to develop in 2022 than in 2021.  Data from field trials conducted by UGA Extension demonstrates that where areolate mildew occurs early enough in the season, judicious use of fungicides increases yields by as much as 400 lb/lint per acre.
  4. Bacterial blight became established in some fields very early in the season on varieties known to be “susceptible” to this disease.  Statewide, bacterial blight has been a minor issue in 2022, demonstrating that the development and spread of a disease can be difficult to predict.  Growers are reminded to be careful in their selection of varieties for 2023 as resistant varieties are THE most important measure for managing bacterial blight.
  5. Fungal boll rots have been devastating in some fields in Georgia this season, especially in fields with excessive, rank growth.  Growers are understandably frustrated at the losses, and even more frustrated at the fact there is little to be done to reduce the threat from boll rot.  Fungicides are not an effective management tool for control of boll rot.  We in UGA Extension continue to see to develop improved recommendations for management of is complex of diseases.
  6. Fusarium wilt is becoming an increasing problem in some of Georgia’s cotton fields.  This disease is recognized by stunting of the plants, “tiger striping” on the leaves, and brown discoloration of the vascular tissue of the lower stem.  I don’t know if this is because the problem is spreading or simply because growers are paying greater attention to it.  Nonetheless, Fusarium wilt can ONLY be managed in our fields by managing the parasitic nematodes associated with it, often by treating the field with a nematicide.
  7. Nematodes in general (root-knot, reniform, sting and lance) continue to be a significant problem in our cotton fields.  Growers are encouraged to make time after harvest and before cold weather hits to take soil samples from areas of poor growth in order to determine if nematodes are indeed a problem.  Growers can also look for the tell-tale “tiger striping” on leaves of affected plants that give good indication that nematodes are present.  In addition to use of nematicides to protect cotton from nematodes in the 2023 crop, growers also can select cotton varieties that are resistant to the southern root-knot and the reniform nematodes.
  8. Cotton leafroll dwarf virus continues to be present in fields across Georgia, though typically of minimal importance.  With the exception of extreme symptoms found on some varieties, such as DG 3615 and DG 3799, this viral disease can be a challenge to confirm based upon visual observations only.  I cannot explain why CLRDV is not more of a problem in our cotton, but it is good that it is not.

Recognizing that one of more of these eight disease and nematode issues are present in your field now may not add value to the 2022 crop, but could help you to make more effective management decisions for 2023.  Effective management of bacterial blight, Fusarium wilt and nematodes occurs before the furrow is closed.  Management of Stemphylium leaf spot requires attention to soil fertility throughout the season.  Management of target spot and areolate mildew could begin as early as the first week of bloom in 2023.  Sadly, there is little to done to manage boll rot.

Bob Kemerait

UGA Extension Plant Pathologist