By Bob Kemerait and Simer Virk
With the arrival of October, cotton growers in Georgia have long anticipated the start of harvest and preparation for the final days of the season. Understandably, much of the focus now is on defoliation and in getting the cotton out of the field and to the gin. If the crop is within four weeks of defoliation, there is little need to further protect against foliar diseases; if a crop is more than a month off from defoliation, then some attention should be given to areolate mildew which may (or may not) be present.
While it will be quickly too late to further protect the 2021 crop from diseases and nematodes, there are important steps that should be taken now in preparation for the 2022 crop. Later in the season, especially while leaves are still on the cotton plants, is a good time to identify problems that have led to “weak” spots in the field, premature leaf drop, and even boll rot. While there is very little that can be done to protect a cotton crop against fungal boll rots, it is still helpful to identify the important pathogens that were responsible for the damage to the bolls and to differentiate the rots they cause from bacterial boll rot. Below are signs and symptoms to look for now in a cotton field and recommendations for next season.
- “Weak spots” in a field, often characterized by stunted and even dead plants, are frequently associated with plant-parasitic nematodes (southern root-knot, reniform, Columbia lance and sting). Submitting soil samples collected directly from the root zone now, or shortly after harvest, and submitting them to a nematode diagnostic lab allows the grower to A) determine if indeed nematodes are the culprit and B) what type of nematodes are present. Such information is critical for 2022 in that it helps to determine best crop rotation, best variety choices (e.g., root-knot and/or reniform resistance) and need for nematicides. Fusarium wilt is also of increasing importance in Georgia and can be best assessed by collecting a soil sample for nematode analysis AND by examining the interior of the lower stem of the affected plants for characteristic “vascular” discoloration. Fusarium wilt can best be managed through crop rotation and use of an effective nematicide. Lastly, dead and dying plants occurring in spots in a field may also be caused be a disease known as “charcoal rot” caused by the fungus Macrophomina phaseolina. Charcoal rot can easily be confirmed in a disease diagnostic clinic. While there is not much that can be done to manage this disease during times of crop stress, identifying the problem can help growers avoid other treatments, such as use of nematicides, that will not help.
- Premature defoliation has been caused by several important diseases in 2021. These include target spot, areolate mildew, Stemphylium leaf spot, Cercospora leaf spot and, to a small degree, bacterial blight. Premature defoliation does not always cause a loss of yield, but these diseases can, when they occur early enough and are severe enough, can increase losses at harvest. Stemphylium and Cercospora leaf spot diseases are a direct result of insufficient nutrients IN the plants, especially potassium. They typically occur in the same spots in a field year after year. Efforts to better manage soil fertility and irrigation in those areas can reduce the risk to both diseases. Target spot and areolate mildew can both be managed with timely and judicious use of fungicides. When they occur late in the season no treatment may be needed as the defoliation is too late to affect yield and may also improve air circulation and reduce bolls rots. Growers are encouraged to identify the cause of foliar diseases in the field so that a) they can scout for them early in the 2022 season and B) so they can deploy effective and appropriate management strategies. For bacterial blight, this would be selection of resistant varieties. For fungal diseases it would be whether use of appropriate fungicides or greater attention to soil fertility issues is the key management strategy.
- Boll rots are a challenging problem, especially when rainfall has been abundant later in the season. Boll rots are especially frustrating to farmers because there is very little that can be done to protect against them. A first step in management for 2022 is to identify the causes of boll rot in a field. If the boll rot is primarily caused by bacterial blight (Xanthomonas citri pv. malvacearum) then it is important to consider planting a resistant variety next season. Impact of all boll rots, whether caused by bacteria or fungal pathogens, can be reduced by managing the cotton crop to increase air flow and reduce humidity within the canopy and to manage insects, such as stinkbugs, that can damage the boll and allow introduction of pathogens and organisms that further rot the bolls.
Attention now to troubled spots and problems in the field may not make more cotton in 2021, but such efforts could significantly increase yields in subsequent seasons.