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Cotton Leaf Spot Severe

Target spot/Corynespora leaf spot disease has increased in recent weeks and many fields in the county have been treated at least once with either Twinline or Headline fungicides.  The frequent showers we’ve been getting appear to be a critical trigger for disease development and spread. During the past week target spot has caused over 50% defoliation in some older planted fields. From what we’ve seen here, target spot seems to be worse in older planted, rank fields, regardless of the variety. It has generally started at the bottom of the plant and worked its way up the stalk. Be aware that this disease can be a significant problem and that late fungicide applications are not much better than not applying fungicide sprays. Target leaf spots usually are up to ¼ to ½ inches in diameter and have a distinct “target spot” pattern with alternating light and dark brown bands of dead tissue. According to Dr. Bob Kemerait, University of Georgia Extension Specialist, research has CLEARLY shown both the benefits of applying fungicides to reduce defoliation and the importance of timing. Fungicide applications that are too early (this year, first square) or too late (later weeks of bloom) have not worked as well as those applications around the early weeks of bloom.  Dr. Kemerait says he MIGHT spray fields at this time if a field is at risk- based upon conditions described above -and defoliation and disease are present but not to advanced in the field, cotton is within the first 4-6 weeks of bloom, and cotton is still 4-6 weeks away from defoliation. Headline (6 fl oz/A) is Dr. Kemerait’s current standard fungicide treatment followed by Twinline (8.5 fl oz/A). Quadris is also an effective fungicide but we have less experience with it.

 We have also seen minor leaf spotting caused by Stemphylium, Alternaria, and/or Cercospora fungi in cotton. Leaf spotting attributed to these fungi is associated with a potash deficiency and has not been shown to respond to fungicide treatments. Individual leaf spot lesions of Stemphylium, Alternaria, and Cercospora are much smaller and circular in shape than Corynespora leaf spots.  [References:  A. Hagan. Alabama IPM Communicator. Vol.3, No. 14. and B. Kemerait. UGA Extension Row Crop Disease Updates for 3, 17, and 23 August 2012.]

Permanent link to this article: http://santarosa.ifas.ufl.edu/blog/2012/08/24/cotton-leaf-spot-severe/