Target spot/Corynespora leaf spot disease has increased in recent weeks. Many Santa Rosa County fields have been treated at least once with either Twinline or Headline fungicides.
The frequent showers appear to be a critical trigger for disease development and spread. During the past week target spot has caused over 50 percent defoliation in some older planted fields.
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 this disease can be a significant problem and late fungicide applications are not very effective. 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 shown both the benefits of applying fungicides to reduce defoliation and the importance of timing. Fungicide applications which 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 said he would consider spraying fields at this time if a field is at risk based upon conditions described above, and defoliation and disease are present but not too advanced in the field, cotton is within the first four to six weeks of bloom, and cotton is still four to six 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.
Minor leaf spotting caused by Stemphylium, Alternaria, and/or Cercospora fungi in cotton has appeared. 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.]
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