Monday, August 6, 2012

Corynespora Leaf Spot

In those areas that have gotten good rain or are irrigated, Corynespora leaf spot has become an issue in cotton.  Much of the cotton acreage in Baldwin Co, an area that has gotten good rain, has been treated at least once with either Twinline or Headline fungicides.  While Phytogen varieties, which previously were shown to be susceptible to Corynespora leaf spot, have been damage, this disease is also a threat to defoliate other cotton varieties as well.

While it’s getting late, producers and consultants should be advised to check cotton for symptoms of Corynespora leaf spot as they scout weekly for insect pests.  Labels for both Twinline and Headline specify that the first fungicide application may be made when first symptoms appear in the lower leaf canopy.  Dryland or irrigated cotton with a 2+ bale/A yield potential that’s getting good rain should be targeted for treatment with a fungicide.

In research trials at outlying units, noticeable disease development has been seen in fungicide trials in dryland cotton at the Gulf Coast Research and Extension Center and irrigated cotton at the Plant Breeding Unit.  At GCREC, heavier leaf spotting was noted in Phytogen 499 as compared with DPL 1050, which suffered only light disease development.   In the PBU study, noticeable leaf spotting with some (10%) defoliation was noted in Phytogen 499, while some spotting on the lowest leaves was found in DPL 1050.  Hard to tell in both trials whether or not the single application of Headline has had much impact on disease spread.  My intention is to evaluate disease development in cotton variety trials at both of those locations as well as Wiregrass Research and Extension Center sometime in the next week.   In the good looking dryland fungicide and variety trials at the Field Crops unit at EV Smith, disease development is minimal.  So, frequent showers or irrigation appears to be a critical trigger for disease development and spread.  If you want to look at trials at the outlying units, get an outline from the station superintendent.

I have also seen minor leaf spotting caused by Stemphylium and Alternaria fungi in cotton.  Leaf spotting attributed to these fungi are associated with a potash deficiency.   Individual leaf spot lesions are much smaller when compared with Corynespora leaf spot.  On Phytogen 499, the target or zonate pattern leaf spots associated with Cercospora leaf spot have a diameter of up to ¾ inches and are circular compared with the unevenly shaped, and much smaller Stemphylium or Alternaria leaf spots.

If you have any questions with the diagnosis of this disease, send a sample overnight fedex or UPS to the Plant Diagnostic Laboratory, S Donahue Dr., Auburn University, AL 36849.

By: Austin Hagan