Cotton Foliar Spots Appearing Mid-July

DR. TYSON RAPER AND DR. HEATHER MARIE KELLY

JACKSON, TENNESSEE

The leaf spot complex is having a heyday.  In this blog, Dr. Kelly and I explain why we may be seeing a relatively high level of incidence this year and what you should do about it.  Spoiler alert, the best action for it is, fortunately, inaction.

 Several across the area have shared images of mainstem leaves with relatively high levels of leaf spots.  Agronomists commonly lump several of the diseases which generate leaf spots together and refer to them as the ‘leaf spot complex’.  Those in that category include alternaria, cercospora, and stemphylium.  These diseases are all generally associated with nutrient deficiencies or general plant stresses.  I believe we are seeing them creep in a little earlier than normal due to the large amounts of rainfall we have received this season and the shallow rooting structures that are subsequently under our crop.  We have not plated any of these out, but if we were guessing, most of these are likely stemphylium.

 Foliar fungicides are effective at reducing the foliar incidence and foliar severity of the leaf spot complexBUT even when the leaf spot complex shows up early in the season,the use of foliar fungicides to combat the leaf spot complex does not result in a yield increase.You can read more about our general lack of yield response when using fungicides to manage these diseases on a blog Dr. Kelly and I posted a few years back.  

 There are other diseases we should consider as we move deeper into July which occasionally do warrant a foliar spray.  They include corynespora (Target Spot) and ramularia (Areolate Mildew).  Again, fortunately for us, neither of these have been reported in the area this year and proactively spraying fungicides for these diseases is rarely recommended in Tennessee.

 I have seen one series of images that looked like Bacterial Blight. Keep in mind that those spots will generally have a water soaked appearance on the underside of the leaf.  Fungicides are not effective for managing that disease.  Our best management strategy there is to select a resistant variety for the following year.   ∆

DR. TYSON RAPER AND DR. HEATHER MARIE KELLY

UNIVERSITY OF TENNESSEE