Cotton: Foliar Fungal Diseases Beginning to be Observed Throughout Delta

Bacterial blight on a cotton leaf. Notice the dark color and angular appearance of the lesion.

Well, it is that time of year again.  Seems over the past 5 seasons we begin to encounter foliar cotton diseases during this particular week in August.  2011 appears to be the year when the majority of these leaf spots are the worst they’ve been compared to the previous 4 seasons.  In addition, there seems to be a little confusion as to the specific organisms causing some of the foliar leaf spotting present throughout MS.  Some of this minor confusion has occurred due to the presence of bacterial blight (or angular leaf spot).  At present, it is increasingly difficult to determine if bacterial blight is the cause of the leaf shed or any one of the number of foliar fungal leaf spot diseases.  Bacterial blight will manifest itself with several different foliar symptoms.  On leaves the spots produced by bacterial blight are considered to be “angular” in appearance (see photos).  Presently, one of the more obvious symptoms that I search for to determine if the leaf spots are the result of bacterial blight or a foliar fungal disease is the presence of the angular shaped lesions along one of the main veins (see photo).  Closer observation of the groupings of lesions along the vein will show that the lesions are in fact squared off (angular) and typically small in relation to most of the foliar fungal diseases.  Lesions of bacterial blight will be dark and a clear halo or “ring” will be absent.  The halo or ring around the lesion will be more prominent when foliar fungal diseases are occurring.  However, spots on bolls, bracts and petioles have a different appearance and will more often than not be round.  Focus on the presence or absence of the angular shaped lesions on the leaves. 

Bipolaris leaf spot.

Several different fungi can cause foliar leaf spots in cotton (Bipolaris, Cercospora, Corynespora, Stemphylium).  Determining the causal organism of the foliar fungal diseases at the field level is almost impossible.  The lesions produced regardless of organism (unless Corynespora leaf spot is present) all appear similar and a proper laboratory diagnosis is necessary.  If the fungi are sporulating then determining the root cause can be much easier.  Almost all of the fungal lesions have a maroon margin around a grey to tan center that depending upon the age of the lesion and the fungus involved can fall out of the lesion (think frogeye leaf spot of soybean).  All lesions will be more rounded than angular and can range in size from 1/16^th of an inch to ¾ of an inch depending on the causal organism.  Typically, the larger spots will be those caused by Corynespora.  In the past we have referred to the target shaped spot on cotton leaves as Alternaria leaf spot (see photo).  While this is not considered to be wrong, the organism has been renamed and now the disease is referred to as Corynespora leaf spot (the same causal organism and type of leaf spot as target spot of soybean).  Keep in mind, that in some rare cases lesions of foliar fungal diseases can be bordered by the leaves’ veins that will make them appear to be angular.

One of the more common lines of thinking regarding some (but not all) of the leaf spot diseases has to do with foliar potash (K; potassium) deficiency.  Now, this is not a result of low soil potash concentrations so don’t mistake the two divergent topics.  Proper soil concentrations of potash may be present but when cotton plants go to setting bolls the leaves serve as sources of the nutrient and bolls become the sink.  However, there is conflicting information available regarding whether or not a foliar potash deficiency is responsible for Corynespora leaf spot.  In general, as potassium is translocated out of leaves into bolls the amount of foliar fungal disease appears to increase.  Essentially this results in a leaf that can be extra susceptible to a foliar fungus that requires the nutritional deficiency to cause infection.

Corynespora leaf spot, notice the target-shaped lesions.