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Allen, T.

Cotton Foliar Leaf Spots: Determining the Underlying Cause is Most Important Management Option

Much has been made regarding the whole foliar disease complex of cotton.  Over the past 5 years numerous fields have been encountered throughout MS whereby a foliar disease has been observed on the cotton foliage.

Bacterial blight of cotton. Note the angular shape of the leaf spot with a water soaked area around the lesion.

First and foremost, not everything on the cotton leaves in the lower canopy is diagnosed as bacterial blight.  Over the past few weeks I’ve had several conversations with seed company personnel, farmers, and consultants regarding the presence of bacterial blight.  One of the hardest things to distinguish regarding a disease called “angular leaf spot” has to be the presence and general shape of the observable lesions on the leaf.  Many foliar pathogens and/or abiotic disorders can produce a symptom on a leaf that will appear to be angular.  Fungi, when the causal organism of the leaf spot observed, will almost always produce a rounded lesion.  More often than not, fungal reproduction will occur within the center of the lesion since this is the point of infection and oldest portion of the leaf spot.  Black fruiting bodies within the lesion will distinguish the lesion as having been produced by a fungus.  In addition, fungi do not produce a symptom whereby one of the main veins on a leaf will have a black, water soaked appearance.  But, as I’ve said a few times this week, exceptions seem to be the rule in plant pathology since there are situations where a fungus will grow along the vein.  In most cases the fungal growth can be wiped off the underside of the leaf.

 

Boll rot complex

Boll rot symptoms present on a rotted boll in the lower canopy.

Just about any organism can produce a boll rot of cotton.  Over 175 organisms, some bacterial and some fungal, can cause a rot of cotton bolls.  In addition, in some cases insects have been proven to produce an opening that will allow microbes (either bacterial or fungal) into the boll and subsequently produce a rotten boll.  Boll rots typically occur in the lower plant canopy but in years where excessive moisture is present boll rots can occur anywhere in the plant canopy.  One thing is certain, in situations where bacterial blight has infected bolls a secondary rot can occur.  In most cases the rot will develop at the base of the boll after the bacterium has entered the plant.  Last year in particular this was a devastating situation in some fields.  Once the secondary boll rotting organisms invaded determining the presence of Xanthomonas axonopodis pv. malvacearum is increasingly difficult since secondary organisms will colonize the area where the bacterium initiated the rot.

Corynespora leaf spot

Corynespora leaf spot, note the concentric rings present in the leaf spot lesion.

Hopefully, within the not so distant future, the name of this particular leaf spot will be changed to target leaf spot.  Essentially, the fungus that causes this leaf spot is the same fungus that causes the target leaf spot of soybean.  One of the most distinguishing features of this particular leaf spot are the concentric rings starting in the center of the leaf spot and working towards the edge of the lesion.  Typically, the leaf spot will be observed in the lower canopy but there are instances whereby the leaf spot will be observed in the mid to upper canopy.  In addition, one of the other observations regarding this disease has to do with the nature of its presence.  Coreynespora leaf spot is not a result of a potassium deficiency in the foliage.  Georgia has recently suggested that a fungicide may be beneficial when Coreynespora leaf spot is present.  However, MS data is lacking on this particular subject.

Additional leaf spots

Several other fungi can cause leaf spots on cotton.  However, as stated above, determining what the exact organism is in the field is difficult.  The other fungi include:

-Alternaria

-Cercospora

Much like Stemphylium leaf spot, the fungi involved with these particular diseases are more of an issue when potassium is limiting in the leaf tissue.

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