From Cotton Grower Magazine – August/September 2016
As the cotton specialist for the state of Tennessee, I am constantly on the lookout for potential issues that may impact Mid-South cotton production. Over the past several years, I have occasionally observed several “target spots,” or Corynespora leaf spots, on the lower leaves of rank cotton plants. Although the number of spots and number of affected leaves are typically low, many growers have asked if the disease might be able to cause the 200-400/lb lint per acre yield penalties reported along the Gulf coast.
Before we get to that question, let’s cover some background on the disease.
Target spot is occasionally observed in cotton plants which have experienced excessive canopy humidity. Caused by the fungus Corynespora cassiicola, it has been reported in almost every cotton-producing state within the Southeast and Mid-South in the past three years. Target spots are typically found on mature main stem and subtending leaves close to the plant base. This disease was dubbed target spot due to the irregular concentric rings contained within the dark brown to light brown lesions. Although lesion size and number per leaf can vary substantially, lesions are commonly larger than 1/4-inch in diameter. Affected plants will also prematurely shed mature leaves starting from the base of the plant as the disease progresses.
The disease is most common in fields with excessively tall cotton due to inadequate plant growth regulation and/or excessive nitrogen, very high levels of canopy humidity from overhead irrigations or frequent rainfall events (like we experienced in late July and early August), large amounts of cotton residue common in continuous cotton rotations, and late maturing, tall-growing varieties.
Fortunately for producers along the northern edge of the Cotton Belt, this disease is currently in the crosshairs of several regional Cotton Incorporated projects. One of the field crops pathologists currently working on the disease is Dr. Heather Kelly with the University of Tennessee. From her research, Tennessee has initially been predicted to be at low risk to target spot, since Tennessee’s environment is not generally conducive for early onset and rapid disease development.
Studies conducted within Tennessee over the past few years have noted numerically lower yields in plots impacted by the disease, but these reductions are often not statistically significant. Still, Dr. Kelly’s research has highlighted several steps which can be taken to reduce the impact of target spot:
- Rotate to a non-host such as corn if possible.
- Plant multiple, high-yielding, stable-performing varieties adapted to the field environment. Although differences in variety susceptibility have been noted, the highest yielding varieties in a given trial have also occasionally been characterized by the highest disease ratings and greatest percent defoliation.
- Properly apply nitrogen and plant growth regulators to produce sufficient vegetative growth to support fruiting bodies without allowing the plant to become rank.
- When considering fungicide applications, keep in mind that while two applications of fungicide have consistently reduced disease and defoliation due to target spot, yield responses have been variable in Tennessee.