July Conditions Favorable for Target Spot
Target spot is a foliar disease of cotton caused by the fungal pathogen Corynespora cassiicola. The symptom most characteristic of this disease are numerous lesions, often with concentric rings (hence the name “target spot”) that develop first on the leaves lower in the canopy. The target-like spots initially develop lower in the canopy for several reasons:
- First, these are older leaves have had longer exposure to the fungal spores.
- Second, the pathogen survives in the debris from previous cotton crops, so the spores can be spread from the debris to the lower leaves through rain splash and irrigation.
- Third, spores of Corynespora cassiicola require a prolonged leaf wetness periods (perhaps as long as 12-14 hours) for infection to occur, which is most likely deeper in the canopy where humidity is higher.
Significant defoliation can happen within two weeks after the first spots are observed if conditions are favorable (warm, humid, and moist). Boll development can be affected when pre-mature defoliation removes the leaves feeding younger bolls.
The initial development of target spot disease deeper in the canopy helps to differentiate this disease from other foliar diseases that affect cotton. Ascochyta “wet weather” blight can also produce spots with concentric rings. However, this disease often develops in younger cotton and is not restricted to leaves in the lower canopy. Stemphylium leaf spot and Cercospora leaf spot are most often associated with a nutrient deficiency in the plant (typically a potassium deficiency) and appear first in the upper leaves (often associated with reddening and yellowing of the leaves) rather than in lower leaves.
Frequent rains can increase the risk for all types of spots on cotton, but potentially for different reasons. Rain and high-humidity increase create more-favorable conditions for infection and spread of target spot. Heavy rains can leach potassium from the root-zone of the cotton crop, thus increasing the risk for Stemphylium leaf spot and Cercospora leaf spot. Target spot is a disease that most often affects cotton with a thick canopy and high yield potential. It rarely (very rarely) affects cotton with poor growth or cotton under drought stress.
While some cotton varieties may be more susceptible to target spot than others, no cotton varieties are resistant to this disease. As previously mentioned, target spot is most likely to affect a crop when conditions (irrigation, rainfall, soil fertility) produce lush plants with substantial canopies.
One of the biggest challenges in protecting a cotton crop with a fungicide against target spot is that coverage is needed deep the canopy. However, getting sufficient fungicide coverage there is a real challenge but can be improved by 1) increasing spray volume, 2) increasing spray pressure, and 3) making the first fungicide application early enough before the canopy of leaves is fully closed.
UGA Extension does not have a blanket recommendation for treating a cotton crop with a fungicide to control target spot, though when this disease occurs early enough, growers can expect losses of lint to reach 200 lb/A (or more). Growers are advised to begin carefully scouting a cotton crop at the approach of the first week of bloom, checking the lower canopy for tell-tale symptomatic leaves. If target spot is not identified after careful scouting, growers may delay a fungicide application and scout again in the coming weeks.
From our research, the best timing of a fungicide application is often during the third week of bloom. Where disease pressure is high, a second application may be beneficial two weeks later. Fungicides are not needed after the sixth week of bloom.
A number of fungicides are labeled for use on cotton to protect against target spot and also areolate mildew. For target spot, Priaxor has been the most important fungicide for growers to consider. Headline has also been quite effective, and now Miravis Top seems to be among the most effective fungicides for management of target spot.
Because of current conditions, to include passage of Tropical Storm Elsa, and the growth and development of the crop, now is the time cotton growers should be carefully assessing the need to protect their crop from target spot disease. Once defoliation has reached 20% or more, there is likely little chance to manage this disease.
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