Mid- to Late-season Cotton Leaf Diseases

Mid- to late-season foliar cotton diseases occur when a crop is under stress. Stress can come from all directions—too dry, too wet, insects, storms, fertility, and others all play a part in the health of a crop. Mitigating conditions conducive for disease development and understanding if and when a fungicide application is necessary is important to help maximize yield potential.

Target Spot

Symptoms  

Target spot (Corynespora cassiicola) often occurs after extended periods of leaf wetness. Lesions are marble-sized with concentric rings (Figure 1). Infection typically begins on lower leaves as water splashes from infected debris on the soil to lower leaves on the plant. Infection can quickly move upward and result in premature defoliation. Lesions may also appear on bolls and boll bracts.1 

Diagnostic Notes 

Target spot is commonly found in Georgia, Florida, and Alabama, and can also be found in other parts of the Southeast and Midsouth. Target spot can be differentiated from Alternaria and Stemphylium because target spot lesions lack a dark margin and premature defoliation occurs in the interior of the canopy and at the bottom of the plant first.3 

Potential Yield Loss and Control 

Defoliation of lower leaves is not likely to negatively impact yield potential and may help increase airflow within the canopy. If the infection moves upward to the middle of the plant where leaves are actively supporting boll development, then yield potential can be negatively impacted. Defoliation in the middle of the plant can cause boll abortion.1 Infection at the top of the plant can lead to arrested boll development and unharvestable bolls.3 

Rank growth contributes to infection. Controlling growth with a timely plant growth regulator application and irrigation scheduling is recommended. A fungicide application can help reduce infection and subsequent premature defoliation. Maximum control has been observed when strobilurin fungicides are applied between the first and third weeks of bloom. A second application should be considered if infection continues to spread three weeks after the initial fungicide application. Yield response to fungicide application for target spot can be extremely variable. There can be as little as no yield advantage observed to as much as 200 lb/acre.1 

Figure 1. Target leaf spot lesions with concentric circles.

Bacterial Blight/Angular Leaf Spot

Symptoms 

Bacterial blight (Xanthomonas citri pv. malvacearum) tends to occur after prolonged rainfall. Symptoms begin as small water-soaked spots and progress into lesions that are defined by the veins of the leaf, giving an angular appearance (Figure 2). Lesions eventually become black and can lead to premature defoliation. Infected bolls can form sunken, water-soaked lesions and rot.

Diagnostic Notes

Bacterial blight has been observed across cotton-growing areas. Lesions are often darker than other cotton foliar diseases but can also have a “shot-hole” appearance.2Boll lesions may be hidden by the bract or the calyx, attached at the base of the boll, near the peduncle.1 

Potential Yield Loss and Control

Plant tissues are infected via wounds caused by sandblasting, storm damage, or insect damage. Plants can be further infected by opportunistic fungal pathogens that are present in the field. This disease is caused by a bacterial pathogen, so fungicide application is not an effective means of control.1 The pathogen can easily survive in infected crop residue, so destruction of residue, use of clean equipment, and crop rotation are recommended as management tools. The most effective means for management is planting a resistant variety (Table 1). 

Figure 2. Bacterial blight lesions defined by leaf veins.

 Table 1. Bacterial blight ratings for Deltapine® cotton varieties.

Alternaria Leaf Spot

Symptoms

Alternaria leaf spot (Alternaria macrospora) is usually attributed to potassium (K) deficiency in the leaves during boll fill as K is moved from the leaves to developing seeds.1,3 Small, brown, circular lesions with purple margins form on senescing leaves (Figure 3). The centers of the lesions become gray and can detach from the leaf leaving a “shot-hole” appearance.1,2

Diagnostic Notes

Alternaria and Stemphylium have similar foliar symptoms. However, Alternaria is typically observed in Texas and the Midsouth. Stemphylium can be found across cotton-growing areas but is more commonly found in the Southeast.2   

Potential Yield Loss and Control

There are few regions and situations where Alternaria alone can lower yield potential enough to merit a fungicide application. The use of a fungicide application or a foliar K application in a curative manner has been largely unsuccessful.1,3 

Limiting stress on the crop can help prevent initial infection. Amacrospora can survive on undecomposed cotton residue, so incorporating any crop residue into the soil can help speed up decomposition and reduce subsequent inoculum production.3

Figure 3. Alternaria leaf spot lesions with purple margins.

Stemphylium Leaf Spot

Symptoms 

Like Alternaria leaf spot, Stemphylium leaf spot (Stemphylium solani) is usually related to a K deficiency. Plants deficient in K have weaker leaf cells, making them more susceptible to fungal infections.4 Foliage quickly turns red and lesions appear with purple margins and ash-gray centers. Centers of lesions can detach and result in a “shot-hole” appearance.1 Earlier in the growing season, infection may increase under droughty conditions as dry soils limit K uptake. Symptoms may also appear around the fourth week of bloom for plants with a heavy boll load as the demand for K can outpace available K.4

Diagnostic Notes

This disease can be found in all cotton-growing areas. Stemphylium leaf spot can be differentiated from target spot by where symptoms occur on the plant. Stemphylium is usually coupled with nutrient deficiency symptoms and found on the top of the plant first, while target spot occurs on green leaves and within the canopy.

Potential Yield Loss and Control

Yield loss can be high for a cotton crop infected with Stemphylium, but this is due to the related stress from the nutrient deficiency that helped to initiate infection.As such, fungicides are largely ineffective.1 

Cercospora Leaf Spot

Symptoms

Cercospora leaf spot (Mycosphaerella gossypina) lesions appear reddish at first and grow larger with light centers as the disease progresses. Infection is related to nutrient deficiency, and a disease complex may form with Alternaria and Stemphylium leaf spots.1

Diagnostic Notes

This disease is present in all cotton-growing areas and can be difficult to diagnose in the field from other leaf spots. Lesions are concentric like target spot and distribution of lesions is similar to Alternaria and Stemphylium leaf spots.2 This disease may be more prevalent in poorer areas of a field.  

Potential Yield Loss and Control

Lower yield and fiber quality can occur when this disease is part of a disease complex or when plants are under stress.2 Fungicides are not considered to be effective as this disease is a result of a nutrient deficiency.

Areolate Mildew

Symptoms

Areolate mildew (Ramularia areola syn. Ramularia gossypii) forms small lesions on leaves and a white mildew on the bottom of partial or entire leaves. Symptoms appear first on the lower canopy and move upward as the disease progresses. Lesions can become necrotic and, in fields with substantial damage, early defoliation can occur.2,5,6 

Diagnostic Notes

This disease can be mistaken for bacterial blight; however, areolate mildew includes a white, powdery growth on the underside of leaves.This disease has been observed in Georgia, South Carolina, and North Carolina, and it is recommended to confirm the disease through leaf sampling.

Potential Yield Loss and Control

This disease usually occurs late in the growing season and does not largely impact yield potential.2 It is recommended to apply a fungicide if the disease is prevalent in large areas of the field and the cotton crop is within the sixth week of bloom or earlier and poor weather conditions are forecasted (cloudy and wet).5

Ascochyta Blight

Symptoms 

Lesions can develop on cotyledons, leaves, stems, and bolls as a result of infection by Ascochyta blight (Ascochyta gossypii syn. Phoma exigua). Lesions on leaves are round and white to light brown with dark margins (Figure 4). Stem cankers are reddish purple to black or gray in color. May also be referred to as “wet weather blight” as a result of the extended cloudy, cool, and wet conditions that are conducive for disease development.2

Diagnostic Notes

This disease has been observed across cotton-growing areas. Symptoms may resemble target spot, but this disease usually occurs earlier in the growing season.2 Small, black fruiting structures that resemble pepper may be found in the lesions.1 

Potential Yield Loss and Control

Ascochyta blight infection generally does not impact yield potential. However, young cotton can be more severely affected.2 Fungicides can be effective at controlling this disease, but application is rarely warranted.1

Figure 4. Symptoms of Ascochyta blight. Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Sources:

1 Whitaker, J., Culpepper, S., Freeman, M., Harris, G., Kemerait, B., Perry, C., Porter, W., Roberts, P., Liu, Y., and Smith, A. 2018. 2019 UGA Cotton Production Guide. The University of Georgia. http://www.ugacotton.com/2019/03/uga-cotton-production-guide-for-2019/

2 Cotton foliar diseases. The University of Tennessee Institute of Agriculture. https://guide.utcrops.com/cotton/cotton-foliar-diseases/

3 Woodward, J. 2016. Increased incidence of Alternaria leaf spot of cotton. Texas A&M AgriLife Extension. https://agrilife.org/texasrowcrops/2016/09/06/increased-incidence-of-alternaria-leaf-spot-of-cotton/.

4 McGriff, E. 2016. Stemphylium leaf spot in cotton. Alabama Cooperative Extension System. https://sites.aces.edu/group/timelyinfo/Lists/Posts/Post.aspx?ID=836

5 Thiessen, L. 2018. Areolate mildew confirmed in North Carolina. North Carolina State Extension. https://plantpathology.ces.ncsu.edu/2018/08/areolate-mildew-confirmed-in-north-carolina/

6 Nichols, K. 2019. Target spot, areolate mildew damaging to cotton.  Alabama A&M and Auburn Universities Extension. https://www.aces.edu/blog/topics/cotton/target-spot-areolate-mildew-damaging-to-cotton/

 

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