Premature Senescence in Cotton

• Premature senescence in cotton is characterized by early leaf yellowing and drop, resulting in reduced photosynthesis that can negatively impact yield potential and lint quality.
• There are many factors that can contribute to premature cotton senescence like drought, temperature fluctuations, disease, and nutrient deficiency.
• Plants deficient in potassium are more prone to early leaf drop, either because there is not enough potassium available or because the plants are unable to absorb it.
• Protecting a cotton crop from premature defoliation is crucial during flowering and boll development.

Causes of Premature Senescence

There are several factors that can contribute to premature senescence in cotton. First, limited uptake of potassium (K)—either because there is not enough K in the soil or because the plants are unable to absorb it—can lead to secondary pathogen infection and subsequent leaf drop (Figure 1). In Texas, this phenomenon has been observed in plants with moderate boll production when dry conditions during boll fill are followed by several days of wet and cloudy weather after cutout. Soil that is too dry or too wet can lead to decreased root function, limiting plants’ abilities to absorb enough K to meet their demand. However, plants with limited boll production may be unaffected by low K levels as their K requirement is lower.¹

Second, cotton roots also play a large role in cytokinin production, which is important for senescence regulation. With decreased root function, leaves senesce because the plant is unable to produce enough cytokinin.

A third major cause of premature senescence is disease, especially in areas of the Southeast where target spot (Corynespora cassiicola), and areolate mildew (Ramulariopsis spp.) are prevalent. Frequent rains can lead to widespread infection and premature leaf drop can occur. Plants that are nutrient deficient are more susceptible to disease, and it is suspected that fungal pathogens—like those from the genera Stemphylium, Cercospora, and Alternaria—cause a secondary infection that further contributes to leaf drop.^1,2

For example, target spot (Figure 2) favors rank growth and typically appears first on leaves in the lower canopy. Affected subtending leaves, or the leaves that are closest to developing bolls, may be unable to supply enough photosynthates required for normal boll production. Trials in Georgia and Florida indicated potential yield loss of 150 to 200 lb lint/acre in fields damaged by target spot.³

Like target spot, areolate mildew favors high humidity and can be identified by white to gray spores present on the undersides of leaves (Figure 3). The top of an infected leaf may be purplish in color. Heavily infected leaves can senesce, and the pathogen can continue to produce spores in leaf debris.

The Impact of Potassium Deficiency

Potassium is important for several physiological processes that directly impact cotton yield potential and fiber quality. It is critical for metabolic enzyme activation, translocation of photosynthates, and fiber elongation and development—which contributes 60% of the total plant K requirement.¹ Adequate potassium can also increase resistance to disease, as mentioned above.

Symptoms of K deficiency include yellowing between leaf veins which quickly changes to orange, red, or bronze coloring in the top third of the plant. The leaves will turn brown with necrotic margins. The affected plants become vulnerable to disease, especially target spot and Stemphylium, Cercospora, and Alternaria leaf spots.^1,2 Observations in the field may indicate a higher incidence of leaf spots on leaves with K deficiency symptoms than leaves that appear healthy. Lower plant vigor and reduced boll size and quantity may also be observed in plants with low K.

Management

It is important to know what is causing premature defoliation to determine which treatment options may be beneficial and if they are economically favorable.

If potassium deficiency is the primary source of premature senescence, then management should begin with trying to determine the cause of the deficiency. Reviewing soil test results and in-season leaf tissue sampling can indicate whether the deficiency is a lack of K availability or an inability of the plant to uptake K (due to drought, flooding, root pruning, or poor root development). In fields that have a tendency toward K deficiency, supplemental K application may help.¹ If K deficiency symptoms are starting to show, a foliar application of K may help the crop but results are variable.²

If target spot is identified in the field, a timely fungicide application can help protect yield potential. Fungicide will be most beneficial in high yield-potential fields where scouting indicated the presence of target spot at or shortly after first bloom, if an application can be made by the third week of bloom. Scouting should continue through the sixth week of bloom, with optimal application timings at the first and third weeks of bloom.³

For high yield-potential fields impacted by areolate mildew and have high yield potential, fungicide applications are recommended to protect yield. Timely applications can help prevent a 100 to 250 lb lint/acre loss. However, if 20% or more of the canopy has already senesced or if the crop is within four weeks of defoliation, it is not recommended to apply a fungicide.⁴

Sources

¹Lemon, R., and McFarland, M. 2005. Premature senescence syndrome. Texas AgriLife Extension, The Texas A&M University System. SCS-2005-23. https://agrilifeextension.tamu.edu/

²Isakeit, T., and Morgan, G. 2017. What is causing the premature defoliation in cotton? Texas AgriLife Extension, The Texas A&M University System. https://agrilife.org/texasrowcrops/2017/07/14/what-is-causing-the-pre-mature-defoliation-in-cotton/#:~:text=Cotton%20fields%20in%20Upper%20Coast,associated%20with%20these%20leaf%20spots

³Small, I., Kemerait, B., and Burger, M. 2021. Time to protect your cotton from premature defoliation due to foliar disease. University of Florida Extension. https://nwdistrict.ifas.ufl.edu/phag/2021/07/30/time-to-protect-your-cotton-from-premature-defoliation-due-to-foliar-diseases/

⁴Kemerait, R.C. Protecting Georgia cotton against leaf diseases: It’s not what it used to be. Georgia Cotton Commission. https://georgiacottoncommission.org/kemerait-protecting-georgia-cotton-against-leaf-diseases-its-not-what-it-used-to-be/

Web sources verified 08/21/24. 1414_444869