Stem blight of cotton leaves is a physiological disorder caused by potassium deficiency. It typically occurs during the mid to late growth stages of cotton plants. The condition tends to be more severe in dry years, as drought conditions exacerbate the problem. Symptoms usually begin to appear during the bud stage and become most pronounced during the boll-opening period. The disease primarily affects the leaves, with symptoms progressing from the top to the bottom and from the outer edges toward the center of the plant.
Initially, the leaf margins turn yellow, then develop red spots, and eventually the entire leaf turns red. The leaf tissue becomes thickened, shriveled, and brittle, while the veins remain green. In severe cases, all the leaves on the plant turn red or brown after chlorosis, and they may become scorched and fall off, leading to early wilting of the plant. Importantly, the vascular bundles in infected plants do not change color, which helps distinguish this condition from cotton brown wilt, a similar but different disease.
The main causes of stem blight include:
(1) Potassium-deficient soil, especially low levels of organic matter and available potassium. In areas like Haimen, farmers often apply urea and nitrogen-based fertilizers in corn and vegetable fields, but rarely use potassium chloride as a top dressing, leading to serious potassium deficiencies in cotton fields.
(2) High temperatures and drought stress can interfere with potassium uptake. Cotton relies heavily on soil moisture for nutrient absorption. During July and August, prolonged droughts can cause soil potassium to become fixed, reducing its availability to the plant. Heavy rain following a dry period can also leach potassium from the soil, further weakening the root system’s ability to absorb nutrients and increasing the risk of red leaf blight.
(3) Bt cotton varieties are particularly sensitive to potassium. With over 90% of cotton grown in Haimen being insect-resistant, these varieties require high levels of potassium. Their sensitivity makes them more prone to stem blight when potassium is lacking.
To manage and prevent the disease, several strategies can be employed:
(1) Apply potassium fertilizer during the flowering and budding stages. Farmers can use compost, manure, or sheep ash in furrows to increase soil potassium levels. Combining this with Shihua Bell Fertilizer and applying 15 kg of potassium chloride per mu is effective. Additionally, foliar sprays of 400 grams of potassium dihydrogen phosphate per acre can be applied every 7–10 days, focusing on the upper and middle leaves. This treatment may need to be repeated 2–3 times for best results.
(2) Implement proper irrigation practices. If the soil is dry during the early stages of the disease, timely watering and drought prevention should be carried out. Furrow irrigation is preferred over flood irrigation to avoid waterlogging. After rainfall, drainage and soil cultivation should be done promptly to improve soil aeration and enhance root activity.
(3) Choose resistant cotton varieties. Selecting appropriate cotton varieties that are less susceptible to the disease, especially in years with fewer bollworm infestations, can help reduce the occurrence of stem blight.
By addressing both the nutritional and environmental factors contributing to this issue, farmers can significantly reduce the impact of stem blight on their cotton crops.
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