Stem blight in cotton leaves is a physiological disorder primarily caused by potassium deficiency. It typically occurs during the mid to late growth stages of cotton plants. The condition tends to worsen in dry years, leading to more severe damage. Symptoms usually begin during the bud stage and reach their peak during the boll-opening period. The most noticeable signs appear on the leaves, starting from the top and moving downward, and from the outer edges toward the center. 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, the entire plant’s leaves turn red and brown, with some leaves becoming scorched and falling off, leading to early wilting. Notably, the vascular bundles in affected plants do not change color, which helps distinguish this condition from cotton brown wilt.
The primary causes of stem blight include: (1) Potassium-deficient soil, especially in areas where organic matter is low. In places like Haimen, farmers often use sheep manure and human waste in corn and vegetable fields, but apply little potassium chloride as a top dressing, resulting in significant potassium deficiency in cotton fields. (2) High temperatures and drought conditions can hinder potassium uptake. Cotton’s nutrient absorption depends heavily on soil moisture levels. During July and August, when droughts occur, soil potassium becomes more tightly bound, reducing its availability to plants. Additionally, heavy rain following prolonged dry periods can leach out available potassium, 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 large amounts of potassium, making them more prone to stem blight when potassium levels are insufficient.
To prevent and manage this issue, several measures can be taken: (1) Apply potassium fertilizers during the flowering and budding stages. Farmers can use compost, ash, or human waste in furrows to increase available potassium. Alternatively, they can mix it with other fertilizers, applying 15 kg of potassium chloride per mu. Foliar sprays of potassium dihydrogen phosphate (400 grams per acre) can also be effective, applied to the upper and middle leaves every 7–10 days, up to two or three times if needed. (2) Implement scientific irrigation practices. During the early stages of the disease, ensure timely watering if the soil is dry, using furrow irrigation instead of flooding. After rainfall, drain excess water and cultivate the soil to improve aeration and enhance root activity. (3) Choose resistant varieties. Selecting appropriate cotton varieties is crucial, especially in years with fewer bollworm infestations, to reduce the risk of potassium-related diseases.
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