Symptom Identification and Control Methods for Cotton Angular Leaf Spot

Symptoms: This disease not only damages cotton seedlings but also harms the stems, leaves, and developing bolls of mature plants. When seedlings are infected, the cotyledons develop water-soaked irregular or round lesions that are dark brown; in severe cases, the cotyledons wither and drop off. When true leaves are infected, small dark green spots first appear on the underside of the leaves, then expand into oil-soaked lesions; on the upper surface of the leaves, the lesions are mostly angular, and sometimes the lesions extend along the veins in irregular streaks, causing the leaves to turn yellow and drop off. When stems are infected, water-soaked main lesions appear, then expand and turn black or rot, the diseased area becomes sunken, and the pathogen bends to one side. When the terminal bud is infected, it forms a "rotten top," causing the entire plant to die. When humidity is high, the diseased areas exude sticky yellow bacterial ooze, which under dry conditions becomes a thin film or breaks apart into powder. When cotton bolls are infected, oil-soaked dark green small spots first appear, then expand into nearly round lesions or multiple lesions merge into irregular shapes, brown to reddish-brown, the diseased area becomes sunken, young bolls drop off, and some locules of mature bolls rot.

Pathogen: Xanthomonas campestris pv. malvacearum (E.F. Smith) Dowson, known as the mallow pathovar of Xanthomonas campestris (cotton angular leaf spot Xanthomonas), belongs to the bacteria. The bacterial cells are rod-shaped, measuring 1.2-2.4 x 0.4-0.6 (micrometers), with 1-2 flagella at one end, capable of swimming, and possessing a capsule. Gram stain negative. On PDA medium, it forms pale yellow round colonies. The bacterial cells often combine in chains of 2-3. The optimal growth temperature for this bacterium is 25-30°C, maximum 36-38°C, minimum 10°C, and it is killed after 10 minutes at 50-51°C. However, under dry conditions, it can withstand high temperatures of 80°C and low temperatures of -21°C. This bacterium has physiological differentiation, and 18 physiological races have been identified.

Transmission routes and disease development conditions: The pathogenic bacteria mainly overwinter on seeds and in diseased bolls and other diseased plant residues in the soil. After cotton is sown the following spring, they spread and diffuse via rain splash and insect carriers. On cotton bolls, the bacteria enter through stomata or cracks in the epidermis and insect wounds, aided by the water film on the host surface after rain, multiply in the intercellular spaces, destroy the tissues within the leaves, and produce symptoms after 8-10 days. The pathogen often invades the vascular bundles through diseased tissues, then reaches the seeds, causing seed contamination; the pathogen on the boll shell falls into the soil with diseased residues, becoming the primary infection source for the following year. This disease is mainly transmitted by seeds, with a seed contamination rate of 6%-24%, and survives inside seeds for 1-2 years. In the cotton regions of the Yangtze River basin, it occurs from June to September, entering the peak period in July-August. During this time, if the relative humidity is above 85%, with frequent rainfall and heavy rainfall, the disease is prone to occur; when hit by typhoons and torrential rain, the disease is severe. Sea island cotton is highly susceptible to the disease.

Control methods: (1) After picking is completed, promptly remove diseased plant residues from the cotton field and compost them in a concentrated pile or burn them. (2) Select cotton seeds carefully, plant at a reasonable density, drain promptly after rain to prevent moisture retention, and remove diseased plants promptly when found during thinning and final singling. (3) Use formula fertilization techniques, advocate applying compost fermented with enzyme bacteria, and avoid partial or excessive application of nitrogen fertilizer. (4) Advocate ridge planting or high beds, scientific irrigation, and strictly prohibit flood irrigation and continuous-flow irrigation. Carry out timely intertillage to release soil moisture. (5) Seed treatment. Delinting with concentrated sulfuric acid can eliminate bacteria carried on cotton seed short fuzz; for specific methods, see Cotton Verticillium Wilt. The traditional "three boiling and one cold" warm water (55-60°C) soaking of seeds for half an hour can also be used. (6) Select disease-resistant varieties. In upland cotton, the Deltapine cotton system has strong resistance, and Chinese cotton is also relatively resistant. (7) Strengthen field management. After typhoons and heavy rain, apply supplementary fertilizer promptly, and spray 1:1:120-200 Bordeaux mixture or 25% Yekuzuo wettable powder at 500 times dilution or 72% agricultural streptomycin sulfate at 4000 times dilution.