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Control Methods for Cotton Anthracnose in Cotton Seedlings

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Symptoms The disease can occur at the seedling stage and the adult plant stage. Infection at the seedling stage: damage after germination but before emergence can cause seed rot; after emergence, red-brown cracked streaks appear at the base of the stem, and as they expand and constrict they cause seedling death. In damp conditions, orange-red sticky material (the pathogen's conidia) forms on the lesions. Round or semicircular yellow-brown spots appear on the edges of the cotyledons, which then dry up and fall off, leaving the cotyledon edges incomplete. When cotton bolls are infected, small dark red dots appear at first, and after expanding they become brown lesions; the affected part is sunken and contains orange-red powdery material, namely the pathogen's conidia. In severe cases the whole boll rots, fails to split open, and the fibers become black hardened, discolored locks. Leaf lesions are irregular and nearly round, and tend to dry out and crack. Stem lesions are red-brown to dark black, oblong, sunken in the center, with the epidermis often breaking open to expose the xylem, and the stems break easily in wind. In production, cotton bolls infected in the later stage suffer severe damage and cause great losses.

Symptoms The disease can occur at the seedling stage and the adult plant stage. Infection at the seedling stage: damage after germination but before emergence can cause seed rot; after emergence, red-brown cracked streaks appear at the base of the stem, and as they expand and constrict they cause seedling death. In damp conditions, orange-red sticky material (the pathogen's conidia) forms on the lesions. Round or semicircular yellow-brown spots appear on the edges of the cotyledons, which then dry up and fall off, leaving the cotyledon edges incomplete. When cotton bolls are infected, small dark red dots appear at first, and after expanding they become brown lesions; the affected part is sunken and contains orange-red powdery material, namely the pathogen's conidia. In severe cases the whole boll rots, fails to split open, and the fibers become black hardened, discolored locks. Leaf lesions are irregular and nearly round, and tend to dry out and crack. Stem lesions are red-brown to dark black, oblong, sunken in the center, with the epidermis often breaking open to expose the xylem, and the stems break easily in wind. In production, cotton bolls infected in the later stage suffer severe damage and cause great losses.

Pathogen Colletotrichum gossypii Southw., known as the cotton anthracnose fungus, is a fungus of the subphylum Deuteromycotina. Its sexual state is Glomerella gossypii (Southw.) Edgerton, known as the cotton anthracnose Glomerella, a fungus of the subphylum Ascomycotina, and is rarely seen. The perithecia are dark brown, spherical to pear-shaped, 100-160 x 80-120 (μm), and are embedded in the host tissue. Each ascus contains 8 ascospores, single-celled, oval, slightly curved, 12-20 x 5-8 (μm). The asexual-state conidia are borne on conidiophores, arranged in a shallow dish shape; the acervulus has setae, which are dark brown and have 2-5 septa. The conidiophores are relatively short, and conidia can be produced continuously on them. The conidia are colorless, single-celled, long oval or short club-shaped, 9-26 x 3.5-7 (μm), mostly clustered together and appearing pink. When conidia germinate, they often produce 1-2 septa, and each cell grows a germ tube; an appressorium forms at the tip of the germ tube, producing an infection hypha that invades the host. The optimum temperature for conidial germination is 25-30°C; at 35°C germination is low and growth is slow, and at 10°C there is no germination. The soil temperature suitable for disease development is 24-28°C, with a relative humidity above 85%.

Transmission routes and conditions for disease development The pathogen mainly overwinters as conidia on the short fuzz of cotton seeds, and a small portion overwinters as mycelium latent in the seed coat of cotton seeds or in the cracks of the cotyledons. Seed-borne inoculum is an important source of primary infection. The pathogen's conidia can survive on cotton seeds for 1-3 years. Because the initial germination temperature of cotton seeds and the initial germination temperature of the spores are both around 10°C, the pathogen can easily invade when the cotton seeds germinate, and afterward the conidia produced on the diseased parts spread through wind and rain, insects, and irrigation water. When cotton bolls are infected, the pathogen invades the cotton seeds, with a seed-borne rate of 30%-80%. Diseased leaves, stems, and bolls falling into the soil cause the soil to carry the pathogen, which can both trigger disease at the seedling stage and, through rain splash, infect cotton bolls and cause boll disease.

Control methods Selecting disease-free seed and disinfecting seed are the two keys to controlling this disease. (1) Select high-quality disease-free seed or seed from the previous year. (2) Treat the seed before sowing. Mix 0.5 kg of 40% carboxin-thiram wettable powder with 100 kg of cotton seed; or mix 0.5 kg of 70% thiophanate-methyl wettable powder with 100 kg of cotton seed; or mix 0.5 kg of 70% mancozeb wettable powder with 100 kg of cotton seed. In addition, 1 kg of 10% carbendazim-thiram mixture can be used to coat 50 kg of cotton seed; all have good control effects. (3) Sow at the proper time, cultivate strong seedlings, promote early development of cotton seedlings, and improve disease resistance. (4) Use a reasonable planting density to lower field humidity, prevent excessive vigorous growth of cotton seedlings, and take care to prevent premature senescence of bolls. (5) At the early stage of disease, spray 70% thiophanate-methyl wettable powder at 800 times dilution, or 70% chlorothalonil wettable powder at 600-800 times dilution, 70% mancozeb wettable powder at 400-600 times dilution, 50% benomyl wettable powder at 1500 times dilution, or 25% carbendazim wettable powder at 500 times dilution.

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