How are cotton seedling diseases diagnosed and controlled?

Diagnosis and Control of Cotton Seedling Diseases

Cotton seedling diseases are numerous, but they can be grouped into two main types: root diseases and leaf diseases. Root diseases often cause seed rot and sprout rot before emergence, and root rot and stem-base rot after emergence, and are the main cause of missing plants and broken rows in cotton fields. Common ones include damping-off (Rhizoctonia), anthracnose, red rot, and Pythium damping-off. Leaf diseases damage the cotyledons, true leaves, and stems of cotton seedlings, and in severe cases the stem tips and leaves wither and drop off. Common ones include black spot, brown spot, cotton seedling blight, and stem blight. This film focuses on summarizing and introducing the occurrence patterns, identification and diagnosis, and integrated agricultural and chemical control measures for several seedling diseases that occur commonly and cause severe damage, for the reference and application of our many member cotton farmers in production practice as appropriate.

Disease Types and Symptoms

Damping-off (Rhizoctonia): Damping-off, also called root rot or black root disease, occurs in all cotton regions of China, and causes severe damage in the Yellow River cotton region. Before cotton seedlings emerge, it often infects young roots and sprouts, causing seed rot and sprout rot. After cotton seedlings emerge, yellow-brown spots appear on the young stem base near the ground, gradually enlarge, and produce a sunken, pinched-in appearance, finally forming a wasp-waist-shaped sunken rot around the tender stem; in severe cases it can cause death or wilting and collapse. After the adult plant stage is affected, brown spots appear on the leaves and later drop off, leaving holes. Cotton plants at the squaring and flowering stages can also be affected. In rainy years, after the stem is affected, dark brown lesions form at the stem base, and after the epidermis breaks open, strips of woody fiber are exposed; in severe cases the stem breaks and the plant dies, and sometimes a tumor-like swelling forms at the affected part of the stem.

Anthracnose: Anthracnose occurs in all major cotton regions of China, is relatively common in the Yangtze River basin, and is the main seedling disease in the southern cotton region. Damage is heavier at the seedling and boll stages. When cotton seedlings are affected, mild cases affect growth, and severe cases cause patchy seedling death; when cotton bolls are affected, it causes boll rot. The anthracnose pathogen attacks as soon as the cotton seed begins to germinate, often causing the cotton seed to rot in a water-soaked manner after sprouting in the soil, so it cannot emerge and dies. After the seedling emerges, the lesions on the cotyledons are yellow-brown with red-brown edges, bearing an orange-red sticky substance, namely the pathogen's conidia. After the young stem base is affected, red-brown spindle-shaped streaks appear, later enlarge and turn brown, are slightly sunken in the middle, and bear an orange-red sticky substance on the lesions, which can cause the seedling to die. When true leaves are affected, small black spots first appear, and after enlargement they become dark brown, round or irregular lesions with purple-red edges; if the weather is dry they often crack, and the leaves become tattered. Red-brown elongated lesions, slightly sunken, can also appear on petioles and stems, and the affected parts break easily. Under humid conditions, a red-brown or orange-red sticky substance, namely the pathogen's conidial mass, forms on the surface of the lesions. Lesions on bolls are initially dark red dots, later gradually enlarge and become sunken, with the middle turning gray-brown, also bearing an orange-red sticky substance on the surface; diseased bolls rot and can form hard locks.

Red rot: Red rot, also called root rot, occurs in all cotton regions of China. The occurrence rate of red rot at the seedling stage is highest in the Yellow River cotton region, with an incidence generally of 20%-50%, and up to more than 80% at the highest. In the northern cotton region, seedling-stage damage is severe, and in serious cases it can cause large patches of seedling death; in the southern cotton region, boll-stage damage is severe. The red rot pathogen can cause damage before cotton seedlings emerge, causing sprouts that have not emerged to turn red-brown and rot in the soil. After emerged seedlings' roots are affected, the root tips and lateral roots first turn from yellow to brown and rot, then it spreads to the whole root, and can also spread to the aboveground part of the young stem. Severely diseased seedlings die. The lesions do not sink in. Thickening of the affected tender stems above the soil surface and young roots is an important feature of red rot, and afterward the affected parts can become dark brown and rot. Besides attacking roots and stems, red rot can also attack cotyledons, true leaves, and cotton bolls. After cotyledons or true leaves are affected, gray-red irregular or nearly round lesions mostly form on the edges; after the lesions enlarge they often crack, and under humid conditions a pink mold layer, namely the pathogen's spores, forms on the surface. Lesions on cotton bolls are irregular in shape, with red powder outside, and later often stick together into pinkish block-like masses; severely diseased bolls do not crack open and become hard locks.

Pythium damping-off: Pythium damping-off occurs in all cotton regions of China, and is especially severe in humid and rainy areas. It is a common cotton seedling root disease, often causing cotton seedlings to wilt suddenly, lodge, and die in patches, greatly affecting cotton emergence and growth. Pythium damping-off generally infects first from the young, fine roots and damages seedlings, and can also attack seeds and sprouts that have just germinated, causing seed rot and sprout rot, so that seedlings emerge and develop poorly. When it attacks seedlings, water-soaked symptoms first appear on the part of the young stem base close to the ground; in severe cases it becomes edematous, spreads, turns yellow, and rots, becoming soft like scalded by water, and rapidly rots and lodges. When underground fine roots are affected, they turn yellow-brown and absorb water poorly, causing the entire seedling to die; the cotyledons also fade and become water-soaked and soft. Under high humidity, white flocculent material, namely the pathogen's hyphae, sometimes appears on the affected parts. The difference between Pythium damping-off and damping-off (Rhizoctonia) is that in Pythium damping-off there are no brown sunken lesions on the cotton seedling stem.

Black spot: Black spot, also called ring spot, occurs in all cotton regions of China, and is heavier in the northern cotton region than in the southern cotton region. It can damage cotton roots, stems, leaves, and young bolls. When cotton seedlings are severely affected, it causes large areas of seedling death. In recent years, with the increase in the area of transgenic insect-resistant cotton in China, black spot has worsened year by year. In 2003-2004, black spot occurred severely over large areas in the Yellow River cotton region, with an incidence as high as 100%. In severely affected areas, by early September all leaves in the cotton field were damaged, and the whole field was pitch black. When black spot affects cotyledons or true leaves at the seedling stage, red-green small dots first appear on the leaf surface, then gradually expand into red-brown lesions 10-15 mm across. The cotyledons of diseased seedlings often stick together, and there is a black mold layer at damaged parts of the shell. Affected cotyledons and true leaves initially have light yellow oil-soaked small round spots sunken on the back; as the lesions expand, the edges become purple-red, nearly round or irregular, with no obvious concentric rings. When the weather is humid, an obvious black mold layer, namely the pathogen's conidia, forms on the surface of the lesions. When cotyledon petioles are affected, dark brown streaks appear, often causing the cotyledons to drop off. When leaves of adult plants are affected, the lesions are mostly round or nearly round and have concentric rings, which is a characteristic of black spot. The lesions can dry, crack, and break apart. When the disease is severe, lesions on a leaf can number in the dozens, causing the diseased leaves to wither and drop off. Sometimes affected petioles show yellow-brown streaks, and in severe cases they often girdle the petiole, causing the cotyledons to wither and drop off, leaving bare stems. After leaves and petioles wither and die, the hyphae spread to the cotyledon node, causing the stem tissue and even the growing point to turn black and die; those that do not die also grow slowly and stop growing for as long as 1 month.

Brown spot: Brown spot occurs in all cotton regions of China, and is relatively common in East China. Brown spot can occur at both the seedling stage and the late stage of cotton, but mainly damages cotton seedlings. When the disease is severe, it causes late development of cotton seedlings, and even leaf death. In the early stage of brown spot on cotton seedlings, purple-red small spots appear on the cotyledons. When the weather is humid, the lesions expand and merge into irregular large lesions with purple-red edges, slightly raised, and yellow-brown centers, scattered with small black dots, namely the pathogen's pycnidia. The lesions break apart easily into holes. When damage is severe, the cotyledons drop early and the cotton seedlings die. The brown spot pathogen does not attack the stem or growing point, and diseased plants can still produce true leaves. When true leaves are affected, purple-red small spots first appear, then gradually expand into yellow-brown to gray-brown round lesions with purple-red edges. The lesions are brittle and easily perforated, and in severe cases can cause defoliation.