Occurrence Patterns and Control Methods of Cotton Seedling Diseases

Cotton seedling diseases are characterized by a concentrated onset period, generally occurring 15-30 days after sowing. The primary infection source comes mainly from the soil, with a small portion from seeds. Around April 20 this year, many provinces experienced continuous rainy and low-temperature weather, so seedling diseases are bound to be severe, and timely control measures should be taken in all areas.

I. Symptom Manifestations

(1) Damping-off. When cotton seedlings are affected before emergence, it causes seed and sprout rot. When disease occurs after seedling emergence, lesions initially appear as yellowish-brown and later turn black and sunken at the base of the young stem near the soil surface. The lesions gradually expand, surround the entire stem base, and eventually cause the seedling to shrink and die. When diseased seedlings are pulled up, mycelium is often seen adhering to the diseased area and surrounding soil. After cotyledons are affected, irregular yellowish-brown lesions mostly appear in the middle, and in the later stage the diseased part falls off, forming holes.

(2) Anthracnose. When disease occurs before cotton seedling emergence, the hypocotyl and young roots turn brown and rot. When disease occurs after seedling emergence, small reddish-brown spots appear at the stem base, later expanding into purple-brown sunken spindle-shaped streaks. The lesions often split longitudinally and sink, with constriction around them, and cotton seedlings easily break at the middle and die. Under humid conditions, the lesions can develop inward, reaching the top of the young stem to form top rot. After the roots are infected, they become dark brown and semi-wet-rotten, and do not break easily when pulled up. Cotyledon disease mostly produces semicircular light brown to brown lesions at the leaf margins, with reddish-brown edges; under humid conditions, orange-red sticky spore masses form on the lesion surface.

(3) Red rot. When affected before cotton seedling emergence, the young sprouts turn yellowish-brown and rot. When roots are affected after seedling emergence, the root tips first turn light yellow, then brown and rot, and gradually spread to the entire root; when the young stem base is affected, it first turns yellowish-brown, then dark brown and rots, with lesions not sunken; when the main root and tender stem at the soil surface are affected, they become swollen and produce short brown streaks, which is the main characteristic of this disease; when cotyledons are affected, grayish-red irregular or nearly circular lesions mostly appear at the margins, and the lesions often rupture. Under humid conditions, pink mold layers form on the diseased parts.

(4) Damping-off (Pythium blight). The main symptom is that seedlings soften and fall over from the base upward in a light brown, water-scalded manner, and the cotyledons immediately become water-soaked, softened, and discolored. The disease progresses rapidly, hence the name damping-off. The pathogen mostly invades first from the young fine roots and spreads to the young stem and the entire seedling. When soil moisture is relatively high, seeds and newly germinated sprouts can also be infected, affecting emergence or forming weak seedlings. On collapsed dead seedling clusters, white fine flocculent matter can sometimes be seen.

II. Conditions for Disease Occurrence

(1) Climatic conditions. The growth, reproduction, and infection of seedling-stage pathogens all require relatively high humidity. Therefore, if rainy weather occurs during the seedling stage or if soil moisture is high while soil temperature is low, it favors the occurrence and development of the disease. For example, low temperatures below 10°C will cause seed rot. Two weeks after cotton seedling emergence, the nutrients in the cotton seed itself are nearly exhausted, and the seedling's ability to produce nutrients is weak. At this time, the seedling's disease resistance is extremely poor, making it susceptible to disease.

(2) Seed quality. Seeds with low maturity, or those that have become damp and moldy, are prone to seed and sprout rot, which not only is unfavorable for emergence but also mostly produces weak seedlings after emergence that are susceptible to disease.

(3) Sowing quality. Plump cotton seeds with strong vitality emerge quickly, and the seedlings are strong and disease-resistant. Conversely, emergence is slow, and the seedlings are weak and not disease-resistant.

(4) Tillage and cultivation measures. In continuous cotton cropping fields, pathogens accumulate in the soil year after year, resulting in severe disease. In low-lying cotton fields with poor drainage, soil moisture is high, ground temperature is low, aeration is poor, and the cotton seedling root system develops poorly, making disease more likely; rough land preparation or tillage with many clods, or too thick soil covering at sowing, all affect cotton seedling emergence and cause severe disease; sowing too early or too deep causes slow seedling emergence and makes disease more likely.

III. Control Methods

Because there are many types of cotton seedling diseases, often with several diseases occurring together, and because they are affected by environmental conditions and the disease resistance of the cotton seedlings themselves, control should, on the basis of good seed disinfection and soil treatment, focus on cultivation management during the seedling stage and chemical control measures.

(1) Cultivation management. After cotton emergence, the ability to absorb inorganic nutrients is relatively weak, the leaf area is small, and the organic nutrients produced by photosynthesis are insufficient. Coupled with the influence of harsh climatic conditions, root growth becomes even more difficult, which aggravates the occurrence of disease. Therefore, intertillage during the cotton seedling stage to raise ground temperature is a necessary means of solving seedling-stage diseases.

(2) Chemical control. After cotton emergence, spraying 1-2 times with a 600-800 times dilution of the drought-resistant and seedling-strengthening Tianda-211 solution can improve the cotton seedlings' resistance to low temperature and disease. In the early stage of disease, spray 50% carbendazim or 70% thiophanate-methyl wettable powder at 800 times dilution plus 70% mancozeb at 600 times dilution, once every 7 days, for 2 consecutive sprays. Using fungicides together with foliar fertilizer gives better results.