What causes premature senescence in cotton seed production fields? How should it be prevented and controlled?

As cotton seed production fields are planted year after year, premature senescence occurs from time to time, and has become an important factor restricting the increase of per-unit yield and the improvement of quality in seed production fields.

1. Causes of premature senescence

1. Continuous successive cropping of seed cotton fields

In cotton-growing areas, cotton has been planted continuously on the same land for many years, resulting in a lack of organic matter in the plow layer, which cannot satisfy the soil's sustained fertilizer supply effect. At the same time, successive cropping causes the accumulation of pathogenic bacteria in the soil and a reduction in the number of insect-resistant bacteria, blocking the normal growth of the cotton root system, thereby affecting the upper growth of cotton and causing premature senescence.

2. Premature senescence of the cotton root system

Planting with wide film and ultra-wide film causes the main root of film-mulched cotton to be distributed shallowly, with many lateral roots and a large root volume. According to measurements, the root volume in the 0-35 cm soil layer accounts for 85% of the total root volume, and the root volume in the 0-10 cm soil layer accounts for 35% of the total root volume. In the late growth period of cotton, the root volume drops rapidly, some roots gradually senesce, the root-shoot ratio becomes small, root activity declines, the ability to absorb fertilizer declines, and the plant becomes sensitive to water and fertilizer demand. If water and fertilizer supply is insufficient at this stage, the root system quickly fails. In particular, drought or waterlogging during the bud and boll stage causes poor cotton growth, damages the root system, and thus causes premature senescence.

3. Imbalance of nutrient supply and demand

Heavy application of chemical fertilizers without organic fertilizer, emphasis on nitrogen fertilizer while neglecting phosphate fertilizer, and no application of farmyard manure, potassium fertilizer or micronutrient fertilizers lead to a serious imbalance of soil nutrients. The soil lacks staying power, causing an unbalanced nutrient supply to the cotton plant, uneven distribution of nutrients at different growth stages, disordered nutrient absorption by the plant, a marked decline in disease resistance, and the appearance of premature senescence.

4. Extensive use of plastic film

The large-scale annual use of plastic film has seriously affected the properties of the soil, reducing permeability and tilth, and creating a natural barrier for the root system to absorb nutrients and water. It has seriously affected the effects of fertilization and irrigation, weakened stress resistance, caused full-layer fertilization to be applied on the surface of the plastic film, affected the root system's absorption of fertilizer, and reduced fertilizer use efficiency. At the same time, residual film also affects cotton seedling emergence in the field and the downward growth of the root system.

5. Occurrence of pests and diseases

The diseases commonly occurring in cotton seed production fields are mainly Fusarium wilt and Verticillium wilt. After cotton becomes diseased, nutrient supply is blocked, affecting normal growth and development of cotton, thereby causing premature senescence in the later stage. In recent years, there have been many cotton varieties, and the poor disease resistance and poor premature-senescence resistance of some varieties themselves are also one cause of premature senescence in cotton.

6. Improper management measures

Sowing cotton seed at a suitable sowing date can achieve the technical goals of uniform emergence, complete stand, strong seedlings, uniform seedlings, and early development of strong seedlings. If cotton is sown too early, temperatures are low, root growth is slow, and diseases are severe. Early fruit setting is early and abundant, leading to insufficient nutrient supply in the middle and late stages and the occurrence of premature senescence. If the soil moisture before sowing is insufficient during the seedling stage of cotton, the root system penetrates shallowly into the soil and lateral roots are few in distribution. Then, after budding, flowering and boll setting, vegetative growth and reproductive growth proceed together. Once there is sufficient water, cotton will grow rapidly, causing delayed maturity with excessive vegetative growth, too many bolls set, disordered nutrition in the cotton plant itself, and reduced plant resistance. In addition, this is also a period when Fusarium wilt and Verticillium wilt occur frequently, making the cotton field prone to premature senescence.

7. Lack of water during irrigation periods

During the cotton growth period, water demand is about 9.26% at the seedling stage, 11.38% at the bud stage, 56.4% at the flowering and boll stage, and about 22.93% at the boll-opening stage. When drought weather occurs in the middle and late stages of cotton, the cotton plant cannot carry out normal physiological metabolism, affecting yield formation and accelerating premature senescence. As the area under autumn and winter irrigation gradually decreases and drip irrigation technology is used, drip irrigation applies small amounts of water but frequently, which also causes the root system to be distributed within a smaller range. The cotton root system penetrates shallowly into the soil, lateral roots are few in distribution, and the root system is highly dependent, showing poorer stress resistance and adaptability under unsuitable climatic conditions and causing premature senescence in cotton.

2. Prevention and control countermeasures

1. Crop rotation and stubble changing

Cotton seed production fields should be selected on flat terrain with fertile soil, convenient drainage and irrigation, a consistent previous crop, and uniform soil fertility. A stubble change should be carried out every 3-5 years, especially grain-cotton rotation and alfalfa-cotton rotation, which have obvious effects. Intercropping and relay intercropping for stubble changing may also be appropriately carried out, and cotton fields should be cleaned in time to eliminate residual film in cotton fields. At the same time, crop rotation and stubble changing and deep plowing of cotton fields can change the ecological conditions of cotton fields, improve soil structure, reduce sources of bacteria, increase the accumulation of soil organic matter, improve the soil's water storage and fertilizer retention capacity, promote deep rooting of the plant root system, promote deep roots and luxuriant leaves, robust growth, and prevent and control premature senescence.

2. Soil testing and fertilizer application

In order to achieve high quality, high yield and high efficiency in cotton, combined with the local situation of nitrogen deficiency and phosphorus shortage, balanced fertilization should be carried out, with nitrogen and phosphorus used together. The nitrogen to phosphorus ratio in cotton fields is 1:0.3-0.7. Apply 45 tons of base fertilizer per hectare. Organic fertilizer has complete nutrients and decomposes slowly in the soil to supply the needs of the cotton plant, while also improving the soil's physical and chemical properties and soil structure, which is beneficial to the development of the cotton root system and meets the nutrient needs of the middle and late stages.

Chemical fertilizer application should follow the principle of stabilizing nitrogen, increasing phosphorus, increasing potassium, and supplementing micronutrients. Apply an additional 300 kg of potassium fertilizer, 15 kg of zinc sulfate, and 7.5 kg of borax per hectare. Ensure good seed fertilizer, stable bud fertilizer, heavy flowering and boll fertilizer, and supplementary top-covering fertilizer.

3. Recovery of residual film

In cotton seed production fields, the film should be removed at the appropriate time during the growth period, and film raking and film picking should be done well during spring land preparation and before autumn plowing. Gradually reduce the amount of residual film in cotton fields, reduce pollution, promote downward growth of the cotton root system, and prevent premature senescence in cotton.

4. Integrated control of pests and diseases

During the cotton growth period, intensify integrated control measures and do a good job in preventing Fusarium wilt and Verticillium wilt. Strengthen pest and disease investigation, control their spread and damage, and deal with discovered disease promptly. Prohibit excessive spraying of powders and high-concentration pesticides to prevent damage to leaves and effects on photosynthesis. Eliminate the use of organophosphorus pesticides and extend the functional period of leaves. Especially in the middle and late stages of cotton, do a good job in integrated control of cotton aphids, cotton spider mites, and cotton bollworms to ensure normal growth and development of cotton and slow down senescence.

5. Intertillage and soil loosening

During the cotton seedling stage, carry out intertillage 1-2 times, with an intertillage depth of 14-16 cm and a seedling protection strip of 8-10 cm, with the intertillage depth achieved in one pass. During intertillage, leave no uncultivated strips, do not raise large clods, do not pull furrows, do not bury seedlings, and do not cut seedlings, achieving the quality requirements of flatness, looseness and fragmentation. For cotton fields with compacted soil, carry out timely intertillage to break up compaction, enhance soil permeability, promote downward growth of the root system, and ensure steady growth of cotton. For cotton fields with lower terrain, ensure smooth drainage and irrigation in the field, prevent water accumulation after rain, and carry out timely intertillage and soil loosening so that cotton roots grow deeper, thereby enhancing the stress resistance of cotton.

6. Rational irrigation

The pressurized drip irrigation method under film provides a better soil environment for the normal growth and development of cotton, which can avoid deep seepage and also control and reduce evaporation between crop roots to the greatest extent, so that the soil basically has no phenomenon of early waterlogging or drought, greatly reducing bud and boll shedding. In order to prevent and control premature senescence in the later stage of cotton, if drought occurs during the cotton flowering and boll stage from July to mid-August, drip water in time, shorten the irrigation cycle, and enhance the cotton's ability to resist premature senescence. Drip water 8-12 times throughout the entire cotton growth period. Each time, drip 225-375 cubic meters per hectare. According to soil moisture and growth stage, determine the appropriate time to stop watering, ensuring that cotton does not senesce or grow excessively, and opens white bolls with green branches and leaves.

7. Chemical regulation and branch pruning

In the early stage of cotton growth, focus on promotion to promote vegetative growth and build a high-yield framework, with control within promotion, strong but not excessive, avoiding excessive vegetative growth that causes rampant growth. In the later stage, focus on control, combining promotion and control, that is, controlling vegetative growth while also having promotion within control, to prevent premature senescence. On the basis of water and fertilizer control, carry out chemical control at the appropriate time and reasonably, so that cotton is vigorous and robust in the early stage, and steady and not senescent in the later stage, adhering to the chemical regulation principle of "early, frequent, light". Top the plants in time, adhering to the principle of "do not wait for the time when branches are sufficient, and do not wait for branches when the time is sufficient". When the visible fruit branches per plant are 8-9, topping can be carried out, and after topping retain 7-9 fruit branches. When topping, remove the growing tips on both sides of the cotton plant together. Prune branches at the appropriate time. Generally, it is required to use flowers as the boundary on August 10, remove the excess upper part of the cotton plant, and finish all of it before August 15.