Analysis of the Causes of Premature Senescence in Drip-Irrigated Cotton and Control Measures

Since 2002, Regiment 141 has widely promoted drip irrigation technology for cotton cultivation, with yields increasing year by year and remarkable economic benefits. However, in recent years, premature senescence has occurred to varying degrees in cotton fields, causing poor boll opening, shell clamping, and an increase in hardened and stiff boll segments and tips, reducing quality.

I. Causes of Premature Senescence

1. Long continuous cropping years

Newly reclaimed cotton fields and those with a short history of cotton planting mostly show green branches and leaves; old cotton fields with 10 or more years of continuous cropping grow well in the early stage, but once they encounter low temperatures and continuous rain in the middle and late stages, red leaf symptoms occur on a large scale. The main reasons: ① Due to long-term continuous cropping of cotton and straw return to the field, the pathogens of cotton Fusarium wilt and Verticillium wilt accumulate in the soil year after year, while the number of antagonistic bacteria is low, leading to increasingly severe disease occurrence and thus causing premature senescence. ② Due to different biological characteristics, various crops absorb different types and amounts of soil nutrients; their root exudates have different effects on the rhizosphere microbial flora and microbial energy substances; planting the same crop continuously for a long time will cause the soil ecosystem to lose balance, greatly reduce soil fertility, and cause premature senescence of cotton. ③ The accumulation of residual film in cotton fields is not conducive to cotton root growth and development, directly affecting the downward penetration and extension of cotton roots, causing root deformity and distortion, which is unfavorable for the root system's absorption of nutrients and water, thereby affecting the normal growth and development of the aboveground parts of cotton, leading to increased bud and boll shedding and causing premature senescence of cotton.

2. Shallow root distribution

Cotton drip irrigation has the characteristic of small amounts and multiple applications, and cotton roots have the characteristic of tending toward water and fertilizer, making the root distribution of drip-irrigated cotton fields relatively shallow, which is not conducive to the cotton root system absorbing water and nutrients from deep soil layers, weakening drought resistance. Once high temperature and drought occur, or water is insufficient, premature senescence is very likely to occur. Surveys show that cotton fields with early drip irrigation at the seedling stage and those with one film and three tapes are more prone to premature senescence. The reasons are that early water entry at the seedling stage results in a short seedling-stabilizing period for cotton, poor root development, small root growth volume and shallow distribution, poor stress resistance of cotton, and weak ability to absorb nutrients and water during the flowering and boll stage. The average drip irrigation volume per 667 sq m in cotton fields with one film and three tapes is about 14 cubic meters less than that with one film and two tubes, the cotton root distribution is shallower, drought resistance is poorer, and premature senescence is more likely.

3. Improper water and fertilizer management

Cotton takes 70-90 days from squaring to boll opening, accounting for more than 2/3 of the entire growth period. During this period, nutrient distribution within the plant is tight, and improper water and fertilizer management can cause premature senescence of cotton. From early June to early July, cotton is dominated by vegetative growth; from mid-July to mid-August, cotton is dominated by reproductive growth. Applying too much fertilizer by drip in the early growth stage and too little in the middle and late stages leads to excessive vegetative mass aboveground, with many buds, flowers, and bolls, but due to insufficient nutrients in the middle and late stages, a large number of buds and bolls shed, the reproductive organs of the cotton plant develop poorly, causing premature senescence.

4. Late topping time

The topping time in our regiment is from June 25 to July 10. The early flowering stage is a period when both vegetative growth and reproductive growth of cotton are vigorous, and the daily growth of the main stem reaches its maximum. If cotton grows to a certain height without timely topping, reproductive growth will lag, the aboveground vegetative mass will be too large, and leaf transpiration will be relatively high. If the cotton's water requirements cannot be met in time, it is very easy to suffer drought, causing premature senescence of the cotton plant.

5. Unreasonable chemical regulation

Chemical regulation is carried out at the emergence stage, the two-leaf stage, and one week after topping when the top fruit branch extends to 8-10 cm. Improper timing will make the middle and upper fruit branches of cotton extend too long, affecting ventilation, light penetration, and leaf photosynthesis of the lower fruit branches, causing bud and boll shedding on the lower fruit branches. Even if they do not shed, the bolls formed are smaller, and the single boll weight is lighter, causing premature senescence of cotton.

6. Damage from insect pests

Cotton aphids and cotton spider mites use piercing-sucking mouthparts to suck sap on the underside of cotton leaves, destroying leaf photosynthesis, causing leaves to discolor, curl, and even shed, causing premature senescence. Excessive concentration of pesticides and foliar fertilizers and frequent spraying can damage leaf mesophyll cells, causing round or irregular water-soaked patches on leaves, or leaves turning red and scorched, leading to severe bud and boll shedding and causing premature senescence.

7. Poor resistance of 3 types

There are many cotton varieties with rapid renewal, and the poor disease resistance and stress resistance of cotton itself can also cause premature senescence. In 2007, Biaoza No. 1, planted on a large scale in our regiment, as well as early-maturing varieties planted on a small scale, after continuous rainfall from August 12 to 14, 80%-90% of cotton fields showed large-scale red leaves, seriously affecting leaf photosynthesis and causing premature senescence of cotton.

8. Climatic factors

Continuous rainfall and low temperatures during the flowering and boll stage cause large-scale red leaves in cotton, and even drying and shedding, causing premature senescence of cotton.

II. Control Measures

1. Reasonable crop rotation

In conjunction with the development of animal husbandry on the regiment farm, adjust the planting structure, expand the planting area of grain and grass, and implement rotation and stubble reversal of grain, cotton, and grass to maintain a long-term stable cycle of soil nutrients.

2. Timely film removal

Removing the film at the early flowering stage of cotton at the end of June and the beginning of July can promote downward root growth, improve drought resistance of cotton, increase the recovery rate of residual film, and basically have no impact on the growth, development, and yield of cotton.

3. Proper seedling stabilization

The first drip irrigation of drip-irrigated cotton is from the end of May to the beginning of June. At this time, cotton begins to square successively and enters the squaring stage. The squaring stage is the period when cotton root growth is fastest; the growth rate of the main root is 2-3 times faster than that of the main stem, about 1 time faster than that of lateral roots, and a large number of new branch roots are produced. Delaying drip irrigation at this time is conducive to downward root growth and a certain amount of growth, ensuring that cotton has a sufficiently large root system during the flowering and boll stage to meet the needs of growth and development in the middle and late stages.

4. Water and fertilizer management

Apply sufficient base fertilizer at each growth stage, apply no or light squaring fertilizer, and after entering the early flowering stage, adhere to one irrigation with one fertilizer, heavily apply flowering and boll fertilizer and top-covering fertilizer to ensure uniform boll formation on the upper, middle, and lower parts of the cotton plant. Nitrogen fertilizer application in the early stage should not be too large to avoid excessive growth and unstable boll setting. Phosphorus has poor mobility in soil and low utilization rate. If diammonium phosphate is applied as base fertilizer, the condition of cotton throughout the growth period will be significantly improved, which is beneficial for preventing premature senescence of cotton.

5. Control of diseases and insect pests

For cotton Fusarium wilt and Verticillium wilt, in lightly diseased areas, diseased plants should be pulled out to reduce pathogen accumulation. In heavily diseased areas, rotation of @#@245@#@, corn, and other gramineous crops with cotton has a clear effect on reducing disease. Due to **straw return to the field, the rotation period should be increased, that is, at least 5 years or more. For insect pests, early investigation and early control should be carried out, striving to eliminate the damage at the central plant or spot-stage occurrence. In control, low concentration and large water volume should be used to reduce the number of pesticide sprays. Pesticides that are highly lethal to natural enemies are strictly prohibited to create a good ecological environment for natural enemies.

**The frost-free period is relatively short, so early-maturing varieties with strong disease resistance, stress resistance, and outstanding high yield should be planted. It is best to select multi-resistant varieties approved by relevant institutions. It is strictly prohibited to transfer in seeds that have not passed quarantine from heavily diseased areas, and delinting and coating treatment should be carried out.