What to do about premature senescence in high-density drip-irrigated cotton? Main causes and preventive measures

I. Main causes of premature senescence in high-density cotton planting

1. Variety factors

Varieties are not resistant to premature senescence. In recent years there have been many cotton varieties; some have aged and degenerated, and some are inherently poor in disease resistance and premature senescence resistance, or are unsuitable for high-density planting, leading to premature senescence in cotton.

2. Environmental factors

① Temperature For mid-early maturing cotton, the suitable sowing period is when the 5 cm soil temperature stably passes 12°C. If sowing is too early or the seedling stage encounters low-temperature chilling injury, because the air temperature is relatively low, root growth is slow and diseases are severe, which easily damages the cotton root system function, thereby causing premature senescence in cotton.

If cotton encounters low-temperature chilling injury below 15°C for a long time during the flowering and boll stage, physiological imbalance and metabolic disorder in the cotton lead to weakened growth function, making premature senescence likely to form.

② Water Insufficient soil moisture, or drought stress at the first or second irrigation of cotton, affects root absorption and normal metabolism of the cotton plant, causing the cotton root system to age prematurely, decline in function, and weaken in absorption capacity, resulting in premature senescence in cotton.

Excessive soil moisture or heavy rain after irrigation causes waterlogging in the field; the root system carries out anaerobic respiration under oxygen-deficient conditions and is poisoned by accumulated alcohol, affecting the normal metabolism of the cotton plant, damaging and aging the cotton plant root system, weakening root absorption function, and leading to premature senescence in cotton.

③ Soil The widespread use of chemical fertilizers, especially the fertilization habit of heavy nitrogen fertilizer, light phosphorus fertilizer, no farmyard manure, and no potassium fertilizer or micronutrient fertilizer, easily leads to soil nutrient imbalance, causing uneven nutrient supply to the cotton plant and the appearance of premature senescence.

When cotton is planted on excessively compacted soil, once the soil dries slightly, cotton rooting becomes difficult and root growth is hindered; the nutrients and water absorbed by the root system during the flowering and boll stage cannot meet the needs of the cotton plant, easily causing premature senescence in cotton.

④ Diseases and insect pests Long-term continuous cropping of cotton leads to an increased cumulative amount of pathogens such as Verticillium wilt and Fusarium wilt in the cotton field soil, damaging nutrient transport and causing premature senescence in cotton.

After cotton is harmed by diseases and insect pests, if disease and pest control is incomplete or unscientific, leaf function is damaged, leaf photosynthesis is affected, leaf lifespan is short, and leaves fall early, easily causing premature senescence in cotton.

⑤ Special climate After cotton suffers drought, an excessive drip irrigation amount or a sudden heavy rain easily causes physiological imbalance and metabolic disorder in cotton, leading to premature senescence in cotton.

If the cotton flowering and boll stage encounters consecutive cloudy days or consecutive heavy rain followed by sudden bright sunshine, it easily causes physiological imbalance and metabolic disorder in cotton, forming premature senescence in cotton.

3. Management factors

① Improper irrigation For drip-irrigated cotton, if there is no pre-winter stubble irrigation or winter irrigation, due to insufficient subsoil moisture, early water entry at the seedling stage, and a short cotton seedling-hardening period, root growth is small and distributed shallowly, resulting in poor stress resistance in cotton; the nutrients and water absorbed by the root system during the flowering and boll stage cannot meet needs, and premature senescence is likely.

Dripping water during high-temperature summer weather causes the soil temperature to drop suddenly, root metabolism to become disordered, and absorption to be hindered, leading to premature senescence in cotton.

The root system of drip-irrigated cotton is distributed relatively shallowly; if the drip irrigation cycle during the flowering and boll stage is too long, it easily causes premature senescence in cotton due to drought stress.

② Unreasonable fertilization In recent years, high-density cotton planting has been adopted, and harvest has reached 15,000 to 18,000 plants per 667 sq m; the biological yield of cotton is also increasing, so fertilizer input also needs to increase. Insufficient fertilization or an unreasonable ratio of phosphorus, nitrogen, and potassium also easily causes premature senescence in cotton.

③ Unscientific chemical regulation If chemical regulation in the early and middle stages is too light, the field is too shaded, the lower leaves of cotton receive little light and manufacture few nutrients, and cannot meet the needs of the lower buds, bolls, and root system, causing the root system to age prematurely due to malnutrition and resulting in premature senescence in cotton.

In early-developing cotton fields, if bolls set early and abundantly in the early stage, with concentrated boll load and a relatively heavy physiological burden, too-heavy chemical regulation or insufficient water and fertilizer supply easily leads to premature senescence.

If chemical regulation is too heavy in the late stage of cotton, the upper internodes become too compressed, leaves are small and flat, and ventilation and light transmission in the middle and lower parts are poor, easily causing premature senescence in cotton.

II. Main countermeasures to prevent premature senescence in high-density drip-irrigated cotton

1. Select good varieties

Choose mid-early maturing varieties with high disease resistance, resistance to continuous cropping, strong late-stage leaf function, and suitability for dense planting.

2. Sow at the appropriate time

The cotton sowing period should be determined according to comprehensive factors such as the local climate, variety growth and development characteristics, and drip irrigation conditions. Generally, sowing and drip irrigation can begin when the 5 cm soil temperature under the film stably passes 12°C for two consecutive days.

3. Timely intertillage

Timely intertillage once cotton rows are visible can break crusting, loosen the soil, increase soil permeability, raise soil temperature, and benefit normal root growth.

4. Seedling hardening at the right time

High-density drip-irrigated cotton must have winter irrigation or stubble irrigation to store sufficient subsoil moisture. In spring, the cotton seedling-hardening time should be no less than 35 days, so as to allow a certain amount of root growth, ensure the cotton root system is large enough during the flowering and boll stage, and meet the needs of cotton growth and development in the middle and late stages.

5. Scientific fertilization

For a cotton lint yield of 120 kg per unit area, apply about 130 kg of standard fertilizer per 667 sq m, with nitrogen : phosphorus : potassium = 1 : 0.4-0.5 : 0.1-0.15. Apply 30%-40% of the nitrogen fertilizer, 50%-60% of the phosphorus fertilizer, and 50% of the potassium fertilizer as base fertilizer with deep plowing. If growth is normal at the seedling and bud stages, no fertilizer may be applied. During the flowering and boll stage, apply the remaining nitrogen, phosphorus, and potassium fertilizers in split applications with drip irrigation.

6. Reasonable drip irrigation

The first irrigation should not be too early; for autumn-winter irrigated cotton, drip irrigate in early to mid-June. During the flowering and boll stage, cotton must not suffer drought or flooding; it is best not to drip irrigate before and after heavy rain. Reducing the irrigation amount and shortening the irrigation cycle can be adopted to prevent drought stress.

7. Scientific chemical regulation

For high-density drip-irrigated cotton, spray mepiquat chloride at the 2-3 leaf stage and 5 leaf stage, adding urea and monopotassium phosphate, to improve the leaf's ability to manufacture nutrients and create good conditions for root growth.

Adhere to the principle of giving priority to water control and supplementing with chemical regulation, and achieve small amounts applied multiple times to ensure even internode distribution: lower internodes 1-4 should be 2-4 cm long, internodes 5-7 should be 4-5 cm long, internodes 8-10 should be 5-6 cm long, and internodes 11-13 should be 6-7 cm long, with leaves held upward and forming an angle of 30-45 degrees with the main stem.

When cotton encounters high-temperature or low-temperature weather during the growth period, when heavy rain falls suddenly after drip irrigation, or when the flowering and boll stage encounters consecutive cloudy days or consecutive heavy rain followed by sudden bright sunshine, appropriately spraying field cotton leaves with foliar fertilizer containing biological regulatory substances, organic substances, and trace elements, plus urea and monopotassium phosphate, can prevent and control premature senescence.

8. Disease and insect pest control

Adjust the production layout and advocate reasonable crop rotation. For cotton fields with severe Verticillium wilt and Fusarium wilt, reasonable rotation and stubble change should be carried out, with a rotation cycle of 3-5 years, which has a certain control effect.

For cotton fields where Verticillium wilt and Fusarium wilt occur sporadically, conduct a general survey of the cotton field twice, in June and August, pull out diseased plants, carry them out of the field, and burn them. Supplement with foliar spraying of foliar fertilizers such as Haizhuangyuan 818 and Lüfeng 95 containing biological regulatory substances, organic substances, and trace elements, plus monopotassium phosphate.

Increase integrated control of insect pests, and achieve early detection and early control. For red spider mites, conscientiously implement the control policy of "inspect, insert, wipe, pick, pull, smear, spray." For cotton bollworm and aphids, on the basis of careful investigation, closely monitor the number of natural enemies and the development dynamics of pests, and spray only after reaching the threshold, eliminating large-area and high-dose pesticide application so as to protect natural enemies.

9. Other measures

For early-developing cotton fields or cotton fields showing a trend of premature senescence in the late stage, apply sufficient fertilizer for the flowering and boll stage, and combine with chemical regulation to spray foliar fertilizer containing biological regulatory substances, organic substances, and trace elements, plus urea and monopotassium phosphate, for a total of 2-3 sprays.

When cotton encounters high-temperature weather, do not drip irrigate at noon. For drought-stressed cotton fields, the first drip irrigation amount should not be too large; adopt the method of reducing the irrigation amount and shortening the cycle to alleviate the drought.