What causes cotton square and boll shedding?

In cotton production, heavy square and boll shedding often occurs, seriously affecting cotton yield and cotton farmers' income. Since the causes of square and boll shedding are many-sided, only by treating the specific cause can shedding be effectively reduced and yield and benefits improved.

1. Variety factors. Tarnished plant bugs and cotton bollworms like to lay eggs on cotton varieties with dense fuzz, and dense-fuzz varieties increase the adhesion of eggs, making them less likely to be blown off by wind or washed away by rain, so damage is severe and large numbers of squares and bolls are shed. Smooth-leaf varieties can have 40.3% fewer squares and bolls damaged than dense-fuzz varieties.

2. Cultivation factors. At present, a very prominent problem in cotton production is excessive density, which easily causes heavy square and boll shedding. At the same time, excessive density often triggers a chain reaction of worsened pests and diseases and severe premature senescence in the late stage.

3. Fertilizer factors. These are mainly caused by improper fertilization, potassium deficiency, boron deficiency and molybdenum deficiency.

4. Pest factors. Besides the square and boll shedding caused by cotton bollworms, cotton stink bugs and cotton boll weevils can directly cause damage leading to square and boll shedding. Indirect damage from cotton aphids, red spider mites, tobacco whiteflies and others can also cause square and boll shedding through plant malnutrition.

5. Disease factors. The occurrence of Fusarium wilt, Verticillium wilt and red-leaf stem blight (especially under high humidity conditions) often leads to heavy square and boll shedding.

6. Chemical control factors. Many cotton farmers wait until the cotton grows excessively before applying chemical control, which leads to an imbalance between vegetative growth and reproductive growth, elongated fruiting nodes, an undesirable plant shape, severe square and boll shedding, and induced premature senescence (Note: some cotton farmers use products marketed as "eight-no-care" and similar; because the mepiquat chloride content is too low, the chemical control effect is not achieved, resulting in excessive plant growth).

7. Climate factors. Drought, high humidity, high temperature and insufficient light can all lead to square and boll shedding.

8. Physiological factors. 1. Failure to fertilize. During flowering, adverse environmental conditions such as rainfall, high temperature, high humidity and drought can damage the pollen and the pollination and fertilization process, so the ovary cannot be fertilized and is shed. 2. The role of hormones in the cotton plant. Cotton bolls contain endogenous hormones such as abscisic acid and ethylene. The content of abscisic acid increases greatly 3 days after flowering and peaks 10 days after flowering, so boll shedding is greatest at this time and then decreases significantly afterward; ethylene peaks on the day of flowering and 1 to 2 days before boll opening, often causing young boll shedding in the early stage and promoting boll maturity in the later stage.

Based on the above causes, the following preventive measures are recommended:

1. Choose smooth-leaf varieties with many squares and strong boll setting, and try not to plant dense-fuzz varieties.

2. Blindly increasing density to obtain yield is not advisable; reasonable sparse planting must be carried out according to the soil fertility level and the characteristics of the variety planted.

3. Base fertilizer should be applied as a balanced formula according to the fertilizer requirements of cotton. For base fertilizer, diammonium phosphate combined with potassium fertilizer is best, and where conditions allow, use as much organic fertilizer as possible.

4. Topdressing should be mainly nitrogen fertilizer, following the principle of "light at the square stage, heavy at the boll stage" (apply potassium fertilizer as a supplement at the square stage if it is insufficient, and apply phosphorus fertilizer as a supplement at the flowering and boll stage if it is insufficient). Note that nitrogen topdressing should be timed according to the weather forecast to avoid rainfall and kept away from the cotton root zone, and watering should be done promptly after topdressing.

5. Use mepiquat chloride (mepiquat) for chemical control, following the principle of "small amounts, multiple applications, light control and frequent adjustment."

6. Inspect pests and diseases frequently and carefully, and apply pesticides promptly at the initial stage of occurrence. At the same time, note that spraying should be done after 5 p.m. to prevent pollen grains from rupturing and becoming sterile due to contact with the spray solution during the morning peak period of pollination and boll setting, which would affect normal pollination.

7. Water promptly in case of high temperature and drought.

8. Under adverse climatic conditions such as cloudy and rainy weather with little sunshine or high temperature and drought, while controlling pests or preventing diseases, spray brassinolide at a concentration of 0.02 mg/kg (or 0.01% gibberellin) + 0.1% boron fertilizer + 0.1% 25% complexed high zinc + 600 times 40% chelated high potassium (or high-quality 0.5% monopotassium phosphate), which can significantly reduce square and boll shedding and markedly increase yield.

9. At the full flowering stage and the flowering and boll stage, spray once each with 20% Baoying (benzylaminopurine acid) at 300 to 400 times dilution, which can play a good role in protecting squares and bolls.