Why do young bolls on the middle and upper parts of the cotton plant crack open prematurely?
Recently, some farmers called to report a strange phenomenon in cotton production: the large bolls and early-developing bolls on the lower part of the plant were growing normally and had not yet opened, while the tender bolls on the middle and upper parts cracked open prematurely, and some of the cracked bolls then rotted. The Wuqiao County Agriculture Bureau immediately organized technical personnel to go into the fields to find the cause.
1. Field symptoms
In early-sown cotton fields where the top 1-3 fruiting branches had no buds or bolls, or very few buds and bolls, due to rain or pest and disease damage, the number of cracked bolls on the middle and upper parts was relatively high, that is, the outer bolls on the 7th-8th fruiting branches counting from the bottom and the inner bolls on the 9th-10th fruiting branches, with a diameter of about 2.5-3.1 cm. The boll shell surface shrank, became thinner, and cracked, turning brown from the top of the boll shell to more than two-thirds of the way down, and the bracts withered and turned yellow. Some cracked bolls, because rainwater entered them and they were then infected by pathogens, showed rot. In such cotton fields, there were on average 2-3 cracked bolls per plant, with as many as 4-5 on some plants; in ordinary cotton fields, there were on average 0.5-0.8 cracked bolls per plant, and late-developing cotton fields had the fewest cracked bolls.
2. Cause analysis
1. It is related to how early or late the sowing date was. In early-sown cotton fields, because development was early and they were affected by continuous high temperatures and sufficient light from late June to early July, there were as many as 2-3 or even more pre-dog days bolls, making reproductive growth vigorous while vegetative growth was relatively weakened. From late July to early August, the average temperature was 1.3°C-1.5°C lower than in the same period in previous years, and there were many consecutive rainy days (cotton is a warmth-loving and light-loving crop and requires relatively high accumulated temperature and strong light). The continuous rainy weather led to insufficient light and reduced photosynthetic products. The fiber and seeds inside the tender bolls on the middle and upper parts grew normally, but because the boll shell received less nutrient supply, and in particular synthesized less of the hydrophilic substances such as soluble carbohydrates that fill out the boll shell, its growth rate lagged severely, causing the boll shell to dehydrate and crack prematurely.
2. Variety factors. The degree of cracking differed somewhat among varieties in the cotton fields. Varieties with thin boll shells and high boll weight, such as 7886, had a more severe degree of cracking than varieties with thick boll shells and relatively low boll weight.
3. Different numbers of sprayings and different application methods also caused differences in the degree of cracking. Fields sprayed more often were clearly more severely affected than fields sprayed less. Analysis shows this is closely related to the spraying method. Farmers are accustomed to mixing several pesticides together, and even spraying some fertilizers and foliar fertilizers together. When the pesticide mixing method or procedure is incorrect, more pesticide and fertilizer land on the middle and upper cotton bolls, and the pesticide and fertilizer accumulate on the surface of the boll shell. The tender bolls on the middle and upper parts are in a period of rapid growth. On the one hand, the accumulation of pesticide and fertilizer affects photosynthesis, and on the other hand it accelerates dehydration and aging of the boll shell surface, prompting it to crack early.
4. Climatic factors. Climatic factors are the most important factors causing cotton boll cracking. From late July to early August, there were as many as more than ten consecutive rainy days, the temperature was 1.3°C-1.5°C lower than in the same period in previous years, and light was severely insufficient. Photosynthesis and photosynthetic products declined, leading to unbalanced internal and external development of the cotton boll. The accumulation of internal substances proceeded normally, while dry matter accumulation in the boll shell was inhibited, and the boll shell grew slowly or stopped growing, causing a loss of proportion and leading to cotton boll cracking.
5. It is related to how well aphids were controlled. Because summer aphids occurred severely this year, in all fields where summer aphids were poorly controlled for various reasons, and where plant vegetative growth was weakened, there were more cracked cotton bolls than in cotton fields where aphid control was effective.
3. Preventive and remedial measures
1. Sow at the appropriate time and not too early. According to local climatic characteristics and combined with previous years' planting experience, sowing at the appropriate time and not too early ensures a full stand from one sowing, reduces the severity of seedling diseases, promotes robust seedling growth, improves the plant's disease resistance and stress resistance, reduces the number of pre-dog days bolls, ensures the proportion of the three types of bolls, and achieves balanced yield increase.
2. Choose varieties with strong stress resistance and make a good varietal planting combination. For old cotton fields and severely diseased plots that have been continuously cropped for many years, choose varieties with strong early growth and a fast growth and development process, to prevent the occurrence of advanced reproductive growth caused by weak seedlings. Farmers with large cotton planting areas may choose 2-3 varieties to plant in combination, to prevent losses caused by various extreme abnormal climatic effects.
3. Control pests and diseases in a timely manner and scientifically mix pesticides. Important pests and diseases in cotton fields include red spiders, aphids, plant bugs and cotton bollworms, and all localities should monitor them closely. When the occurrence reaches the control threshold, control should be carried out promptly without delay, to prevent severe damage to cotton plants caused by missing the key period for pest and disease control, and to prevent premature senescence of cotton plants. When mixing pesticides, the following points should be noted: ① A compound preparation with good simultaneous control effects may be selected and used alone to control multiple pests; ② Prepare a mixing bucket, and first dissolve the single agents or fertilizers to be mixed in the bucket at the required concentration for later use; ③ First add half a bucket of water to the sprayer, then put in the diluted pesticide solution or fertilizer, fill with water, stir repeatedly, and then spray; ④ Pesticides should be mixed and used immediately.
4. Remove the plastic film in a timely manner and do good intertillage to promote downward rooting of the root system and improve the plant's drought resistance, disease resistance and stress resistance.
5. Appropriately widen row spacing and retain leaf branches to increase the vegetative body area and improve stress resistance. According to experiments, one more vegetative branch on cotton means one more root. Only with many roots and deep rooting can deep roots and luxuriant leaves be achieved, for the purpose of disease resistance and senescence resistance. According to investigation, cotton fields planted this year with uniform row spacing of 80-100 cm have so far basically had no occurrence of premature cotton boll cracking.
6. Treatment of cracked cotton bolls. For currently cracked cotton bolls, as long as there is no more continuous rainy weather, they may temporarily not be removed. Nutrients in the leaves and fruiting branches can still be transported into the cotton boll through the boll stalk, which is beneficial for appropriately increasing boll weight. Cotton bolls into which rainwater has entered and which have then been infected by pathogens should be removed in a timely manner, taken out of the field for centralized treatment or drying.