Recently, many cotton farmers have come up to inquire about the problem of cotton yellow leaves. Field investigations have found that the yellowing off of cotton leaves this year is widespread and serious. The lighter parts of the lower 2 to 3 yellow leaves fall off, and the heavy 4 to 5 leaves fall off, even more. The main reasons for the loss of yellow leaves and prevention measures are as follows:
One, the main reason
1. The fields are shaded and the humidity is high. Some cotton farmers still follow the small ridge and row spacing cultivation mode. Due to multiple rains at the seedling stage, the temperature of the seedlings drops, and the seedling disease is serious. Because of the fear of seedlings, the density of seedlings is generally too large, and the density of some lands is as high as 5,000 to 6,000. Strains. Since June, the temperature was high and rain was high, cotton plants grew too profusely and rapidly, and the control measures were not in place. As a result, the fields were closed and the ventilation and light transmission were poor, the field microclimate was deteriorating, the soil was severely compacted, and the field was hot and stuffy after rain. The lower leaves yellow off in advance.
2. Deficiency caused by falling leaves. Since the middle and late June, the rainfall in all parts of our province has been more than 30% higher than the same period of the previous year. The cotton plants have grown rapidly and prosperously, consuming more nutrients, and most of the land has been covered with ridges at a high density. No fertilizer has been applied, and the flowering period has not occurred. Timely catching up with fertilizers, there is a phenomenon of de-fertilization, which causes the yellowing of the lower leaves. The phenomenon of deficiency includes the following situations:
1 Nitrogen deficiency. The nitrogen deficiency of cotton plants showed that the stalks were weak, the upper leaves became thinner and smaller, the leaf color became lighter, the old leaves were whitish green and yellow-green, and the yellowing was dead and shedding.
2 potassium deficiency. Cotton is a high potassium crop, and insufficient potassium fertilizer can cause defoliation of insect-resistant cotton due to lack of potassium. In the middle and lower leaves, the tips of the leaves showed yellowing and scorching of the leaf margins, and the leaves shrank. Then yellow spots appeared between the veins and gradually expanded into brown spots. Finally, the whole leaf became red-brown, the leaf margins drooped, and the leaves died.
3 magnesium deficiency. The flowering period and the late period are prone to occur. Magnesium deficiency is characterized by the formation of yellow patches in the chlorosis of the lower lobes, some of which gradually turn purple, and even the entire leaf is purple, but the veins remain green and clear vein patterns are visible.
4 lack of manganese. Cotton is sensitive to manganese, due to the amount of pre-growth plus some of the province's soil is a potential lack of manganese, some plants show symptoms of manganese deficiency, the performance of the leaf chlorosis luster was gray yellow or gray red, late leaf loss of premature aging.
3. Disease caused by yellowing off.
1 Verticillium wilt. Since late June, Verticillium wilt has been seriously affected in all parts of the province, especially in the old cotton region. The symptoms of defoliation-type Verticillium wilt are: chlorotic wilting in the leaf veins or leaf margins of diseased plants. The diseased leaves are light yellow. Quickly turned brown, the leaves gradually fall from the bottom up.
2 spot disease, ring spot disease. Both of these diseases can also cause yellow leaves to fall off.
Second, prevention and control measures
1. For the phenomenon of yellowing and shedding caused by the shortage of nitrogen and potassium fertilizers, cover top dressing should be applied before August 10th. Generally, 5~7.5 kg of urea is applied per mu and 5~7.5 kg of potassium sulfate or potassium chloride is added to meet the requirement. Need for later growth. For the buddhist cotton field to focus on doing a good job of chemical control, the general Mu with shrinkage Ann 3 to 5 grams or Zhangzhou Qibao Zhuang Tao Xiao Ling 60 to 70 grams of water to 40 to 50 kg evenly spray, can effectively control the bud sprouting and The upper fruit branches are excessively elongated and enhance the ventilation and light transmission in the lower part.
2. Do a good job of foliar spray fertilizer, for the ridge can not be fertilizer on the plot should be done a good foliar spray fertilizer, spraying 1% ~ 2% urea solution to supplement nitrogen fertilizer; spray 0.2% ~ 0.3% or 1% ~ 2% Calcium phosphate leachate plus 1% to 2% potassium chloride leaching solution supplemented with phosphorus and potassium fertilizers; 0.1% to 0.2% manganese sulphate was sprayed on the detached plots due to lack of manganese; 0.3% ferrous sulfate spray was used to supplement iron deficiency; spray 0.1 %-0.2% make up for the lack of magnesium sulfate on cotton. No matter which nutrient elements are sprayed, they must be sprayed 2 or 3 times in succession. The interval should be 5 to 7 days. The water consumption should not be less than 50 kilograms. It is advisable to spray the leaves wet to the desired drip.
3. Dehumidify and increase ventilation and light transmittance. After taking rain, the crop is cultivated by 5 cm or so, and the humidity in the field is reduced. In order to reduce canopy closure, the lower leaves, yellow leaves, diseased leaves, empty fruit branches and empty trees should be promptly removed and brought out of the field for centralized treatment to eliminate some germs and eggs. The closure of serious plots will also require pushing and ridges.
4. Fungicides prevent disease. In the past two years, through screening, the agents that are more effective in preventing Verticillium wilt have chitin, ethyl allicin (or thiram) plus hymexazol, trichloroisocyanuric acid (or Greenhenge 6) and so on. The above medicines should be used in rotation, and disease prevention can be combined with pest control and sprayer fertilizer to reduce the field labor intensity and field humidity. The amount of liquid medicine per mu should be no less than 40 to 50 kg; and 70% of mancozeb manganese 800 should be used. 1000 times liquid spray to prevent other diseases such as keratoses and ring leaf spot.
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