What causes yellowing leaves in watermelons after transplanting?

Yellowing leaves in watermelons after transplanting may be caused by the use of unrotted human or animal manure, or long-term heavy use of chemical fertilizers, a deficiency of potassium, calcium or boron in the plants, improper use of herbicides, or damage from red spider mites. Therefore, when using fertilizers and herbicides, you must master the right amounts and techniques, and for diseases you must apply the right remedy promptly.

Causes of and control methods for yellowing leaves in watermelons after transplanting:

1. Yellow leaves caused by root burn from improper fertilizer use

Because unrotted human or animal manure is used, or chemical fertilizers are used heavily over a long period, too much fertilizer damages the roots. Some growers use too much foliar fertilizer; since many foliar fertilizers contain hormone-like substances, this causes hormone and trace-element toxicity in watermelons, making the leaves stiff, chlorotic, brittle, deformed and yellow.

Control methods:

1. Strictly prohibit the use of unrotted human or animal manure. Chicken and cattle manure must be fully fermented and rotted in advance before use.

2. When using chemical fertilizers, it is best to adopt formula fertilization and appropriately increase the application of phosphorus and potassium fertilizers. When applying base fertilizer, nitrogen fertilizer should not be used in excess, to prevent excessive nitrogen from burning the roots.

3. If fertilizer burn to the roots is found, microbial agents should be applied promptly by flushing, at 5-10 kg per mu, flushed with water. For watermelons grown in greenhouses where root rot and yellow leaves are caused by the use of raw manure, the method of breaking the film and ventilating should also be adopted to reduce gas damage.

2. Yellow leaves caused by herbicide damage

In watermelon fields, post-emergence herbicides are generally used. When transplanting, the order of operations must be strictly followed: spray the herbicide on flat ground before ridging, then dig holes and set the plants. In this way, when transplanting, the herbicide-containing soil layer on the surface can be turned aside, avoiding damage to the watermelon root system. For watermelons grown in small arched sheds in the north, when using herbicides, double mulching should be used as far as possible, that is, one layer of plastic film and one layer of arched shed. In this way, the plastic film can effectively press the fumes evaporated from the herbicide under the film and will not affect watermelon growth. For the planting method of using one film for two purposes, that is, as an arched shed in the early stage and as plastic film in the later stage, herbicides should be used very cautiously. In a closed environment, water vapor transpiration plus herbicide fumigation will cause harmful gases to inhibit the growth of watermelon seedlings and make the leaves turn yellow.

Control method: ventilate promptly, and drench the roots with 5 kg per mu of Jinwei microbial agent, then water; the yellow leaves can gradually turn green again.

3. Yellow leaves caused by nutrient deficiency

In recent years, because watermelon cultivation has good returns, continuous cropping of watermelons has become quite prominent, which easily causes nutrient deficiency. The more common cases are potassium, calcium and boron deficiency in watermelons, all of which easily cause yellowing of watermelon leaves to varying degrees.

Control methods:

1. Try not to practice continuous cropping.

2. For continuously cropped watermelons, while applying nitrogen, phosphorus and potassium fertilizers, 50 kg per mu of Shenliu 54 mineral fertilizer can be added to supplement the deficiency of medium and trace elements; for watermelon seedlings that already show nutrient deficiency symptoms, spraying 300 times diluted compound microbial fertilizer or 350 times diluted Jinwei multi-purpose plus 500 times diluted Indian calcium has a clear effect on preventing nutrient deficiency symptoms in watermelons.

4. Yellow leaves caused by soil salinity damage

Soil salinity damage mostly occurs in greenhouse watermelon sheds, while in open-field watermelons it mostly occurs in continuously cropped plots. Long-term use of chemical fertilizers or excessive use of chicken, cattle and duck manure often makes salinity damage more severe. Soil with heavier salinization generally shows a white or red surface. Watermelons growing in salinized soil show slow growth, stunting, dark green leaf color, and withered, water-loss leaf margins.

Control methods:

1. Rotate crops. It is best to rotate with crops other than cucurbits, especially paddy-upland rotation. Use chicken manure and cattle manure appropriately.

2. Improve the soil with microbial agents. Microbial agents or soil conditioners can be used to reduce salinity and neutralize alkalinity, so as to promote watermelon root growth; this is quite effective in preventing yellow leaves caused by salinity damage in watermelons.

5. Yellow leaves caused by red spider mite damage

Adult and larval red spider mites gather on the undersides of young tender leaves to pierce and suck the leaf sap. Especially the unopened buds, young leaves and flower buds are the main damaged parts, causing watermelon leaves to be damaged and unable to grow normally. Leaves pierced and sucked by red spider mites lose their green color and turn yellow, and in severe cases the leaves can dry up.

Control method: remove weeds around the watermelon field (shed) to cut off the source of the pests. Red spider mites have a short life cycle and strong reproductive capacity, so early control should be emphasized. Spray 1.8% Chongmankexing aqueous solution at 2000-3000 times dilution or 2% abamectin at 1500-2000 times dilution on the leaves for control.

The above is the relevant introduction to yellowing leaves in watermelons after transplanting. I hope it is helpful to everyone.