What disease causes yellowing cucumber leaves? How to treat it?

Cucumber is one of our most common vegetables . In some places, cucumber is called green melon. In the process of growing cucumber, some diseases often occur. Yellowing leaves is one of the most common diseases. So what disease is the yellowing of cucumber leaves? How to treat it? Let’s take a look below.

1. Infectious diseases

1. Downy Mildew

Downy mildew is the main infectious disease in most cucumber-growing areas, and it develops quickly. If not prevented and controlled in time, it will not only seriously affect the yield, but may also cause serious consequences such as garden destruction.

Downy mildew can occur throughout the entire growth period of cucumber. In the early stage, chlorotic yellow spots appear on the leaves. The diameter of the yellow spots is generally about 1 cm and they are polygonal. As the infection deepens, the contrast between the chlorotic yellow spots and the surrounding healthy leaves becomes clearer and clearer. On rainy days or mornings with high humidity, the back of the leaves will appear waterlogged and a gray-black mold layer will appear. In severe cases, the chlorotic yellow spots will dry up.

Prevention and control methods : Choose disease-resistant varieties, cultivate on high ridges during planting, and increase row spacing appropriately; strengthen ventilation to avoid a high humidity environment of more than 85% in the greenhouse, and choose drip-free film to prevent water film from forming on the leaves; The key to using drugs to prevent downy mildew with half the effort is to use drugs early, at the early stage of the appearance of chlorotic yellow spots. Agents that can be used include mancozeb, mancozeb, cymoxanil, mancozeb, etc. The drugs should be mainly sprayed on the back of the leaves, and the frequency is about once a week.

2. Bacterial angular spot

The onset of bacterial angular leaf spot usually starts from the middle and lower leaves. The initial symptoms are chlorotic yellow spots on the mesophyll between the veins. When the environmental humidity is high, the back of the leaves will also appear water-soaked. Unlike downy mildew, the spots of bacterial angular leaf spot are smaller, generally not exceeding 0.5 cm. In good ventilation and low humidity conditions, the water-soaked spots disappear, and looking through the spots at the sky, there will be more obvious light transmittance, but the light transmittance of downy mildew is poor. In the later stages of the disease when it is more serious, pus will flow out of the lesions, and when the air is dry, it is easy to break and cause perforation.

Prevention and control methods: Choose resistant varieties and soak the seeds in warm water or potassium permanganate solution before sowing to kill the pathogens in the seeds themselves. Try not to plant in the same crop or alternate cropping, use high ridges or high beds for planting, pay attention to strengthening ventilation, and keep the environmental humidity below 80%. For chemical control, you can choose Chunlei Wangtong, agricultural streptomycin, and Kesaidi, and the frequency of spraying is once every 7-10 days.

3. Fusarium wilt

Fusarium wilt is often called "dead seedlings" and wilt disease in production. Fusarium wilt can occur at all stages of cucumber growth, and is most common during the flowering and fruiting period. In the seedling stage, the base of the seedlings usually shrink and then fall over. In the adult stage, the typical manifestation is that under good temperature, light and ventilation conditions, the leaves wilt and gradually turn yellow, which is sometimes mistaken for lack of water. The wilt symptoms will recover in the morning and evening, but after repeated occurrences, the plant will wilt and cannot recover and will eventually die.

Cucumber plants infected with Fusarium wilt often have gum oozing on the stems near the ground. The color is amber . When the soil humidity is high due to irrigation or the relative humidity of the air is high due to poor ventilation, the gum oozing parts will appear pink or white. Wilt disease is common and serious in continuously cropped plots, causing a yield reduction of more than 30%, and even causing a total loss of yield in severe cases.

Prevention and control methods: Choose disease-resistant varieties, disinfect and sterilize seeds and seedlings, and it is best to use grafted seedlings for continuous or successive planting. In the management process, in addition to strengthening ventilation and dehumidification, it is also necessary to avoid excessive nitrogen fertilizer application, which may weaken the resistance of plants. Chemical control can include quinoline copper, metalaxyl, and Biotide. The main method of application is root irrigation. After diluting the concentration according to the instructions, the amount of medicine for each plant should be more than 200 mg, once every 3-5 days, for 3 consecutive times.

2. Deficiency

1. The leaves in the middle and upper parts turn yellow

When cucumber plants are deficient in nutrients, if the younger and tenderer leaves in the middle and upper parts turn yellow first, the elements that may be in short supply are mainly zinc, calcium, and iron, which have poor recycling ability within the cucumber plants. If the leaves show yellow stripes or necrotic spots, but there is no obvious change in the growth of the entire plant, it is a zinc deficiency; if the new leaves appear scorched at the growing point, the leaves begin to turn yellow from the edges and droop in a parachute shape, it is a calcium deficiency; if the taproot is normal, but the leaves appear yellow-white, and the veins remain clearly green, it is an iron deficiency.

Prevention and control methods: When these less mobile nutrients are in short supply, first increase the ambient temperature appropriately to enhance the transpiration pull to improve the effective utilization rate of nutrients in the soil; secondly, use foliar spraying for emergency supplementation. When zinc is deficient, spray 0.2% zinc sulfate, when calcium is deficient, spray 0.2-0.3% calcium chloride, when iron is deficient, spray 0.5% ferrous sulfate.

2. Yellowing of the middle and lower leaves

If the old leaves in the middle and lower parts turn green and yellow due to nutrient deficiency, it may be due to insufficient supply of elements such as potassium, nitrogen, and magnesium. The main ways to distinguish them are: potassium deficiency is mainly manifested at the edges of the leaves, and occasionally yellow spots or necrotic spots may appear in the middle of the leaves, but the growth of the plant will not change significantly; if the leaves are yellow as a whole and the growth rate of the plant is significantly slowed down, this is caused by nitrogen deficiency; if the leaf changes appear in the mesophyll between the veins and the growth of the plant is not significantly affected, then it is magnesium deficiency.

Prevention and control methods: First, apply sufficient base fertilizer, especially organic fertilizer ; second, during the yield formation stage when the plant enters the vigorous growth period, increase topdressing to ensure adequate fertilizer supply to the roots; third, increase foliar fertilizer supplementation. To supplement potassium fertilizer, spray 0.4% potassium dihydrogen phosphate; to supplement nitrogen fertilizer, spray 1.5% urea on the leaves; and in case of magnesium deficiency, spray 1-2% magnesium sulfate on the leaves.

That’s it