Detailed Explanation of Key Technical Points for Watermelon Pest and Disease Control

Watermelon is an annual trailing vine of the genus Citrullus in the Cucurbitaceae family. Its main diseases include watermelon anthracnose, watermelon damping-off, watermelon viral disease, watermelon powdery mildew, watermelon phytophthora blight, watermelon cottony leak, watermelon southern blight, watermelon gummy stem blight, watermelon brown rot, watermelon root-knot nematode disease, watermelon leaf blight and watermelon fusarium wilt, etc. Let us now take a look at watermelon pest and disease control techniques!

Watermelon Anthracnose

[Symptoms] At the seedling stage, semicircular or circular brown lesions appear on the edges of the cotyledons, on which small black dots or pale red sticky matter are produced. When young stems are affected, the diseased part turns dark brown and constricts, causing the seedlings to topple over. Leaf lesions are actually circular pale yellow water-soaked small spots, which later turn brown, with purplish-brown edges and a pale brown center, bearing concentric rings and small black dots. The lesions tend to perforate.

[Occurrence pattern] The optimum temperature for pathogen growth is 24-25°C, with humidity above 90%; under high temperature and drought conditions, the disease is relatively mild.

[Control methods] First use disease-free seeds, or disinfect the seeds by soaking them in warm water at 55°C for 15 minutes, or soaking them in a 100-fold dilution of 40% formaldehyde for 30 minutes, then rinse with clean water and germinate. In the early stage of the disease, spray a 400-500 fold dilution of 70% mancozeb, or a 500-fold dilution of 65% zineb or carbendazim, spraying once every 5-6 days, for 2-3 consecutive sprays.

Watermelon Damping-off

[Symptoms] In the early stage of the disease, the area near the ground on the seedlings appears yellow and water-soaked, then turns yellowish-brown, dries up and shrinks, and the seedlings break off as soon as they are pulled.

[Occurrence pattern] The pathogen spreads with rainwater and running water. Low soil temperature (10-15°C), high humidity in the seedbed, cool nights and insufficient daylight during the day are conducive to the disease. More pathogen carried on seeds and in the seedbed soil means more chances for the seedlings to become infected.

[Control methods] (1) Agricultural control: Strictly select soil that is high-lying, well-drained and has not previously grown cucurbits. Strengthen seedbed management, and do a good job of heat preservation, ventilation and humidity reduction. When the seedbed humidity is too high, a small amount of plant ash or dry soil can be scattered, and ventilation used to reduce humidity. (2) Chemical control: As soon as diseased plants appear, immediately remove the diseased seedlings and spray copper-ammonia mixture to prevent the disease from spreading. A 700-800 fold dilution of 75% chlorothalonil can also be sprayed.

Watermelon Viral Disease

[Symptoms] On melon plants, symptoms first appear on the upper leaves, showing a mosaic mottling of alternating dark and light green; the leaves become smaller and curled, the stems twist and shrink, and the plants are stunted. The fruits become smaller, and they also bear mottling of alternating dark and light green. When watermelon is affected, yellow-green mottling appears on the leaves, the leaf surface becomes uneven, newly grown leaves are deformed, infected leaves are narrow and long, shrunken and twisted, the floral organs develop poorly, and no fruit is set or the fruit is deformed.

[Occurrence pattern] High temperature and strong sunlight favor the disease, and melon fields lacking fertilizer and with weak growth are often severely affected.

[Control methods] Kill aphids promptly; effective agents include a 3000-fold dilution of 20% fenvalerate emulsion, a 2000-3000 fold dilution of 2.5% deltamethrin, and a 1500-2000 fold dilution of 40% dimethoate emulsifiable concentrate. In severe cases, use 4 vials of 800,000-unit penicillin diluted in 15 kg of water and spray. Alternatively, using 2 vials of antitoxin (20 mL) plus 50 g of zinc sulfate diluted in 15 kg of water and spraying on the leaf surface also has a notable effect.

Watermelon Powdery Mildew

[Symptoms] In the early stage of the disease, small circular white spots appear on the leaves, which later expand into white powdery patches; the lesions connect into sheets, covering the leaves and stems with white powder.

[Occurrence pattern] When field humidity is high and the temperature is 16-24°C, the disease is severe. When watermelon and melon are grown in greenhouses and plastic tunnels, because of poor air circulation and high humidity, the disease occurs earlier and more severely than in open-field cultivation.

[Control methods] Pay attention to field sanitation, promptly remove diseased leaves and burn them in a concentrated manner. Strengthen cultivation management, apply more phosphorus and potassium fertilizer, and improve plant resistance. In the early stage of the disease, spray a 120-200 fold dilution of Agricultural Antibiotic 120, or a 1500-2000 fold dilution of 15% triadimefon, or an 800-1000 fold dilution of 70% thiophanate-methyl wettable powder.

Watermelon Phytophthora Blight

[Symptoms] Both seedlings and mature plants can be affected, damaging leaves, stems and fruits. When cotyledons are infected, water-soaked dark green circular spots first appear, the center gradually turns reddish-brown, and the area near the ground constricts or dies. When true leaves are infected, dark green water-soaked circular or irregular lesions first form and expand rapidly; under high humidity they rot or look as if scalded by boiling water, and after drying they become pale brown and easily broken. When the stem base is infected, spindle-shaped water-soaked dark green sunken lesions form, encircle the stem and rot, and everything above the affected part dies completely. When fruits are infected, dark green circular water-soaked sunken lesions form, then rapidly spread to the whole fruit, causing the fruit to rot and give off a silage odor.

[Occurrence pattern] The temperature range for disease onset is 5-37°C. The optimum temperature is 20-30°C. During the rainy season and under high temperature and high humidity, the disease develops rapidly; poor drainage, overly dense planting, dense stems and leaves, or poor ventilation lead to severe disease.

[Control methods] From the early stage of the disease, begin spraying a 700-800 fold dilution of 50% metalaxyl-copper wettable powder, an 800-fold dilution of 35% oxadixyl-copper wettable powder, an 800-fold dilution of 72.2% propamocarb aqueous solution, or a 58% ridomil-mancozeb (metalaxyl-mancozeb) wettable powder.