Melons (Citrullus lanatus) is a plant in the Cucurbitaceae family, native to the African continent. It is widely cultivated all over the world due to the sweet and refreshing taste of its fruits. The melon is an annual herbaceous plant, hanging and creeping, with a rough texture and five-lobed leaves. The flowers are large, yellow, and unisexual. The fruit is a huge, fleshy, and juicy berry (about 90% water), spherical and green. The flesh is pink or red, due to lycopene (also found in tomatoes), with a sweet, refreshing taste, rich in water and salts. Melon seeds are black, brown, or white and are rich in vitamins. The melon crop can be affected by pests and diseases, against which it’s important to apply the right treatments.

The main diseases in melons

Viruses

Cucumber mosaic virus

The virus is widespread, being found in zucchini, cucumbers, melons, tomatoes, eggplants, and peppers, as well as other species of wild plants. Symptoms include discolored spots on the leaves. Leaves take on a mosaic appearance, and after a strong attack, they become shriveled. Attacked plants produce small, misshapen fruit with mosaic spots. The virus resists over the winter in spontaneous and cultivated plant species and is transmitted through infected seeds and aphids.

Prevention and control measures:

• cultural hygiene;
• weed control;
• using healthy seeds;
• using resistant melon varieties;
• insecticide treatments to control the aphid population.

Bacteriosis

Angular leaf spot (Pseudomonas syringae pv. lachrymans)

The attack occurs on all aerial parts of plants (stems, leaves, flowers, and fruits). As soon as vegetation begins, small, moist, brown, irregular circular spots appear. Later, these spots turn brown and the tissue around them dries out and detaches. On the fruit, the attack produces small, circular, moist-looking spots whose central area is whitish. Bacterial ooze appears around these spots (in wet weather). High humidity (over 90%) and temperatures between 18-28º C favor the onset of the disease. Angular leaf spot is transmitted by infected seeds or by vectors (wind, irrigation, rain, people, or agricultural tools).

Prevention and control measures:

• using healthy seeds;
• cultural hygiene;
• avoiding sprinkler irrigation;
• melons treatments with specific copper-based fungicides.

Bacterial wilt (Erwinia tracheiphila)

The leaves of attacked plants wither like an umbrella. Subsequently, the mechanical tissues in the stem get destroyed and the plant dies. The wilting symptom is due to the bacteria blocking the plant’s vessels. The bacterium only survives on plant debris for a few weeks but is transmitted by the striped beetle (Acalymma vittata) and the spotted beetle (Diabrotica undecimpunctata).

Prevention and control measures:

• cultural hygiene;
• insecticide treatments against insects that spread the disease.

Mycosis

Cucurbits powdery mildew (Sphaerotheca fuliginea)

It is a common disease of cucumbers, melons, zucchini, and other cucurbits, causing significant damage. The disease is manifested by the appearance of large, white spots on the leaves, which become powdery. Heavily affected leaves turn brown, dry out, and fall off. Fungus is favored by temperatures above 24º C and dry weather.

Prevention and control measures:

• cultural hygiene;
• melons treatments with specific fungicides.

Cucurbits downy mildew (Pseudoperonospora cubensis)

It is the most important disease of cucurbits, causing significant damage to both field and protected crops. Symptoms include yellowish-green spots on the upper leaves, which later turn yellow and finally turn brown. On the underside of the leaves (next to the spots), a purple-grey puff appears, consisting of the fungus fructifications. After the attack, the leaves dry out and fall off, and the fruits remain small. This disease is promoted by the presence of water droplets on the leaves and temperatures between 15-30º C.

Prevention and control measures:

• cultural hygiene;
• avoiding sprinkler irrigation;
• growing resistant varieties;
• melons treatments with specific fungicides.

Fusarium wilt (Fusarium oxysporum)

The first symptoms consist of leaves turning yellow and wilting, after which the disease gradually progresses to the top of the plant. In section through the stem, it can be seen that the vascular tissue is browned. Temperatures between 28 and 32 ºC, slightly acid soil pH, and high soil humidity favor the appearance of this disease.

Prevention and control measures:

• cultural hygiene;
• disinfecting the substrate in the seedbed and treating the seeds before sowing;
• rational fertilization;
• melons treatments with specific fungicides.

Gray mold (Botrytis cinerea)

The attack occurs in greenhouses, and polytunnels, and in years with heavy rainfall, it is also manifested in the field. The most common symptom is sudden wilting of succulent tissues (stems, fruits). On the fruit, the attack consists of deep, moist, brown spots of irregular shape and size. The attack is favored by high humidity (more than 95% several days in a row), lack of ventilation, persistent cloudiness, fertilization, and excessive watering.

Prevention and control measures:

• cultural hygiene;
• rational fertilization;
• carrying out treatments with specific fungicides.

Anthracnose of cucurbits (Colletotrichum lagenarium)

The disease manifests on all aerial parts, in conditions of high atmospheric humidity and temperatures above 25 °C. Yellowish, circular, or colored spots appear on leaves and stems, turning brown. Circular, brown spots appear on the fruit, deep in the tissue. The pulp of the affected fruit turns into a dense mass and can no longer be eaten. The disease can also develop during storage.

Prevention and control measures:

• using healthy seeds;
• gathering and destroying plant debris after harvest;
• melons treatments with specific fungicides.

Damping-off (Pythium spp.)

It is one of the most important seedling diseases. It manifests from the sprouting stage to the stage of 2-3 true leaves. When attacked, the stem tissues at ground level turn black, watery, and decay. The disease occurs at temperatures between 18-30º C and humidity above 90%.

Prevention and control measures:

• removing plants from the crop and disinfecting the soil in the area;
• disinfecting the substrate and treating the seeds before sowing;
• avoiding excess watering;
• carrying out treatments with specific fungicides.

Alternaria leaf blight of cucurbits (Alternaria cucumerina)

It is a common disease of plants in the Cucurbitaceae family (cucumbers, courgettes, melons, etc.). Small discolored spots appear on infected leaves, which may merge. Spots are bordered by a yellow margin while concentric areas appear on the surface of the spots, representing the fungus fructifications. The fungus lives on plant debris on the soil surface and spontaneous cucurbits.

Prevention and control measures:

• correct crop rotation;
• gathering and destroying plant debris after harvest;
• weed control;
• seed treatment before sowing;
• carrying out treatments with specific fungicides.

Gummy stem blight of cucurbits (Didymella bryoniae)

Disease onset is favored by temperatures between 20-25 °C. The fungus penetrates the vessels, where it blocks sap circulation, leading to plant wilting. The attack is best observed on the stem. Moist lesions appear in the node area, which turns into a brown rot. The symptoms can appear on the leaves and fruit as wet spots, which turn blackish.

Prevention and control measures:

• correct crop rotation;
• gathering and destroying plant debris after harvest;
• avoiding sprinkler irrigation;
• growing resistant melon varieties;
• carrying out treatments with specific fungicides.

The main pests of melons

Wireworms (Agriotes spp.)

Wireworms represent the larval stage of Agriotes insects. These larvae attack the root system and the stem of the plant. Attacked plants show yellowed leaves which dry out or vegetate poorly.

Control measures:

• applying soil treatments with specific insecticides, at planting.

Red spider mite (Tetranychus urticae)

It has 6-10 generations per year and overwinters as an adult, mainly as females (males are generally rarer), under the peeling bark of trees, under fallen leaves, on dry weeds, or in the topsoil. Attacked leaves dry up and fall off. Following the attack, the shoots stop growing.

Control measures:

• carrying out treatments with specific acaricides.

Aphids

They are polyphagous species that migrate from one plant to another or from one species to another. They grow on spontaneous flora and then move on to cultivated species. Aphids occur as a colony on the underside of leaves, flowers, or inflorescences and young shoots. Insects sting and suck the sap, causing stress to the plant. In case of a severe attack, it causes the death of the plant.

Control measures:

• using sticky traps;
• melons treatments with specific insecticides.

Greenhouse whitefly (Trialeurodes vaporariorum)

The pest thrives in greenhouse conditions all year round, and in the field only in summer. In the greenhouse, it can develop 3-6 generations when conditions are favorable. Adults and larvae colonize leaves, often also the shoots. Following the attack, leaves turn yellow, dry out, and fall off.

Control measures:

• destroying plant debris after the crop is harvested;
• disinsecting greenhouses and polytunnels before setting up a new crop;
• using sticky traps;
• specific treatments with insecticides.

Thrips 

They are small pests, hardly visible to the naked eye, that attack a very large number of crop plants. They colonize the growth tips, flowers, and leaves, causing flower abortion and plant dwarfing. Thrips are also vectors for virus transmission.

Control measures:

• using sticky traps;
• treatments with specific insecticides.