Diseases and pests of melon

Melon downy mildew, common in humid summer

Melon downy mildew, a recurring disease during particularly rainy and mild summers, is caused by the fungus Pseudoperonospora cubensis. Once established, the disease is difficult to eradicate. Prevention is therefore key.

Symptoms

The disease generally occurs between June and the end of August. Oily spots appear on the upper side of the leaves, starting with those closest to the ground. Initially round, these spots become angular, turn brown, and then necrotise. A greyish to brown felt develops on the underside of the leaves. Subsequently, the leaves completely dry out. The melons develop poorly, remain small, and become deformed.

Downy mildew on melon leaves (©Jerzy Opiola)

Prevention

• Ensure good air circulation between your melon plants by planting them sufficiently spaced apart
• Water at the base without wetting the foliage or install a drip irrigation system
• Strictly adhere to a crop rotation every 4 years
• As a preventive measure during humid summers, a weekly spray of Bordeaux mixture can be applied
• A spray of horsetail decoction diluted to 20% is also effective. Apply every two weeks.
• Systematically remove leaves showing early signs of disease by burning them.

Powdery mildew on melon, especially during hot weather

On my plants, powdery mildew is caused by two fungi: Sphaerotheca fuliginea and Erysiphe cichoracearum. This white disease primarily develops in high temperatures, at least 26 °C, in a somewhat humid atmosphere.

Symptoms

Powdery mildew is easily identified by the white fluff that appears on both sides of the leaves. This type of white powder can also sometimes cover the stems and melons. Eventually, the leaves become stunted and dry out. The plants may die.

A melon leaf affected by powdery mildew

Prevention

• Ensure good air circulation between your melon plants by planting them sufficiently spaced apart
• Avoid excessive fertilisers that are too high in nitrogen
• Water the melon plants regularly while avoiding wetting the foliage. Apply mulch.
• As a preventive measure, spray horsetail decoction every two weeks.

Control

If powdery mildew is established on your melon plants, it is imperative to remove and burn all affected parts of the plant. Then, implementing one of these natural treatments is advisable:

• wettable sulphur in sprays at a rate of 7.5 g per litre of water
• baking soda at a rate of 5 tablespoons diluted in 5 litres of water
• skimmed cow’s milk is very effective against powdery mildew (one volume of milk to one volume of water)

For more details on powdery mildew or white disease, consult Virginie’s advice sheet.

Anthracnose or melon red rot

Anthracnose or melon red rot is a fungal disease caused by the fungus Colletotrichum lagenarium or orbiculare

Anthracnose on fruit

It is also found on cucumber or gherkin. This disease manifests during humid periods with temperatures between 19 and 24 °C.

Symptoms

Brown or black spots surrounded by a lighter circle appear on the leaves. These spots then spread to the fruits, turning black and becoming sunken. The melons eventually rot.

Prevention and control

• Avoid excess nitrogen fertiliser
• Ensure good air circulation in crops by spacing plants during planting
• Water while avoiding wetting the foliage or use drip irrigation
• Clean and disinfect gardening tools
• Implement a strict crop rotation over several years
• Make preventive applications of Bordeaux mixture or horsetail decoctions every two weeks.
• Remove and burn all affected plant parts.

Want to know more about anthracnose? Eva explains everything in her advice sheet on this fungal disease

Cladosporium or grey mould of melon

It is a fungus that causes cladosporiosis. This is the Cladosporium cucumerinum. This disease leads to loss and degeneration of melons. This fungus lives in the soil and overwinters on crop debris. It develops in humid weather.

Symptoms

Glass-like spots surrounded by a yellowish halo appear on the leaves. They then turn brown and undergo necrosis. Meanwhile, the stems and fruits become covered with cankerous lesions that evolve into corky scars covered with olive-green fluff.

Prevention

• Systematically eliminate all crop residues
• Regularly hoe and weed melon plants, removing adventive plants
• Remove and burn the parts affected by the disease.

Fusarium wilt, a fairly common disease on melon

Fusarium wilt is caused by the fungus Fusarium oxysporum which can remain in the soil for many years. It primarily occurs in warm and humid weather.

Symptoms

The veins of the leaves turn yellow, then the lamina wilts and dries out. Next, the stems develop longitudinal necrosis from which drops of brown gum ooze. This stem becomes necrotic, and the young plant dies.

Prevention and control

• Implement a strict crop rotation of 4 to 5 years
• Limit nitrogen fertiliser application
• Water without wetting the foliage
• Clean and disinfect gardening tools
• Burn affected plants and root systems of melon plants suffering from the disease.

Need more information on Fusarium wilt? Check out Eva’s sheet, Fusarium wilt, prevention and treatment

The mosaic of cucurbits also affects melon

The cucumber mosaic virus primarily affects cucumber but can also target melon. It is transmitted by aphids.

Symptoms

Disease manifests as chlorotic spots, yellow or green, resembling a mosaic, appearing on young leaves, which can also be found on the fruits. Melon plants do not develop normally.

Prevention and control

• Protect the crop from aphids
• Regularly hoe and weed around melon plants, removing adventive plants that may harbour the virus
• Eliminate the first infected plants.

Melon bacteriosis, a melon rot

It is the bacterium Pseudomonas syringae that causes melon bacteriosis. Temperatures of 20 to 25 °C promote the development of the bacterial disease transmitted by seeds and soil.

Symptoms

Brown, slightly viscous necrosis appears on the leaves and melons. White crusts may also form on the leaves. The fruits are covered with dark green then brown round spots. The flesh develops cavities.

Prevention and control

• Use perfectly healthy seeds or soak seeds in garlic extract for 20 minutes
• Implement a crop rotation of at least 3 years
• Carefully remove diseased leaves

The various pests that love melons

There are many parasitic insects that attack melons, causing varying degrees of damage.

Aphids

There are essentially two types of aphids that target melons: Aphis gossypii and Myzus persicae. Not only do they suck the sap from the plants and weaken them, but they are also often vectors of diseases such as mosaic. The best solution for combating an aphid infestation is repeated spraying of diluted black soap. Also, consider planting melliferous plants to attract the natural predators of aphids.

Spider mites

(©Gilles San Martin Namur-Belgique)

Often referred to as red spiders, spider mites are piercing and sucking mites. They thrive in warm, dry conditions, around 35 °C, and can be seen due to their tiny spider webs. Generally, abundant watering of the foliage is sufficient to eradicate spider mites. Preventive sprays of tansy infusion can be applied during very hot summers.

Whiteflies

These tiny winged piercing-sucking insects have bodies that appear dusted with white wax. They often develop in greenhouses and hide on the undersides of leaves. Often, simply ensuring good ventilation in greenhouses and frames, along with keeping the soil moist through watering or mulching, is enough. In case of a severe infestation, a spray of tansy infusion should be applied.

Whiteflies

Olivier provides more information about whiteflies in his advice sheet for identifying and treating them.