1.1. Fusarium wilt
Watermelons are attacked by Fusarium wilt at all stages of development. Seeds may decay in the soil. Seedlings wilt collapse and die before or after emergence. Older plants wilt, wither, and die any time during the growing season. If melons do form, they are generally small and unpalatable. Wilting of runners usually progresses slowly, showing initially during the midday heat. Such plants recover at night, but after a few days they wilt permanently and die.
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1.2. Anthracnose
Anthracnose is a very destructive disease of watermelons and other curcubits. Anthracnose attacks all above-ground parts of the plant. Watermelon can be infected at any stage of development; however, disease symptoms are first noticed as round to angular reddish brown spots on older leaves.
Spots may later dry, turn almost black and tear out, giving a leaf a ragged appearance. Often the leaves at the center of the plant are killed first, leaving the stem and a portion of the runners bare.
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1.3. Powdery mildew (Erysiphe cichoracearum and Sphaerotheca fuliginea)
Symptoms first develop as a whitish talcum-like powdery growth on lower leaf surface. The powdery growth is composed of fungal spore mass. These areas covered by white powdery growth may enlarge and join up to cover both lower and upper leaf surfaces. Severely affected leaves dry, turn brown and become brittle. Vines can be also attacked. Secondary effects of the disease include sun-burning and premature ripening of fruits.
The powdery mildew fungi are influenced by plant age, humidity and temperature. Foliage is most susceptible 16 to 23 days after unfolding. The fungi reproduce under dry conditions. Infection increases as humidity increases, but does not occur when leaf surface is wet. Optimum temperature for infection is about 27.4º C. However, infection can take place at a temperature as high as 32º C and relative humidity as low as 46%.
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1.4. Downy mildew (Peronospora cubensis)
Symptoms on leaves appear as small, pale-yellow areas on upper leaf surface. Under humid conditions, a purplish, grey whitish growth may be seen on the underside of the yellowish spots. Affected leaves curl, shrivel and die.
Most downy mildew fungi require cool weather for reproduction and development. This is not true of the cucurbit downy mildew fungus. Optimum temperature for infection is at 16 to 22º C. It can survive when temperatures are over 37.8ºC. The most critical factor for infection is a film of moisture and / or long dew periods on leaves. Disease spread is primarily thorough by wind and rain splash. The fungus attacks only members of the cucurbit family, mostly those that are cultivated, although it can infect wild cucumber and a few other weed hosts.
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1.5. Watermelon mosaic virus
Caused by two distinct viruses referred to as Watermelon Mosaic Number I and Watermelon Mosaic Number II. The most common symptom is mottling of the leaf, some plants are stunted with abnormally shaped leaves. The diseases is usually spread by aphids which can spread through an entire planting during the growing season.
Leaf symptoms include a prominent light and green mosaic pattern, mottling (yellow with green islands or blisters), and in severe cases, leaf distortion whereby affected leaves appear fern-like. Diseased fruits are malformed (slightly to severely misshaped with wart-like lumps).
© A.M. Varela, ICIPE
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1.6. The Epilachna beetle (Epilachna chrysomelina)
Adults of the Epilachna beetle, also known as the African melon ladybird are 6-8 mm long, reddish in colour with a number of black spots on the wing covers. The larvae are 7-9 mm in length, soft and covered with dark coloured spines. They pupate on leaves. Both adults and larvae feed on the leaves leaving a fine net of veins.
Damaged leaves shrivel and dry up. They may also gnaw (chew) stems and eat holes in fruits. These beetles are most likely to be a problem during establishment when plants are small, young plants can be entirely destroyed. Older plants can tolerate considerable leaf damage, but during flowering fruit set maybe affected. This beetle is a vector of squash mosaic virus. The Epilachna beetle attacks all cucurbits. They often fly into a crop from nearby crops.
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1.7. Fruit flies (Bactrocera cucurbitae, Dacus spp and Ceratitis capitata)
Fruit flies are important pests of cucurbits. They pierce the fruits and lay eggs in them. The fruit fly maggots feed inside the fruit causing sunken, discolored patches, distortions and open cracks. These cracks serve as entry points for fungi and bacteria, which cause fruit rot.
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