Watermelon

Spots on leaves angular in shape and variable in size due to leaf veins that limit their enlargement. Initially, spots are water-soaked. In moist weather, bacteria ooze from the spots in tear-like droplets, which dry making the tissue white. Affected leaf dries and shrinks, and it may tear away from the healthy portion leaving irregular holes. Leaves approaching maturity are more susceptible than older leaves. Fruits may also be attacked. Fruit spots are small, nearly circular and superficial. The bacteria survive in association with seed. When infected cucumbers are used for seed extraction, the seed can be contaminated during fermentation process. The bacteria can also survive in soil or infected crop debris. Drainage water can spread the bacteria in the soil.

Angular leaf spot disease is favoured by wet conditions, frequently associated with rainfall and overhead irrigation. Optimum temperature for disease development is 23.9 to 27.8⁰C. The disease attacks watermelon, gherkin, muskmelon, pumpkin, squash, and vegetable marrow.

The fungus can attack all the above-ground plant parts. Cotyledons (seed leaves) of affected seedlings droop and wilt. Lesions (elongated spots) may form on stems of affected seedlings near the ground. Spots on leaves start as small yellowish areas that enlarge and turn brown. The affected tissue dries, breaks and the whole leaf dies. On vines, the spots are elongated and may kill the vines. 

Symptoms are most noticeable on fruits. Spots on fruits are circular, black, and sunken. When wet, the centres of the spots become salmon coloured due to a mass of fungal spores. Affected fruits can be destroyed by secondary soft-rot organisms, which enter through broken rind. The fungus is seed-borne. It can survive in crop debris and in weeds belonging to the cucurbit family. Fungal development is promoted by wet conditions, high relative humidity and moderate temperatures (20 to 23.9⁰C). Its host range includes watermelon, cucumber, gherkin, gourd and muskmelon. Cucurbit weeds can also be attacked.

The cotton aphid (Aphis gossypii) is common on cucurbits, including watermelons. Colonies of green to blackish aphids are found on tender shoots, mainly on the lower leaf surface, where they suck sap. The growth of the attacked shoots is stunted and the leaves are curled and twisted. Aphids excrete honeydew, which leads to growth of sooty mould, and may attract fruit flies. Aphids, in particularly winged aphids, transmit virus diseases (e.g. cucumber mosaic virus) when feeding from plant to plant.

They cause reduced plant stand (population). Diseased seedlings exhibit rotting and death of infected roots. A white cottony growth appears on affected tissues. Loss of young plants occurs through damping-off.

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 22⁰C. It can survive when temperatures are over 37.8⁰C. The most critical factor for infection is a film of moisture and / or long dew periods on leaves. Disease spread is primarily through wind and rain splash. The fungusattacks only members of the cucumber family, mostly those that are cultivated, although it can infect wild cucumber and a few other weed hosts.

Adults of this 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 cases. The larvae are 7-9 mm in length, soft and covered with dark coloured spines. Adults and larvae feed on leaves leaving a fine net of veins. Damaged leaves shrivel and dry up. Young plants can be entirely destroyed. Older plants can tolerate considerable leaf damage. This beetle is a vector of squash mosaic virus. Epilachna beetles attack all cucurbits.

They are tiny to small (1.5 to 3 mm) long leaf beetles with well-developed hind legs. They are named for their habit of jumping like fleas when they are disturbed. The colour of the adult beetles varies from black, brown, black and yellow striped or metallic blue-green depending on the species. Adult flea beetles chew small round holes (shot holes) in leaves, giving them a sieve like appearance. On rare occasions, they may feed directly on ripe fruit, just below the calyx. This damage is usually seen only in plants that show extreme foliar stress resulting from lack of water or powdery mildew. The larvae of flea beetles live in the soil and feed on roots, but the damage caused is not of economic importance. Adult flea beetles can be particularly damaging to young plants. Seedlings are most vulnerable to flea beetle feeding when stressed, particularly by inadequate moisture. Older plants can withstand considerable leaf perforation.

Fruit flies are important pests of watermelons and other cucurbits. They are about 4-7 mm long, they pierce the fruits and lay eggs in fruits. The fruit fly maggots feed inside the fruit causing sunken, discoloured patches, distortions and open cracks. These cracks serve as entry points for fungi and bacteria, causing fruit rot.

In seedlings, the cotyledons lose their healthy look (luster) and wilt. This is followed by complete collapse of the plants. Older plants initially exhibit wilting and yellowing of leaves near the crown. Later individual vines and then the whole plant wilt and dies. If the taproot and stem are split open, an orange-brown discolouration of the water conducting tissues will be seen. Fruits from affected vines are small with poor flavour and colour.

The fungus is a soil inhabitant. It enters the roots and grows in the water conducting tissues thereby blocking water movement. The fungus is also carried on the seeds and in soil adhering to farm implements. It can persist in soil for long periods. The disease is favoured by warm weather (optimum soil temperature for infection is about 27.8⁰C) and air humidity of more than 80% over a long period.

Symptoms first develop as a whitish talcum-like powdery growth on upper 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.

Powdery mildew affects watermelons and all other cucurbits. Other hosts include African violets and pawpaws. 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 27.4⁰C. However, infection can take place at a temperature as high as 32⁰C and relative humidity as low as 46%.

Symptoms of infestation by root-knot nematodes are similar in all crops: wilting of plants and if infested plants are pulled from the soil the roots can be seen to be distorted, swollen and bearing knots. The infested roots eventually rot and affected plants die.

It attacks all aboveground plant parts. Initial symptoms on leaves appear as light water-soaked or pale green spots. The spots are numerous and can appear on and between veins. Elongate spots may develop on petioles and stems. The spots later turn grey to white and become angular. The fine veinlets in the spots may be brown and are distinct against a white background. Dead leaf tissue cracks and breaks away until the whole leaf is ragged. Fruits can be attacked at all stages of growth. However, young fruits are most susceptible. Plant tissue near the spots may produce sap, initially watery but later becomes gummy to hard.

The fruit spots are cankerous and with time become darker, sunken until a pronounced cavity is formed. Under moist weather, a dark-green velvety layer of fungal growth appears on the cavities. The fungus survives in crop debris, soil and on seed. It is spread by insects, farm tools and wind. The disease is most severe at 100 % relative humidity and at relatively cool temperatures (21-25⁰C). Its host range includes cantaloupe, gherkin, muskmelon, pumpkin and squash.

Red spider mites attack leaves of watermelons. Adults are about 0.6 mm long. Attacked leaves have a stippled appearance, turn yellowish to whitish and dry up. Often young plants are entirely destroyed. In older plants growth can be severely stunted and the fruit set considerable reduced. Spider mites can be a problem in dry and hot conditions. Plants under water stress (drought) are more likely to suffer damage by spider mites.

Many viruses affect the cucurbit family. The important viruses include cucumber mosaic virus, watermelon mosaic virus and zucchini yellow mosaic virus. Field symptoms of these 3 diseases are similar and therefore it is very difficult to separate them by symptoms alone. 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).

Cucumber mosaic virus (CMV) It is not seed transmitted except through seed of perennial wild cucumber (Echinocytis lobata) and chickweed (Stellaria media). It is mechanically transmitted and in nature it is spread by various species of aphids (e.g. Aphis gossypii, Myzus persicae, etc.). The transmission mode by aphids is of the non-persistent type: the aphids acquire the virus from an infected plant and then transmit it to a healthy plant with their probing. They can acquire CMV from an infected plant in 10 to 30 seconds and can transmit the virus after feeding for as few as 9 seconds. It is also transmitted by striped cucumber beetles (Diabrotica vittata) and 12-spotted cucumber beetles (D. undecimpunctata) by mechanical means in plant sap carried on their mouthparts. It has a very extensive host range including such varied species as bananas, carrots, cowpeas, lupine, lilies, onions, passion fruit, potatoes and tomatoes.

Watermelon mosaic virus [Papaya ringspot virus-watermelon strain (PRSV-W), Watermelon mosaic virus 2 (WMV-2)] [p]This virus is mechanically transmitted and also spread by several species of aphids in non-persistent mode. It is not seed transmitted. Its host range is primarily restricted to cucurbits although one of its strains infects peas.

Zucchini yellow mosaic virus (ZYMV). ZYMV is transmitted from one plant to another by aphids (e.g. Aphis gossypii, Myzus persicae, etc.) in non-persistent mode and by mechanical inoculation. The virus is not seed-borne.

They suck plant sap and excrete honeydew where moulds grow, which may affect plant growth and vigour. The tobacco whitefly is considered a major pest due to its ability to vector various virus diseases, which cause considerable damage to watermelons and other cucurbits.