Cucumber

Spots on leaves are 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 27.8 degC. The disease attacks gherkin, muskmelon, pumpkin, squash, vegetable marrow and watermelon.

It is a very destructive disease. It causes defoliation and lesions on the fruits. 

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 23.9°C ). Its host range includes cucumber, gherkin, gourd, muskmelon, and watermelon. Cucurbit weeds can also be attacked.

The cotton aphid (Aphis gossypii) is common on cucurbits, including cucumber. 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 particular winged aphids, transmit virus diseases (e.g. cucumber mosaic virus) when moving from plant to plant.

Damping-off diseases are caused by a complex of plant pathogens (disease inciting agents) including Alternaria spp., Fusarium spp., Pythium spp. and Rhizoctonia solani. These pathogenic fungi may cause rotting of seeds before emergence or death of seedlings after emergence. The most disposing factors are use of non-certified seeds and excessive watering of seed-beds or field plots.

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 22°C. It can survive when temperatures are over 37.8°C. 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 fungus attacks 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 1.5 to 3 mm long with well-developed hind legs. They are named for their habit of jumping like fleas when 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 cucumber and other cucurbits.

Fruit flies 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.

Fruit rots are of minor importance in Africa; they are primarily post-harvest problems during storage. In most African countries, cucumbers are sold fresh from farm, consumed and are hardly stored for long (exception could be supermarkets in urban areas where fruits are kept on shelves).

The lower leaves of the plant usually turn yellow and die. One or more branches may exhibit such symptoms. Leaflets on one side may be affected while those on the other side are symptomless. Diseased leaves readily break away from the stem. When affected stems just above ground level and petioles are cut diagonally, a reddish-brown discolouration of the water conducting tissues will be observed. The fungus is both seed- and soil-borne. It causes most damage on light, sandy soils. It is most active at temperatures between 25 and 32⁰C. The fungus can survive in the soil indefinitely even when no tomatoes are grown. It can also survive in fibrous roots of weeds (e.g. Amaranthus, Digitaria and Malva species). Acidic soils (pH 5.0 to 5.6) and excessive nitrogen fertilisation promote disease development. Infestation by root-knot nematodes encourages the disease.

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 (21-25°C). Its host range includes cantaloupe, gherkin, muskmelon, pumpkin, squash and watermelon.

Red spider mites attack leaves of cucumber. 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.

Cucumber mosaic virus

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. 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

This virus is mechanically transmitted and also spread by several species of aphids. It is not seed transmitted. Its host range is primarily restricted to cucurbits although one of its strains infects peas.

Squash mosaic virus

It is mechanically transmitted. It is transmitted through seeds of melons and squash. It is also transmitted by spotted, striped and banded cucumber beetles, which attack cucumbers in the Americas. The Epilachna beetle (Epilachna chrysomelina) a pest of cucumber in Africa, is also vector of squash mosaic virus. Its host range includes cucurbits, peas, coriander, and salad chervil.

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 cucurbits.