Aphids (Aphis gossypii)

Colonies of green to blackish aphids are found on tender shoots, mainly on the lower leaf surface, where they suck sap. Under heavy attack the growth of attacked shoots is stunted and leaves are curled and twisted. Aphids excrete honeydew, which leads to growth of sooty mould, and may also attract fruit flies. Aphids transmit virus diseases such as the watermelon mosaic virus to pumpkins.

Whiteflies (Bemisia tabaci)

Whiteflies suck plant sap and excrete honeydew where moulds growth and may affect plant growth. However, the major damage is caused as vectors of various virus diseases, which cause considerable damage to cucurbits.

The Epilachna beetle (Epilachna chrysomelina)

Adults of the Epilachna beetle, also known as the African melon ladybird are 6 to 8 mm long, reddish in colour with a number of black spots on the wing cases. The larvae are 7 to 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 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.

Fruit flies (Bactrocera cucurbitae, Dacusspp. and Ceratitis capitata)

Fruit flies are important pests of cucurbits including pumpkins. Fruit flies pierce the fruits and lay eggs in them. 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, which cause fruit rot.

Anthracnose (Colletotrichum lagenarium (= C. orbiculare))

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

Gummy stem blight and black rot (Didymella bryoniae (= Mycosphaerella citrullina); Phoma cucurbitacearum)

Circular to irregular spots on leaves. On centres of the spots pycnidia (fungal fruiting bodies) develop. The spots may tear and drop out giving the leaves a tattered appearance. Crown and runners turn pale-brown and then crack oozing a reddish gum. The affected areas are studded with pycnidia. Runners may be girdled and die. Affected fruits exhibit black, leathery, sunken spots and under moist conditions pycnidia develop at the centres. On butternut squash, large irregular areas of the fruit become bronzed with distinct concentric rings. Fruits are spotted and rot. Infected fruits have no commercial value.

Fusarium wilt (Fusarium oxysporum f.sp. niveum)

It causes root rot and corky decay of the stem near ground level, leading to wilting and death. Inside wilted stems the vascular tissue may be discoloured. White to pinkish fungus mycelium may grow over affected tissue. Leaves of affected plants turn yellow and eventually wilt and die. It also causes loss of young plants through damping-off, or of older plants through successive wilting. If fruit has formed, it remains small and loose commercial value. Cucurbits are attacked at all stages of growth. When seedlings are invaded, they may damp-off and die. Older susceptible plants wilt initially, occasionally recover at night, but usually wilt quickly and die. In wet weather, a white or pinkish fungal growth develops on the surface of dead tissues. In advance stages of the disease, roots may decompose.

Choanephora fruit rot (Choanephora cucurbitarum)

C. cucurbitarum mostly attacks tissues that have been damaged by insects or mechanical means. Host tissues have a hairy appearance resulting from the tall fungal filaments that produce a cluster of brown spores (conidia) at their tips.

Powdery mildew (Erysiphe cichoracearum; Sphaerotheca fuliginea )

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 cucumber, gourd, muskmelon, pumpkin, squash and watermelon. 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 27.4º C. However, infection can take place at a temperature as high as 32º C and relative humidity as low as 46%.

Downy mildew (Pseudoperonospora 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 37.8º C. 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 cucumber family, mostly those that are cultivated, although it can infect wild cucumber and a few other weed hosts.

Scab (Cladosporium cucumerinum)

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

Virus diseases

Many important virus diseases affect cucurbits. These include:

Cucumber mosaic cucumovirus (CMV)
Watermelon mosaic 2 potyvirus (WMV-2)
Watermelon mosaic 1 potyvirus
Zucchini yellow mosaic potyvirus (ZYMV)
Squash leaf curl bigeminivirus (SLCV)

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.