The most common pests and diseases

Identification of pests and diseases

Proper identification of pests and diseases is important. Whether the pest is an insect, rodent, phytopathogenic fungus, or other organism, correct identification makes controlling it easier and more effective. A mistake in identification can lead to improper control tactics that cost time and money. It may also lead to unnecessary risks to people, to the fish, or to the environment. To identify a potential disease, one should follow the steps described in Figure 5 and 6. Sometimes disease symptoms are similar to plant nutrient deficiency symptoms. In case of doubt, one should consult a specialist. If this is not possible, describe the symptoms and take photos (which will also serve for future reference). Then search on the internet to find photos and descriptions of disease symptoms that match those of your plants.

Figure 5: Disease symptoms on plants

Figure 6: Procedure to follow when identifying plant diseases

Common plant diseases

Grey mould (Botrytis)

This is the most prevalent fungal disease of lettuce, aubergines, tomatoes and cucumbers (Figure 7) when the humidity levels are too high and there is bad air circulation. Maintain optimum humidity levels through ventilation and temperature regulation. In general, a relative humidity of 75% is good for most crops and is not too humid which promotes diseases. Removing lower, yellowing leaves will assist in keeping the humidity low near the plant base and will allow the air to circulate. Make a clean break or cut at the base of the leaf petiole (where the leaf joins the stem). Botrytis will also affect fruits, stems, and leaves. Cut the fruit during harvesting with pruning shears or a sharp knife to encourage rapid healing of the wound. After flowering remove dead flowers that have not set fruit, as often Botrytis quickly invades these dead tissues.

Figure 7: Symptoms of Botrytis infection on lettuce (A), tomato (B), aubergine (C) and cucumber leaves (D)

Stem rot (Sclerotinia)

This fungus infects the stem of aubergines, lettuce (Figure 8) and tomatoes. Treat it as for Botrytis. Proper sanitation and ventilation assist in preventing this disease.

Figure 8: Symptoms of stem rot on lettuce

Powdery mildew (order Erysiphales)

Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is the most common disease on cucumbers and lettuce (Figure 9). Powdery mildew is one of the easier plant diseases to identify, as its symptoms are quite distinctive. Infected plants display small white powdery spots on the upper leaf surface and stems. The lower leaves are the most affected, but the mildew spreads rapidly on any aboveground part of the plant. As the disease progresses, the spots enlarge and spread to cover the entire leaf surface as large numbers of asexual spores are formed, and the mildew may spread up and down the length of the plant. Proper sanitation and ventilation assist in preventing this disease. The best prevention is the selection of resistant or highly tolerant varieties.

Figure 9: Symptoms of powdery mildew on lettuce (left) and cucumber (right)

Common plant pests

Most pests, such as aphids, larvae of caterpillars and moths, mealybugs, two-spotted spider mites, thrips, and whiteflies infest all crops. However, some are more aggressive on certain crops than on others. Place yellow sticky traps on overhead wires or support strings about 300 mm above the top of the plant to catch and monitor the presence of these pests.

Aphids

These pests are almost always present. They are green, brown, or black depending upon the species (Figure 10). There are winged and wingless forms. One prominent characteristic of their infestation on plants is the presence of ‘honeydew’ excreted from their abdomens as they suck on the plants, which causes stickiness of leaves and plant parts. Often sooty moulds (fungi) infect the leaves as a secondary organism, creating a black film on the leaves.

Figure 10: Green aphids on a leaf

Figure 11: Life cycle of aphids (Drawing courtesy of J.R. Baker, North Carolina Agricultural Extension Service)

Whiteflies (family Aleyrodidae)

Whiteflies are small hemipterans that typically feed on the undersides of plant leaves (Figure 12). More than 1550 species have been described. This is one of the most troublesome pests associated with tomatoes. These insects can be identified by their white wings and body. They are most prevalent on the undersides of leaves, and they fly quickly when disturbed. There are beneficial insects as well as pesticides available for their control.

Figure 12: Whiteflies

Figure 13: Life cycle of whiteflies (Drawing courtesy of J.R. Baker, North Carolina Agricultural Extension Service)

Two-spotted spider mite or red spider mite (Tetranychus urticae)

Mites are related to spiders and ticks (Figure 14). They have four pairs of legs in contrast to insects that have only three pairs of legs. Two-spotted spider mites have, as the name says, two dark- coloured spots on their bodies. As they suck on the leaves, small yellow spots form that eventually coalesce to give a bronze appearance to the leaves. They also produce webbing on the leaf surface as the infestation increases. If not controlled when numbers are manageable, they will cause complete bleaching and death of the leaves as they suck out all the contents of the cells.

Other spider mites that also damage greenhouse crops are carmine mites (Tetranychus cinnabarinus), and broad mites (Polyphagotarsonemus latus). These, however, are not as prevalent as the two-spotted mite and they differ in colour. The carmine mite is bright red, while the broad mite is translucent and can only be seen with a hand lens. Broad mites cause leaf and fruit deformation.

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| | Figure 14: Two-spotted spider mite (adult and egg) | Figure 15: The life cycle of two-spotted spider mites (Drawing courtesy of J.R. Baker, North Carolina Agricultural Extension Service) |

Leaf miners

A leaf miner is the larva of an insect that lives in and eats the leaf tissue of plants (Figure 16). The vast majority of leaf-mining insects are moths (Lepidoptera), sawflies (Symphyta, close relatives of wasps) and flies (Diptera), though some beetles also exhibit this behaviour. Adult leaf miners deposit eggs in the leaves that show as white swellings. As the larvae hatch, they eat ‘tunnels’ through the leaf between the upper and lower leaf epidermis, creating ‘mines’. As infestation increases, the mines coalesce resulting in large areas of damage that eventually lead to the death of the leaf. The mature larvae drop to the ground (surface of the substrate) where they pupate (go through metamorphosis to adults) within 10 days. The cycle then begins all over again. The infestations can be reduced by the removal of badly infected leaves and any fallen leaves from the floor. If the substrate is covered with white polyethylene to prevent the larvae from entering as they fall from the leaves, it will minimize the reproduction of the insects. This is particularly helpful if plants are growing in pots or grow beds. The use of plastic wrapped slabs will restrict the infestation by breaking the life cycle.

Figure 16: Leaf damage caused by a leaf miner

Figure 17: The life cycle of a typical leaf miner (Drawing courtesy of J.R. Baker, North Carolina Agricultural Extension Service)

Thrips (order Thysanoptera)

Thrips are minute slender insects (Figure 18) with fringed wings and unique asymmetrical mouthparts. There are more than 6000 thrips species sucking the life from plants all over the world. These insects are especially attracted to the flowers. Their distinctive feature is the presence of feathery wings. They have rasping mouthparts that scrape the leaf surface and suck the plant sap, causing white, silvery streaks on the leaves. They, like whiteflies and aphids, also carry viruses. Thrips are attracted to blue sticky traps.

Figure 18: Thrips damage on basil (a) and thrips nymph (b)

Figure 19: The life cycle of thrips (Drawing courtesy of J.R. Baker, North Carolina Agricultural Extension Service)

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