When pests or diseases attack garden plants, the first step to fixing the sick plant is to identify the cause. Start by working out whether the problem is caused by an insect pest or a plant disease. Next, narrow it down further by either identifying the specific pest or disease, or working out how the pest is damaging your plant. Pests commonly suck sap, chew or feed by burrowing into a leaf or stem.
Below, we take a look at some of the most common pest and disease problems found on garden plants, and how to control them. Just remember to always follow application instructions when using chemical controls.
The way in which a pest attacks provides the clue to treatment. When you do spot an obvious pest, remove by hand and kill them to reduce plant damage. Large infestations such as scale can often be removed by pruning.
Chewing and rasping pests
All sorts of caterpillars feed on the leaves of garden plants. Some, including budworm, will burrow into flowers and fruit. While all can damage plants, some caterpillars are welcomed as they turn into butterflies or showy moths. Pest species, however, inflict major damage and need to be controlled.
Armyworms (brown and striped caterpillars) travel in large groups and can march across vast tracts of lawn leaving it defoliated, while the soft green caterpillars of the cabbage white butterfly defoliate brassicas. Lily caterpillars burrow down into clivias. Cut worm (some the larvae of bogong moths), destroy new seedlings, slicing them off at the base.
Plants attacked: All plants (usually leaves, flowers or fruit). Seedlings.
Control: Dipel and Success are organic chemicals that control caterpillars that are the larvae of butterflies and moths. Caterpillars can also be handpicked and squashed when damage is noted.
BEETLES AND CURL GRUBS
Adult beetles can damage many types of plants. For example, Christmas beetles chew gum leaves, hibiscus beetle damages flowers of hibiscus and abutilon, and staghorn beetle feeds in the leaves of staghorn and elkhorn ferns. However, it is often beetle larvae that damage plants. Curl grubs (the C-shaped grubs found in soil and potting mix), are beetle larva that eat roots, and the larvae of the African black beetle is known to damage lawns.
Plants affected: A wide range of plants – often roots.
Control: Handpick larvae found in potting mix or under damaged turf. Handpick beetles in ferns. Collect fallen hibiscus buds and flowers containing beetles (place all in a bag, seal and bin). Products containing eucalyptus and tea tree oil are registered for the control of African lawn beetle larvae. Beneficial nematodes can also be used to control beetle larvae.
SNAILS AND SLUGS
Among the most commonly encountered garden pests, snails and slugs damage plants by chewing leaves and young shoots, and decimating new seedlings. They are often active at night and after rain.
Plants affected: All plants, particularly young or leafy vegetables and annuals, as well as succulents, hostas and many shrubs.
Control: Squash any feeding on plants. Snails and slugs can also be killed in a bucket of salty water. Search out hiding spaces, including under the rims and bases of pots. Use snail and slug baits around susceptible plants. Iron-based baits don’t poison pets or wildlife.
Various weevils (types of beetles) attack a wide range of garden plants and weeds. Adult feeding often leaves a notched appearance in leaves. Flower buds may also have a gnawed looked. Many weevils feed at night and hide in leaf litter or mulch at the base of plants, where they also lay eggs.
Plants affected: Lawns (Argentine stem weevil), many ornamentals including gardenias (black vine weevil, Fuller’s vine weevil), roses (Fuller’s vine weevil), vegetables (vegetable weevil).
Control: Squash any that are spotted feeding or hiding at the base of a plant. Use a registered insecticide (usually applied to the soil).
Sap sucking pests
These small, soft-bodied insects can be green, brown or black. They cluster on new shoots, buds and stems and feed on sap. Some mature aphids have wings. Aphids give birth to live young and numbers increase rapidly. Some attack specific plants (for example rose aphid, black citrus aphid, black peach aphid, cabbage aphids), while others are more general. Some aphids spread plant viruses.
Plants affected: A wide range of plants including citrus, rose, peach, pansy, vegetables and many more.
Control: Many predators provide natural aphid control, including other insects such as ladybird larvae and many wasps and small birds. Look for predators or mummies (round ‘balls’ among the colony which are aphids parasitised by the wasp and containing its eggs) before attempting any control measures. Natural control may occur where predators are present. To control physically, gently squash colonies, taking care not to damage new growth, or wash off with a jet of water from the hose. If necessary, use a registered pesticide such as horticultural pest oil, horticultural soap, pyrethrum or a systemic insecticide.
AZALEA LACE BUG
This tiny insect has clear, lacy wings. It feeds on the underside of the leaves, causing yellow spots and a bronze discolouration on the upper side of the leaf. Individual adults and their spiky nymphs are difficult to see without a hand lens, but the black frass (droppings) they excrete are often very obvious under leaves.
Plants affected: Azalea, rhododendron.
Control: Stressed plants are more often attacked. Improve growing conditions (for example more water and shade). Prune off badly affected stems and dispose of in a bag in the rubbish bin. Use a registered system chemical.
Several large bugs, often called stinkbugs, are pests of vegetables, ornamentals and citrus. Commonly encountered are bronze orange bugs (found as both juveniles and adults on orange trees from winter to summer), spined citrus bug (often found on lemon trees) and green vegetable bug (a mainly summer pest of vegetables and annuals). Feeding can lead to fruit drop and damaged fruit, and oranges may be dry when attacked by bronze orange bugs. Adults can fly from tree to tree.
Plants affected: A wide range of plants including citrus, vegetables and flowers.
Control: Handpick and destroy bugs when noticed. Wear eye protection and gloves, as many bugs exude acrid liquid that can burn eyes and skin. Drop bugs into soapy water. Apply horticultural pest oil in winter to control citrus bugs in their juvenile stage.
When this pest insect feeds on the plant’s leaf tissue, the plant defends itself by forming galls (lumps) around the feeding damage. Adults are tiny winged insects, and they lay eggs under the leaves, where the developing nymphs feed. The result is ugly pimple-like lumps on foliage. The damage is unsightly and may stunt growth, but generally doesn’t kill plants. It is the pimples not the insect that are observed.
Plants affected: Many types of lillypillies (Acmena and Syzygium species and cultivars) including related plants such as Eugenia and Waterhousia.
Control: Look for pysllid resistant lillypillies. Prune off damaged growth and protect new growth with a registered systemic insecticide.
This small, soft-bodied insect protects itself with a white mealy covering that resembles small pieces of cotton wool. It sucks sap and can debilitate plants. Mealy bugs feed on stems and roots.
Plants affected: A wide range of plants, including indoor plants and plants under stress.
Control: Small outbreaks can be wiped off a plant or removed by pruning off infected growth. Infestations are usually widespread and difficult to see, so a chemical control is needed for complete control. Apply a registered pesticide including horticultural pest oil, horticultural soap or a systemic insecticide.
These tiny insects are related to spiders and are often invisible to the naked eye. Mites may feed under leaves, in buds or on fruit and are often identified by the damage that they cause. A common pest mite is two-spotted mite (also called red spider mite), which feeds on the underside of leaves causing leaves to become silvery above. It creates webbing and frass (droppings) under leaves. Broad mite is a microscopic insect, which also feeds under leaves, causing them to curl and develop a bronze colouring. Erinose mites cause blistering on hibiscus leaves. Plants affected by mites often grow poorly and may be stunted.
Plants affected: A wide range of plants, including indoor plants, fruit trees, roses, hibiscus, many vegetables and plants under stress.
Control: Mites are resistant to many chemicals. Prune off infected growth. Increase watering – dry conditions favour mites. Wettable sulphur and paraffin oil are registered for mites (apply with care following directions). Azadirachtin (sold as neem) and horticultural soaps are also registered to control two-spotted mites.
Both adults and young passionvine hopper damage plants by feeding. Adults have triangular lacy brown wings. Juveniles are tiny insects with fluffy tails (often called ‘fluffy bums’) and are found in the soil or on plant stems. They jump or spring rather than fly. Sooty mould may form on leaves where hoppers are feeding.
Plants affected: Passionfruit and other vines including jasmine.
Control: Hose insects and sooty mould from plants.
Scale is a soft-bodied insect that protects itself with a hard or waxy cover. It sucks nutrients from its host plant. Scale can be white, brown or black and may be raised or flat. These pests may be found on stems, on leaves or clustered around the base of buds or shoots.
Plants affected: A wide range of plants including indoor plants, ferns, shrubs, fruit trees, roses and plants under stress.
Control: Prune off infected growth or scrape scale off to reduce numbers, and try to improve growing conditions. Follow up sprays may be needed for control. Horticultural spray oils can break down the waxy covering over scales and are usually included in scale treatments along with an insecticide such as pyrethrum.
There are many types of thrips, which is a very small insect found on plants in large numbers. Thrips feed on leaves, flowers, inside buds and on vegetables. As they are small they are often identified by the damage they cause, which includes silvering of leaves and blemishes on flower petals.
Plants affected: A wide range of plants including roses.
Control: Prune off infected growth or flowers, and improve growing conditions. Follow up sprays may be needed for control. Thrips are resistant to many insecticides. Registered sprays horticultural soaps, pyrethrum and insecticidal guns.
This tiny bug, which resembles a narrow white moth, feeds on the undersides of leaves and is often present in very large numbers. It is also a pest of greenhouses. Whiteflies fly up from plants when they are disturbed.
Plants affected: A wide range of plants including vegetables, annuals and shrubs.
Control: This can be difficult, as whitefly is resistant to many insecticides and flies off when disturbed. Whitefly in greenhouses or shadehouses can be captured on sticky yellow boards. Use a registered chemical control such as horticultural soaps, horticultural pest oils, azadirachtin (neem) or pyrethrum. These are contact sprays so they are most effective when they contact the pest rather than just sprayed on foliage.
Boring and tunnelling pests
Tunnelling or boring pests make holes in stems or trunks that are surrounded by webbing and sawdust. They are generally the larvae of beetles or moths. Borers may kill branches or entire trees. Trees may fight back by exuding gum or sap.
Plants affected: Many trees including wattles and gum trees, some shrubs, stems of ornamental and other gingers.
Control: Where possible remove dead stems or branches below the damaged area. If the insect is still feeding in the plant it may be skewered with wire poked into the borer’s hole. For large trees use the services of a professional arborist who can inject trees with systemic insecticides. Borers often indicate stressed trees. Improved growing conditions can help the tree to recover.
CITRUS LEAF MINER
This pest lays its eggs in the soft new growth of citrus trees. As the larvae feed they cause silvery trails that distort leaves as the leaves mature. Other types of leaf miner may attack other plants including cineraria.
Plants affected: Citrus.
Control: Remove affected growth. The pest inside the leaf can’t be controlled with a chemical as it is protected by the leaf tissue, however applying horticultural pest oil to new growth before it is damaged protects growth.
This pest lays its eggs in ripening fruit. The larvae burrow through the fruit causing it to rot and fall from the plant. The larvae then drops from the fruit to pupate in the soil. It is most active in the warmer months of the year, but can also be a problem from spring to autumn. Adults are rarely seen but damage is very evident.
Plants affected: All soft fruits including tomato, capsicum, peach, nectarine, raspberry, mango, mandarin, loquat, clivia berries.
Control: Use exclusion bags or netting as fruit ripens. Green tomatoes can be picked while still firm and ripened indoors. Fruit fly baits containing spinosad give good protection but must be reapplied frequently especially after rain. Follow directions for their use as they are applied to trunks or boards not to the fruit. Collect fallen fruit. To kill larvae in fruit, place the fruit in a sealed plastic bag and ‘stew’ in the sun for several days before placing into the garbage bin.
ROOT KNOT NEMATODE
This pest is invisible to the naked eye but recognised by the damage it causes. It attacks roots blocking the plants ability to distribute water and nutrients from the roots to the stem. Nematode-affected plants may wilt, become stunted and develop yellow leaves. Roots develop lumps known as ‘knots’ or galls.
Plants affected: Many plants including tomatoes, carrots and potatoes. Stressed plants and plants in sandy soils are susceptible to nematode attack.
Control: Remove affected plants. Practice crop rotation when growing vegetables. Add compost to soils. French marigolds (Tagetes patula) and some grasses including oats are resistant to root knot nematodes so growing a crop of marigolds or oats in nematode-infested soils can reduce pest numbers. Chemical soil drenches may be available.
Plant diseases include bacterial disease, fungal disease and viruses. In general, diseases are controlled with fungicides. Virus diseases cannot be treated and are controlled by removing the diseased plant.
AZALEA PETAL BLIGHT
This fungal disease causes the petals of azalea and rhododendron flowers to appear spotted, collapse as if wilted, then turn brown. Flowers then stay on the bush. This disease spreads easily by wind or rain.
Plants affected: Azalea, rhododendron.
Control: Remove affected flowers and put them in a bag and dispose of in the rubbish bin. If lots of flowers are affected, shear over the plant to remove the blooms then rake up the prunings and dispose of in a bag in a rubbish bin. Avoid overhead watering when plants are in flower as this helps to spread spore. To prevent infection spray flowers with a fungicide at bud stage then repeat the spray through the flowering period. Registered fungicides include mancozeb and Zaleton. Follow application rates and times.
This fungal disease attacks buds and flowers causing browning, as well as damaging fruit that results in brown or rotten patches. Fruit damage may appear after harvest.
Plants affected: A wide range of ornamental and productive plants including stone fruit.
Control: Remove affected fruit including old fruit (mummies). Prune out infected twigs and growth. Apply a registered fungicide (such as Bordeaux mix, mancozeb, lime sulphur or copper oxychloride) at bud swell (as the flower buds swell in late winter or spring on deciduous fruit trees). Timings of sprays are important so follow application times and rates. Observe withholding periods (time between spraying and harvest) on edible plants.
This disease appears as rot around the base of trunks, particularly of old or poorly drained citrus trees. Collar rot is also seen around the graft union on other fruit trees. It is usually caused by phytophthora, a soil-borne disease.
Plants affected: Citrus (including lemon, orange, grapefruit), many other fruit trees including apple.
Control: To reduce the likelihood of infection, plant citrus in well-drained soil. In poorly drained areas grow citrus in raised beds. Avoid splashing soil onto trunks when watering. Apply phosphorus acid (sold as Yates Anti Rot) over foliage, affected trunk and soil. Remove rotted material before applying Anti Rot to the trunk. Old and poorly affected trees should be removed. Replant a new tree in a new location.
PEACH LEAF CURL
This fungal disease affects leaves in spring causing them to become thickened, pink and distorted before dropping.
Plants affected: Peach, nectarine.
Control: Action must be undertaken in winter before leaves reappear. Spray buds with a registered fungicide such as Bordeaux mix, mancozeb, lime sulphur or copper oxychloride at bud swell (as the flower buds swell in late winter or spring on deciduous fruit trees). Timings of sprays are important so follow application times and rates.
A range of fungal organisms cause black or brown spots on leaves and fruit. Commonly seen are black spot on roses and ink spot on kangaroo paw. Black spot on roses appears as black or brown spots. Affected leaves turn yellow and drop. Ink spot on kangaroo paw causes black marks on leaves.
Plants affected: A wide range of ornamental and productive plants including roses, fruit, vegetables. Some fungal leaf spots affect a wide range of hosts; others are host specific.
Control: Remove affected leaves or fruit from the plant and the ground. Apply a fungicide registered for leaf spot diseases including copper-based fungicides, mancozeb or potassium bicarbonate (eco-rose, eco-fungicide) and rose sprays. When using a fungicide on an edible plant, check that it is registered to use on edibles and observe any withholding period (the time that must elapse between spraying and harvest).
This disease appears as a white powdery covering, usually on the upper side of the leaves of many annuals, perennials, shrubs and trees. It may also be found on buds, shoots and stems. The disease often attacks plants that are stressed, overly shaded or at the end of a growth period (for example vegetables dying back). It can lead to die back of leaves and shoots.
Plants affected: Vegetables including beans, silver beet, zucchini and pumpkin; annuals including forget-me-not, pansy and viola; biennials and perennials including aquilegia, dahlia and penstemon; fruit plants including strawberry and grape, shrubs such as fuchsia, rose and hydrangea; and trees such as crepe myrtle.
Control: Remove affected leaves when first seen. For annuals including vegetables near the end of their life cycle remove plants. For perennials that are dying back, cut back to remove affected growth. For permanent or young plants spray with a registered fungicide such lime sulphur, potassium bicarbonate (eco-rose, eco-fungicide), triforine, wettable sulphur or mancozeb. Prune to increase airflow around susceptible plants. Look for resistant varieties such as Indian Summer forms of crepe myrtle.
A range of soil-borne diseases can lead to the death of plant roots, which leads to the death or die back of affected plants. Infections may be caused by rhizoctinia and phytophthora.
Plants affected: A wide range of plants including seedlings (referred to as damping off), vegetables including potatoes, shrubs including azalea, protea and native shrubs, lawns (brown patch), trees (dieback).
Control: For seedlings, remove affected plants. Avoid planting susceptible plants in poorly drained soils. Look for certified disease free plants (such as potatoes). Phosphorus acid (sold as Yates Anti Rot) may control some forms of root rots or act as a preventative treatment.
Yellow, orange, brown or sometimes white pustules on the undersides of leaves and on stems indicate various types of rust disease. The pustules contain spores that spread the disease. Severe infestations can cause defoliation or even lead to plant death. There are different fungus diseases that lead to rust. Some cause rust in specific plants; others are more general.
Plants affected: Berry fruits including raspberry and blackberry, calendula, chrysanthemum, frangipani (frangipani rust), hollyhock, native and introduced members of the myrtle family (myrtle rust), rhododendron, rose, snapdragon, turf grasses
Control: Removing infected leaves (bag and bin) when the disease is first seen may limit its spread. When annuals or vegetables are affected, remove entire plant (bag and bin). Apply a registered fungicide such as copper oxychloride, mancozeb, potassium bicarbonate (sold as eco-rose, eco-fungicide), or sulphur. Check that the fungicide is registered for the plant exhibiting rust and follow application times and rates.
This is a secondary fungal infection, not a disease. It grows on sticky exudate or honeydew caused by infestations of sap sucking insects including aphids, mealy bug and scale. Sooty mould forms an unsightly black crusty covering on leaves or stems. As the sooty mould coats leaves it can cause them to die and fall.
Plants affected: Wide range of ornamental and native plants, but usually sooty mould is seen on shrubs, trees or climbers.
Control: Remove sooty mould with applications of horticultural soap spray. If the source of the honeydew on which the mould is growing is removed, the sooty mould will stop growing, dry out and fall off the foliage. To prevent its return, control the sap-sucking pest outbreak, which may be on the same plant or on a nearby plant.