Plant pests and diseases can be a real nuisance for any gardener, but with a bit of knowledge and some basic preventative measures, you can keep them at bay. In this beginner’s guide, we’ll teach you everything you need to know about preventing, identifying and controlling common pests and diseases.
Rust
Garden plant rusts are among the most prevalent fungal diseases. Plants such as grasses, bulbs, fruit and vegetables, herbaceous and bedding plants, trees and shrubs can all be susceptible to developing rust. Rust diseases are unsightly and frequently, though not always, weaken plants. In extreme cases, plants may also die from rust infections.
The main sign of a rust infection to watch out for is the presence of pustules: structures that produce spores – will emerge from pale leaf spots. Most of the time the pustules are on the lower surface of the leaf and produce a lot of tiny spores. Orange, yellow, brown, black or white bumps are all possible, though the most well-known colour is a rusty brown shade, for which the disease is named. On a single leaf, there can be dozens of pustules, though they can also appear on leaf stalks (petioles), stems and occasionally fruits or flowers. A plant’s strength is commonly reduced by severe infection; in extreme cases, they can fail (such as with antirrhinum rust.)
The RHS believes that the first line of defence against pests, diseases and weeds should be good cultivation practices. This includes cultivar selection, garden hygiene and encouraging or introducing natural enemies. If chemical controls are used, they should be used sparingly and specifically.
Non-chemical control
There are in-depth recommendations linked in the “Biology” section below or profiles available for rust diseases of specific crops and plants, though here are some general recommendations:
As long as it only affects a small number of leaves, you can sometimes halt the spread of the disease by removing infected leaves as soon as they appear. It is likely to cause more harm than good if removing a lot of leaves. Make sure that the conditions are right for strong growth. But don’t use too much nitrogen fertiliser as this makes the growth soft and lush – easier for rust to get into. At the end of the growing season, you should get rid of any dead or diseased material.
Some rusts’ overwintering spores are extremely durable; therefore, affected materials should not be composted. Some crops and plants have resistant cultivars, but the rust fungus’ genetic mutations can occasionally overcome this resistance. There are times when no control is necessary. Even though the entire tree may appear yellow or orange in summer due to a large number of rust pustules on the leaves, many rusts on trees develop too late in the summer to significantly impact vigour. The time of infection will dictate which approach to take.
Chemical control
There are currently no fungicides for eradicating rust diseases from edible crops available for home gardeners to purchase, though there are options for non-edible plants.
Approved fungicide treatments for rust diseases in ornamental plants are:
- Tebuconazole (Provanto Fungus Fighter Concentrate)
- Tebuconazole with Trifloxystrobin (Toprose Fungus Control & Protect)
- Triticonazole (Fungus Clear Ultra)
Products that combine insecticide and fungicide make it possible to control disease and harmful invertebrates at the same time, such as acetamiprid-containing triticonazole (Roseclear Ultra and Roseclear Ultra Gun.) If there are no issues with invertebrate damage to the plants, it would be preferable to use an alternative product than one with both insecticide and fungicide.
Lichen
Lichens are multicellular organisms: a fungus that grows alongside algae. The British Isles are home to more than 1,800 different species of lichen. Most gardens will have at least a few different types, but some are extremely rare and restricted to particular locations. Although lichen species are difficult to identify, they can be broken down into three categories for ease of recognition:
Foliose lichen are flat and creeping – almost leaf-like, are a variety of colours and are attached across the whole of their base, to the substrate on which it is growing,
Fruticose lichens typically have a single point at their base which holds them to the substrate. They appear to be bush-like plants, growing in an erect or pendulous pattern.
Crustose lichen is characterised by thin, flat crusts which have distinct margins or none at all. They grow directly on their substrate and do not have a lower surface. These are common on wooden structures like benches and paving – they create interesting mosaics that, given a few years, will help it to blend in with the harsh appearance of new wood, brick or stone.
Most lichen species are silver-grey, grey-green, yellow or orange. Lichens growing on trees and shrubs tend to be grey-to-green in colour.
The most prevalent lichen in turf is Peltigera spp. Lichen (dog); it is brown or grey and made up of flat, lobed structures which have prominent rootlets (rhizines) underneath.
Powdery mildew
Powdery mildews are a group of related fungi which infect a wide variety of plants, leaves, stems and flowers with a white, dusty coating.
It can affect numerous edible garden plants such as fruits or vegetables like apples, gooseberries, blackcurrants, grapes, cucumbers and peas. As well as ornamentals, roses, rhododendrons, honeysuckles, delphiniums and others.
Typically, powdery mildew only inhabits a small number of related plants in their host range. As a result, the powdery mildew that attacks apples and peas will most likely come from different species.
White, powdery, spreading patches of fungus on the upper or lower surfaces of the leaves, flowers and fruits are typical symptoms of powdery mildew. Despite the effect of plant tissue’s discolouration, the fungal growth may occasionally be sparse and difficult to observe.
Infectious spores in spring will be reduced if fallen, infected leaves are destroyed in autumn. Reduce subsequent infection by pruning out infected shoots swiftly after discovery.
Box Tree Moth
Box plants, scientifically known as Buxus, are cherished by many gardeners for their versatile use in hedges, topiaries, and ornamental gardens. However, a relatively recent addition to the list of challenges faced by gardeners is the box-tree moth (Cydalima perspectalis). This invasive pest, native to Asia, poses a significant threat to box plants across the United Kingdom and other parts of Europe.
Identification & Damage
The box-tree moth is a small, white or light brown moth with distinctive dark brown markings on its wings. It has quickly become a cause for concern due to its voracious appetite for box plants. The moth lays its eggs on box leaves, and when the larvae hatch, they begin feeding on the leaves, often leaving behind a skeletonized appearance. Severe infestations can lead to extensive defoliation and weakened box plants.
Prevention & Control
Preventing and managing box-tree moth infestations requires a combination of proactive measures and careful monitoring. Here are some strategies to help protect your box plants:
- Regular Inspection: To detect box-tree moths early, conduct frequent inspections of your box plants, especially during the warmer months when these pests are most active. Look for signs of eggs, larvae, or adult moths on the leaves.
- Hand-Picking: If you notice early signs of infestation, you can remove the larvae and adult moths by hand. Wear gloves and carefully pick off the caterpillars or moths, disposing of them in a sealed plastic bag.
- Pheromone Traps: Pheromone traps are available for box-tree moths. These traps use synthetic pheromones to attract and capture adult moths, helping reduce the population.
- Biological Controls: Some natural predators and parasites, such as parasitic wasps, can help control box-tree moth populations. Encouraging a diverse ecosystem in your garden may attract these beneficial insects.
- Pruning and Sanitation: Prune and dispose of any heavily infested branches or leaves. Proper sanitation practices can help limit the spread of the pest.
- Chemical Control: If the infestation is severe and other methods prove ineffective, consider using insecticides specifically designed to target box-tree moths. Follow the manufacturer’s instructions carefully and avoid overusing chemical treatments, as they can have adverse effects on beneficial insects.
- Quarantine: If you purchase new box plants, consider quarantining them for a period before introducing them to your garden. This precaution can help prevent the introduction of box-tree moths to your existing plants.
Popular plants & their problems
Acers
Leaf scorch is especially common in Acers. This can happen when the environment is stressed by things like dry winds, frost, drought and waterlogging. The affected tree’s leaves will begin to brown, then curl and shrink. Choose a shaded location away from direct sunlight to prevent this. Additionally, you can plant the tree in moisture-retaining soil that is well-draining.
If the leaves of an Acer grown in a container become scorched, move the pot to a safer location. The tree will shed affected leaves at its own pace, so do not remove them. Dead shoots can be cut down in autumn, though.
Buxus/ Box Plants
Box (Buxus) is a very popular plant among gardeners in the UK, though as with every plant, they are susceptible to many different pests and diseases. The most common pests and diseases for Buxus are box blight, box rust and red spider mites.
Box blight is a disease caused by two fungi which attack box leaves and stems. Signs of box blight are leaves turning brown and falling, leaving behind bare patches and black streaks and dieback on young stems. Thankfully, there are fungicides which can be used to remove the fungi, though you should always research any fungicides you are thinking of using.
Box rust is caused by a fungus called ‘Puccinia buxi’ and displays as thick, rusty blister-like pustules on both sides of the leaves. It will often occur in autumn and winter, though can be controlled by clipping off affected shoots or spraying a rust disease fungicide.
Box red spider mite feeds by sucking sap from the undersides of leaves mostly in spring or early summer. By late summer, it will die out and overwinters as eggs which are laid on the underside of leaves. These are difficult to control but do not cause serious damage to the plants.
Camellia
Camellia flower blight is a fungal disease which infects camellias and causes the flowers to rot. The only part of the camellia affected by the disease is the flowers and it presents as brown flecks on any part of the petal, followed by the premature falling of affected flowers. In the years which follow, infection will be less likely to occur if fallen flowers are cleaned up. Spraying fungicides will not work for this as there are no fungicides or soil treatments for at-home use which target this disease.
Camellia gall is another disease caused by a fungus and is prevalent in wet weather. It will only affect camellias, though azaleas and rhododendrons also suffer from similar conditions. Cream-coloured galls appear in place of leaves in the summer if affected by camellia gall. Once spotted, take them out and destroy them; make sure to cover them with a plastic bag before removing them if spores are already present.
Controlling pests & diseases without chemicals
The best way to avoid diseases or pests is to cultivate and keep your plants clean. Unfortunately, despite our best efforts, some diseases or weather conditions can bring about a sudden increase in the number of pests.
Once they become established, diseases are particularly challenging to control, so early intervention is vital. Due to the lack of publicly available fungicides approved for edible plants, fruit and vegetable plants are particularly problematic.
Tips for preventing diseases and pests
- Be on the lookout and consider using pheromones or sticky traps to catch pests. They may not catch many pests but they are a good way to keep an eye out for potential problems.
- Insect populations can fluctuate, so patience is key. Although the damage may be tolerable, insects can help to spread diseases.
- Use a hose to get rid of aphids or to remove slug infestations manually.
- Plant sections and diseased materials should be removed – garden cleanliness is key!
