Pests and Diseases

Garden Pests
Top 10 Pests

1. Viburnum beetle (Pyrrhalta viburni):

This pest has been climbing up the Top 10 ranks in recent years and makes the number one spot for the first time. The principal host plants are Viburnum opulusV. lantana and V. tinus but some other Viburnum species may also be attacked.
Most of the damage is done by the larvae, which can reduce the foliage to lacework during May-June.

The adult beetles also eat the leaves in late summer but less extensive damage occurs at that time.

2. Slugs and snails (various species):

In most years, slugs and snails are the number one problem in gardens and on allotments. The reduced number of enquiries in 2010 may be due to the long dry spell during the summer. Most damage occurs during spring to autumn, affecting seedlings, many ornamental plants and vegetables, especially potato tubers and narcissus flowers.

3. Cushion scale (Chloropulvinaria floccifera):

This sap-sucking insect occurs on evergreen shrubs, especially camellia, holly, rhododendron, Trachelospermum and Euonymus japonicus.
Although long established in Britain, it has become more widespread and troublesome over the last 20 years.
It excretes honeydew, causing infested plants to develop a thick black coating of sooty mould on their foliage over the winter months.

4. Chafer grubs in lawns (various species):

Chafer grubs are the larvae of several species of chafer beetles. Most of the damage in lawns is caused by grubs of garden chafer (Phyllopertha horticola) and Welsh chafer (Hoplia philanthus), which eat the roots.

During autumn to spring other animals, such as foxes, badgers and crows, rip up the loosened turf to feed on the grubs.

5. Harlequin ladybird (Harmonia axyridis):

This foreign ladybird was unknown in Britain until 2004 but has since spread rapidly throughout Britain.
It is not a plant pest but causes concern because of its reputation for eating native ladybirds and other aphid predators. Its prey of choice, however, is greenfly and other aphids, so it is of some benefit to gardeners. It can feed on a wide range of other insects when aphids are in short supply but it remains to be seen whether harlequin ladybirds will reduce the numbers of native beneficial insects. Adult harlequin ladybirds are highly variable in appearance, with many different colour forms and variable numbers of spots and other markings. It over-winters as adult beetles and likes to do so inside buildings; this habit helps to bring it to public attention.

6. Vine weevil (Otiorhynchus sulcatus):

The adult beetles eat notches in the leaf margins of a wide range of herbaceous plants and shrubs. The larvae feed on plant roots, especially those being grown in pots or other containers. It is one of the few pests capable of killing plants and in most years is a top five enquiry.
These days, the majority of vine weevil enquiries concern foliar damage by the adult beetles rather than the grub stage. This may reflect the better control options currently available to amateur gardeners for dealing with vine weevil grubs. Another contributing factor is likely to be the establishment and spread of several other Otiorhynchus spp. in Britain in recent years. These cause identical patterns of leaf damage that cannot be separated from that caused by adult vine weevil.

7. Lily beetle (Lilioceris lilii):

Although established in England since 1939, this pest of lilies (Lilium spp.) and fritillaries (Fritillaria spp.) did not spread out of the south east counties until the 1980s. It now occurs in all English counties with a more scattered occurrence in Wales, Scotland and Northern Ireland. Both the larvae and adults eat the leaves and can cause severe defoliation.

8. Horse chestnut scale (Pulvinaria regalis):

This sap-sucking insect occurs on various trees and shrubs, including horse chestnut, sycamore and maples, lime, magnolia and bay trees. It becomes noticeable in early summer, when the mature scales deposit white egg masses on the trunks and larger branches of infested trees. A common and spreading pest, especially on trees growing in streets and other urban situations.

9. Glasshouse red spider mite (Tetranychus urticae):

A wide range of glasshouse plants and house plants is attacked by this small sap-sucking pest. In late summer, damaging infestations can also build up on garden plants, especially in hot dry summers. Vulnerable plants include cucumber, runner beans, strawberry, raspberry, fuchsia, rose, Brugmansia, Crocosmia, orchids, oleander and dahlia.

10. Ants (mostly Formica and Lasius spp.):

Ants are often abundant in sunny gardens with well-drained soils. They cause little direct damage to plants but the soil excavated from their nests can be a nuisance in lawns, on patios and in flower beds where low-growing plants may become partly buried.

Common Diseases
How To Treat Common Plant Diseases

Rose bushes and flower seedlings, shrubs and trees, vegetable plots and fruit crops, house plants and even the lawn are all in danger of succumbing to the UK’s most common plant and garden diseases, whether they be fungal, bacterial or viral. Here’s how to spot and treat them.

Powdery Mildew

This disease (below) grows mostly on the upper surface of leaves and will occasionally spread to the underside and other parts of the plant.
It thrives in dry soil conditions, but where overhead conditions are a little humid.
A whole host of plants, including herbaceous and many ornamental, can be at risk from a powdery mildew infection.

What to look for:
As the name suggests, it is characterised by a white powdery fungus coating on the leaves.

How to treat: Remove and destroy all infected parts of the plant and be careful not to transfer spores on to healthy foliage. Try not to add too much nitrogen-rich fertiliser as this can promote sappy growth which is more vulnerable to fungi. If opting for chemical solution, choose a fungicide containing penconazole. If looking for a suitable organic treatment, choose a dispersible sulphur spray.


Botrytis (or grey mould) is often found on indoor cyclamen and outdoor flowers, such as lily and chrysanthemum, which are kept in very humid conditions.

What to look for:
It looks like a grey furry mould and can be found on leaves, petals and the base of stalks.

How to treat: Pick off the affected part of the plant and reduce any humidity by increasing space between plants, encouraging air movement. Destroy all plants which are severely affected, and avoid over-feeding with a high nitrogen fertiliser as this encourages soft lush growth that is more susceptible to attack. No fungicide treatments are currently available on the market for home gardeners, but a recommended organic remedy is lime sulphur.

Clematis Wilt

Clematis wilt mostly strikes early, large-flowering varieties.

What to look for:
The top of the clematis can wilt, collapse and eventually die. The diseased shoots turn brown and hang loosely from the main stems.

How to treat: Cut out all affected shoots and, if the entire plant is affected, cut it down to ground level and feed with a good liquid fertiliser. Initial planting of the clematis several centimetres deeper than normal allows for the development of healthy shoots beneath the surface as replacements. There’s no ready-made chemical treatment, but spraying the soil around the plant with a fungicide containing penconazole can help healing.

Damping-Off Disease

This disease (left) is extremely common in seedlings and is usually caused by over-watering, crowding of seedlings, poor air circulation, dirty containers, infected soil or contaminated water.

What to look for:
Damping-off disease causes emerging seedlings to rot off at soil level, collapse easily and lose their leaves early.

How to treat: Sow your seeds thinly in good compost and clean pots, then dampen in moderation with fresh water and make sure there is the necessary ventilation. And avoid sowing seeds too thickly. For a chemical treatment, try one of the many copper-based fungicides on the market in the seed trays.

Leaf Spot

Leaf spot is a common plant disease, easily noticeable, and caused by a combination of bacteria and fungi.

What to look for: Leaf spots can be of various colours such as grey, brown or black, and can affect all types of plants. Spots can sometimes join together to form larger areas of dead black tissue.

How to treat: Immediately remove and destroy all infected foliage, and to reduce the chances of re-infection for the following season, prune shrubs back hard and water sparingly for a few weeks. For a non-organic treatment, spray with suitable fungicides (containing myclobutanil) and continue spraying at fortnightly intervals throughout the season.

Rose Diseases

Black Spot

Black spot is a widespread problem on roses but can be sometimes found on other plants too.

What to look for: Black spots appear on the leaves and can sometimes join together to form larger areas of dead tissue. The spots are in fact dead leaf tissue caused by the fungus which spreads the disease. Some roses also develop smaller black spots on stems.

How to treat: On first sign of black spot, remove and destroy all infected leaves and plant parts. A Bordeaux Mixture gives excellent organic control.
Or alternatively spray with a systemic chemical fungicide such as Rose Clear. To reduce the chances of re-infection for the following season, prune shrubs back hard. Try to select resistant varieties of roses when buying.


This fungal problem mostly develops on the rose leaves, but also on stems, and the rust spores thrive in really moist environments.
What to look for: They can develop either as patches or as pustules (like septic spots) and usually they range in colour from orange to brown, depending on the season.

The presence of a rust infection may point to an already weakened plant, so check for other diseases or pest problems.

How to treat: Once spotted, immediately remove all affected plant parts, and remember to destroy any fallen leaves too. Choose a suitable fungicide if looking for a fast chemical treatment – there are many specialist chemical rose formulations available – and spray at fortnightly intervals throughout the season. If you would prefer an organic solution, try one part white vinegar to three parts water and spray.

Rose Viruses

Viral infections on roses are unusual in the garden, but can be particularly troublesome.

What to look for: Viral infections on rose plants and bushes can appear as veining which gives the foliage unsightly streaks, marks, or patches.

One virus in particular, the Rose Mosaic, will appear as discoloured mosaic patterns in shades of yellow and green.

How to treat: Viruses which strike roses are virtually impossible to cure and will seriously weaken the plant, probably causing its death. It is advisable to destroy roses with a virus to avoid spreading the infection to other plants.

Shrubs, Trees and Lawns
Shrub, Tree and Lawn Diseases

Honey Fungus

Honey fungus affects all woody plants and trees, including ornamentals.

What to look for: The disease causes nasty white growths under the bark and honey-coloured threads in the roots, which is how it gets it’s a name.

Brown toadstools can also appear at the base of the tree.


How to treat: Remove and destroy the affected parts, and dig out as much of the roots as possible because this is where the disease is most easily spread to other plants. And when replanting in the affected area, make sure the soil is replaced first. At present, there are no chemicals available for the treatment of honey fungus for home gardeners.

Silver Leaf

Silver leaf is a vicious tree disease caused when a fungus enters wounds (during a cool and wet season) and slowly kills off parts of the tree by blocking its water-carrying vessels.

What to look for: This infection gives the leaves of trees a silvery sheen, and affected branches can split quickly, turn brown and die.

Similar silvering symptoms may sometimes develop as a result of cold, drought or other non-disease forms of stress, but beware – this is known as ‘false silver leaf’ and can be told apart from true silver leaf by the lack of a stain in the wood.

How to treat: The only treatment for silver leaf disease is to cut down all affected branches completely and burn them.

On susceptible trees, consider painting all pruning cuts with protective wound paint.

Red Thread

Red thread disease is caused by a fungus which grows on infected blades of grass. The threads can extend to around an inch past the lawn height.

What to look for: As its name suggests, red thread causes red, pinkish threads to appear on the lawn.

How to treat: It’s possible to apply a systemic fungicide, but the effect of these is usually fleeting and you will still need to rely on nutrients of a good fertiliser to improve the health of the grass.

The best plan to thoroughly rake out the affected grass and use a high nitrogen lawn fertiliser.

Coral Spot

Coral spot disease appears on the dead and dying twigs of trees and shrubs, and it can quickly migrate over to living wood by entering in pruning snags.

What to look for: Coral spot manifests itself as tiny salmon-pink coloured spots, usually in the winter months.

It can move down into living wood rapidly and will usually attack trees such as beeches and magnolias.

How to treat: A tricky disease to cure organically, the best action is to remove all dead branches (on and off the tree) and always prune in dry weather. Chemical treatment is available if you seal all pruning cuts with a protective dressing like Seal and Heal.