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Pests & diseases
Help solve your gardening problem. Here are the answers to some commonly asked gardening questions about pests and diseases and their control with hints, tips and advice.
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Further information on the control of:
- Aphids, greenfly, blackfly control
- Clematis wilt
- Lily beetle control
- Powdery mildew control
- Scale insect control
- Slug & snail control
- Vine weevil control
- Whitefly control
Over the summer my Robinia pseudoacacia 'Frisia' has lost 95% of its leaves and has failed to produce new shoots. I've noticed that other robinias in my area have also lost their leaves. Is there a disease attacking our robinias?
In the last two years, there have been many reports of this problem on robinia.
At the moment, it is unclear what causes the problem. The symptoms are consistent with a wilt fungus blocking the water-carrying vessels in the trunk, but plant pathologists have found no evidence of this so far, but have found many instances of fungal leaf-spot disease that can defoliate and weaken trees. Some instances of honey fungus and wood-rotting fungi have also been found. Neither the leaf spot or the other diseases are likely to fully account for the damage. So, the cause is as yet unexplained - of course, it could simply be physiological problems caused by the last three cool, wet summers.
Trees suffering the problem last year rarely came back to life this year. I would wait until spring and see what happens. If it has died, then I would certainly remove it. If you want to replace it I'd plant Gleditsia triacanthos 'Sunburst' - this looks similar but, to my mind, is a much better tree.
I've noticed that the leaves on our photinia bushes are covered with dark spots. This has happened in previous winters.Can you advise me on how to deal with this?
From your description it sounds like a fungal leafspot disease. Usually these are only disfiguring and rarely affect the overall health of the plant. However, they are usually a symptom that the plant is stressed and may be suffering from some other problem - probably physiological, and may be something as simple as drought or waterlogging, cold or wind damage.
Spraying with a general fungicide, such as Fungus Fighter or FungusClear, will help protect new growth from getting the marks, but you should try and work out why it is stressed.
What are the little flies appearing from the compost around my houseplants and, if they're a problem, how do I get rid of them?
The flies are known as mushroom or sciarid flies, or fungus gnats. Their larvae live in the compost and the flies breed on the surface. They do little damage when present in small numbers, although they will eat the fine root hairs of your plants and severe infestations can cause damage - especially to already unhealthy plants.
A biological control agent is available mail order from various suppliers, including Green Gardener or you can treat the compost with Provado Vine Weevil Killer.
The flies are worse in peat-based composts which are kept too wet, so make sure you only give the plants as much water as they need and no more.
Can you suggest the best way of dealing with earwigs that are devouring my dahlias, clematis and chrysanthemums?
The best way of controlling earwigs is to set a straw trap for them.
Place bamboo canes among your plants and then place an upturned pot full of straw on top of each one so that the pot is level with the flowers; the earwigs will hide in the straw. Check the pots daily and dispose of any earwigs you find.
Also make sure that any debris surrounding the plants is cleared up regularly as this material provides perfect hiding places for the earwigs.
You can also smear a layer of petroleum jelly on the flower stems of your plants. This acts as a barrier which the earwigs won't cross and so protects the blooms from damage.
Can you give me a list of plants that rabbits won't eat?
Unfortunately, rabbits have a fairly varied diet - that is, they'll eat almost anything!
Even prickly shrubs like berberis, roses and pyracantha are not safe as they will have a go at the young growth - unless it is out of reach.
But there are some plants they are less fond of. Any with tough, leathery leaves - such as mahonia, elaeagnus, aucuba, osmanthus, acanthus, eryngium, phormium - or silvery foliage like lavender and artemisia and any with a lemon scent, are less prone to attack.
You can buy a movement-operated scarer called Scarecrow, which shoots out a burst of water when movement is detected. The water, movement and noise scare most animals - but keep moving it around the garden so they don't get used to it. It costs around £80.
You can also try chemical deterrents such as Get Off My Garden.
You could even grow lettuce in the hope they will eat those and leave everything else alone.
But the only foolproof method is to fence off the garden with wire netting ensuring at least 38cm (15in) is below ground level and angled away from the garden.
I have found limpet-like lumps on the stems and underside of the leaves of several of my shrubs. What are they and should I try getting rid of them?
Your pest is a sap sucker called scale insect. These pests are well protected by the hard scale that covers them.
You can start by gently removing them with thumbnail or similar. Be wary of the white woolly substance underneath some of the scales as this is a mass of eggs which must also be removed. The young scales are much smaller and easier to control as the scale hasn't hardened. They can be sprayed with Bayer Provado Ultimate Bug Killer or similar insecticide, but this may need to be repeated as necessary to completely remove and control them. Spring is a good time to start spraying as the scales tend to breed and hatch at this time of year.
The scales also produce a sticky substance called honeydew - a mixture of sugars and other substances that are excreted by the scales after sucking too much sap. The honeydew - being rich in sugar - is then a site for sooty mould disease to colonise. Wipe off the honeydew with a damp sponge before the sooty mould gets a hold.
Find out more about scale insect control
I have an established camellia that flowers well every year. However, by mid-summer, I noticed a very black sooty-like substance covering the leaves and stems. The deposit could be removed by wiping with a damp cloth. Help please, what is it?
The black deposit is called sooty mould - a fungal disease. It is easy to wipe off - but its cause is more sinister. The sooty mould colonises honeydew, which is excreted by sap-sucking insects. I'm sure if you look on the backs of the leaves and on the stems you will see small brown projections - like limpets. These are scale insects. Other sap-sucking pests may be responsible.
See the answer above for more details.
My garden is over-run with woodlice. Will they damage any of my plants?
Woodlice usually don't do much damage to plants - although they are often given the blame for it. Their main food is already damaged/rotten plant material. However, in large numbers &/or when other food sources are in limited supply they will have a go at, and damage, plants. Unfortunately, there isn't a natural cure for woodlice - apart from clearing away debris - even mulches - and any other cover they may hide under.
The only real way of control is to use an insecticide powder puffed around where you see them. There are many available - most chemical companies make one - usually the sort sold to kill ants. Check on the label first before buying.
I would be grateful for any advice on how to control honey fungus. It appears to be attacking newly planted shrubs. Is there a fungicide that will help?
First make sure it definitely is honey fungus. The easiest way to tell is to check the soil for the black bootlace-like rhizomorphs. And remove the bark at ground level of killed shrubs - those killed by honey fungus will have a sheath of white mycelium under the bark.
I'm afraid there aren't any secret or miracle cures for honey fungus. make sure you dig up and burn affected plants. Always make sure you remove the major roots. And never leave tree stumps in the garden - either dig them out or use a stump grinder to permanently remove them. Keep plants growing as strong as possible by feeding and mulching regularly.
The following plants show some resistance to honey fungus:
Acer negundo, Abutilon, Bamboos, Carpenteria, Catalpa, Celastrus, Ceratostigma, Cercis, Chaenomeles, Clematis, Cotinus, Fothergilla, Hebe, Juglans nigra, Kerria, Lavandula, Passiflora, Phlomis, Pieris, Pittosporum, Quercus, Rhus, Romneya, Sarcococca, Tamarix, Taxus.
How can I stop cats using my garden as a toilet?
There are a number of cat deterrents - using orange peel, tea bags soaked in olbas oil, rose and other prickly cuttings, plastic bottles filled with water and laid on their sides etc, etc. Some have limited success.
When I did a test on cat deterrents a few years ago the ultrasonic one I tested gave the best results. There are a number of them now on the market, but the one I tested is the best of the bunch. It is called Catwatch and is produced by Concept Research.
Another useful tool is the Scarecrow Animal Deterrent, which also detects movement but sends out a spray of water and a hissing noise that scares away cats - and other animals.
I have lost a number of plants such as pansies, lobelias & salvias through attacks by red ants. How I can kill these pests.
The best product on the market for killing ants is Ant Stop - available from any good garden centre/store.
However, like any ant killer it's pointless trying to attack the few ants that are out on patrol - you must follow them back to their nests and treat the nest.
If you want to know more, or if there's a gardening topic you're having a problem with and want help and advice, then send an e-mail to: info@gardenforumhorticulture.co.uk
