This is one of a new series of posts based on real questions I have been asked. Feel free to add to the answers or include your ideas and experiences

I get to answer a lot of questions about pests and diseases. Some plants are rarely affected by any problems but lots of common garden plants suffer from various pests and diseases. In most cases the incidence of these can be reduced by good cultivation, but not always. I am constantly told that people don’t want to use garden chemicals because they want to be organic but my experience is that they don’t want to use chemicals to control problems because of the expense. And although many gardeners don’t want to eliminate all insect life in their garden, others get in a right state if they find a single nibbled leaf or spotted petal on their plants.

The holistic approach, and the one that I feel is best, is to:

Choose resistant varieties, where possible

Know what potential problems your plants might have and be ready to act at the first sign of trouble,

Grow what suits your garden conditions, and keep the soil healthy so your plants are healthy

Use only registered chemicals (which have been tested for sale and safety) and use them as a last resort,

Put up with damage if it is not serious,

Encourage wildlife to help you and

Consider the impact of anything you do.

Now I don’t always follow all these rules to the letter – for example I do try to push the boundaries of what I can grow, but I strongly believe that keeping the soil healthy is a big step to having a healthy and colourful garden.

Which brings me to today’s question: This person had a six year old apple tree that was ‘infected with white froth-like stuff all over the stems, branches and base of the tree’. They wondered if it was cuckoo spit? And they wanted to know how to get rid of it.

Cuckoo spit is the frothy exudate of frog-hopper bugs; small, sap-sucking insects that protect themselves with liquid ‘froth’. They are usually seen in summer on a wide range of plants but seem to particularly like lavender. I also get lots on tall grass and goosegrass (galium). They are usually solitary and never cause a problem and I rather like them, in a sympathetic sort of way. They could have been on the young stems and leaves of the apple tree but would never be on the branches and trunk. They are soft-bodied, delicate little creatures and I can’t imagine them pushing their little stylets into bark.

The culprits in this case are woolly aphids. These form dense colonies that feed on the sap of apple trees (and related shrubs such as chaenomeles and pyracantha) and in summer spread onto young shoots and then produce flying males. Most of the time they are thick colonies that cover themselves in waxy ‘wool’ that hides them from predators. As they feed, they distort the stems which then become lumpy and swollen, further protecting the aphids.

Like most pests, this one is much more severe when the plant is under stress. In the photos (above) the tree (a ‘Cox’s Orange Pippin’) was growing against a sunny wall and the soil was dry. It had been untreated for years and the stems were covered in aphids all year round. ‘Cox’ can be prone to problems which is unfortunate because it is such a great apple to eat, but it is no more prone to woolly aphid than any other apple. Heavy infestations like this seriously weaken the tree and can’t be left.

There are various ways to tackle this. I would firstly prune out seriously distorted branches which will probably be weakened anyway and not very fruitful.

Then you can physically remove the aphids. You can brush the branches with a stiff brush (not a wire brush, which will damage the bark). And, in winter, you could power-wash the branches – don’t do this in summer or you will blast the leaves off. If you are an optimist you could hang bird-feeders among the branches which, in theory, encourage tits to visit the tree and peck off the aphids.

And if you want to use a spray, you can use an organic or chemical spray to kill the aphids. If you do, then use a coarse spray to it penetrates the ‘wool’ and gets to the aphids.

But it has to be emphasised that stressed trees will be worst affected and a healthy tree will be less affected. A healthy young tree of six years should not really be suffering from woolly aphid.