Shrub problems FAQs

One of the shoots of my clematis suddenly wilted and died back to ground level. The remaining shoots are still apparently healthy. What can have caused this dieback?

The situation you describe is typical of an attack by clematis wilt, caused by the fungus Ascochyta clematidina. This is a poorly understood disease, but I can assure you that the trouble is not likely to spread to the remaining healthy shoots. If you cut off the dead shoot you will probably get re-growth from its base. No further remedial action is called for.

Last year the foliage on a branch of my blue atlas cedar (Cedrus atlantica ‘Glauca’) turned grey and then became brown. This year the whole tree has suddenly died. What can have caused this loss?

The most likely cause is an attack by honey fungus (Armillaria mellea). This soil-borne fungus disease penetrates the tree roots and then grows upwards to the base of the trunk, causing the tree to die. You can easily check for the presence of honey fungus by chipping off some bark from the base of the trunk: you should find a sheet of white fungal growth on the surface of the wood. Most types of trees and shrubs, involving hedging plants are subject to attack. Dig up and burn the dead tree, including as much of its roots as possible; then sterilise the soil with a special tar oil emulsion, specially designed for use against honey fungus before replanting.

The tips of some branches of my flowering cherry seem to have died back. The branches were girdled by rough cankers. How can I prevent further damage?

Your tree is infected with cherry bacterial canker (Pseudomonas mors-prunorum). Protect against further attacks by applying a series of three sprays with a copper fungicide applied at intervals of three weeks from the end of August.

The foliage of my pyracantha, which earlier produced a splendid crop of flowers and looked perfectly healthy, has gradually turned brown and now the whole bush seems to be dead. What can have caused this collapse?

The disease affecting your pyracantha is called fireblight and is due to infection by the bacterium Erwinia amylouora. This disease enters through the flowers and then progresses down the shoots. Fireblight can also attack apples, pears, cotoneaster, hawthorn, whitebeam, and mountain ash as well as pyracantha. Report any suspected attack to your county horticultural officer or to your park superintendent, who will advise on what action to take.

The leaves of my Oregon grape have become covered with a grey powdery growth. What is this, and how should I deal with it?

Your mahonia is infected with powdery mildew (Microsphaera berberidis). If tackled in its early stages, this disease can be controlled by repeat sprays of fungicides containing bupirimate, carbendazim, or dinocap.

Many of the flower buds on my rhododendrons turned brownish-black this spring and failed to open. What is the trouble?

The buds have been infected with bud blast, a disease caused by the fungus Pycnostysanus azalaea. This is believed to enter the buds through wounds caused by the rhododendron leaf hopper (Graphocephah coccinea) when depositing its eggs in the buds.

First of all you must pick off and burn any infected buds, and protect against future attacks by leaf hoppers by spraying the bushes with a general insecticide in August and September.

Some of the young leaves of my evergreen azaleas are turning reddish and are swelling into galls. Is this caused by a pest or disease?

This is azalea gall, caused by the primitive fungus Exobasidium vaccinii. Although the galls are reddish at first, they turn a waxy white when spores are produced to spread the disease. To prevent the disease spreading, pick off the galls and then spray the bushes with a copper fungicide.

Some of the buds on my broom bushes have become very swollen, with the bud scales opened out so that they look rather like miniature green roses. What is the cause of this abnormal growth?

This strange growth is broom bud gall, which is produced in response to an invasion of the buds by minute gall mites (Aceria genistae). These galls usually have little effect on the health and vigour of the brooms. Very heavy infestations, however, can cause some suppression of flowering. Normally, therefore, no control measures are needed, although it is helpful to remove and burn as many galls as possible.

Abnormal growths, resembling small pineapples, have appeared at the ends of some of the branches of my sitka spruce tree. What is the cause of this abnormality?

The spruce is infested with spruce-gall adelgids (Aldelges uiridis). These insects are related to aphids but are much smaller and look like darkish speckles on the host plant. To control them, remove and destroy all the galls; in early April give the tree a thorough spraying with a general insecticide.

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