Pests and Diseases that Affect Shrubs
Sadly it is just not possible to grow a range of shrubs without encountering a few problems along the way. The most common of these are pests and diseases, many of which are instantly recognisable. Fortunately, in most cases, remedies are available — and successful.
There are a number of pests that will attack shrubs.
• Aphids (greenfly and blackfly) cause discoloration of the leaves and distorted shoots; they also leave a sticky substance known as honeydew. They should be controlled with a systemic insecticide as soon as they are seen, as large colonies can quickly build up.
• Vine weevils are a particularly serious problem. The grubs attack the roots belowlevel. Modern biological control is available with the use of nematodes applied to the soil. This must be done when the grubs are active and the soil temperature is right, generally in spring and early autumn. Shrubs most susceptible include Camellia, Clematis and Rhododendron.
The adult weevils cut small ‘U’-shaped holes in the leaf edges, feeding after dark. Spraying shrubs and soil with a systemic insecticide is helpful, or alternatively check after dark and destroy any adults that are found.
• Caterpillars should be treated as soon as seen, as they can seriously damage foliage. Spray with a persistent insecticide.
• Birds can also damage shrubs, especially early in the season, removingfrom Forsythia and Prunus.
• Red spider mite is a very serious pest under glass, which can also attack outside, especially in dry conditions. Usual signs include a ‘bronzing’ of the leaves. Check the undersides of the leaves. If you see a fine webbing of silky hairs and tiny spider-like mites, you’ll know that your plant has been attacked. Spray with a suitable insecticide and repeat one month later.
• Another pest that can sometimes be seen on shrubs is woolly aphid, which is indicated by a white waxy wool substance. Brush off and spray with insecticide.
• Other pests that damage foliage include capsid bugs and chafer beetle. Spraying foliage and surrounding soil with a systemic insecticide will help to control them.
There are a number of fungal diseases that can attack shrubs.
• Honey fungus — a white fungal growth appears below the bark at ground level, and black thread-like rhizomorphs grow through and over the roots. These can travel through the soil and attack other subjects, particularly if they are under stress or damaged. Honey-coloured toadstools appear in the autumn.
There is no cure for the disease, although chemical products are available, to treat the soil. Remove all traces of wood, including trunk and roots. Rhododendrons, cherries, lilac and willows are particularly susceptible to this disease.
• Canker will kill any branch it affects. When seen, you should cut it back to clean wood.
• Fireblight will cause the shoots of some shrubs and trees — members of the rose family —to wilt and die. The brown, withered leaves that remain on the plant identify this. Cut out and burn the affected branch.
• Coral spot disease is easily identifiable with its raised pink spots. Here again, cut out all diseased wood and burn it.
•is a common garden problem. When seen, spray immediately with a systemic fungicide and repeat one week later
Virus diseases can occur on a few shrubs. Insects, tools, or even your hands are known to carry or transmit them, from one subject to another. Always buy clean plant stock. Avoid any that look doubtful, with yellow blotched or crinkly leaves and stunted growth. There is no cure for viruses. Remove the virused plants, or parts of plants, and burn the cuttings. Keep all tools clean, especially secateurs and pruners.
One problem that can occur on acid-loving shrubs if they are grown where lime is present in the soil, is chlorosis. This is a lime-induced disorder Leaves become yellow. Treat the plant with a sequestered compound — depending on the severity of the problem, this may cure it.
If the soil lacks certain plant nutrients, this will normally show in a change of leaf colour. The leaves themselves will usually indicate what is missing, for example a lack of nitrogen would cause reddish and yellow tints. Leaf-edge scorch can indicate a potash shortage. Magnesium shortage produces a browning between the veins. Iron deficiency causes yellow leaves. The last two can be improved by watering with a sequestered compound, and foliar feeding in the first two cases.
In spring, sprinkle the soil with a good balanced fertilizer and rake it in.