Pests and Diseases that Affect Garden Shrubs
PESTS AND DISEASES
Another point about the ease of aftercare of shrubs is that they suffer, on the whole, from relatively few attacks by insect pests or fungus diseases. Always follow manufacturer’s instructions when using chemicals.
The ubiquitous greenfly, or the blackfly (both are ) feed on the tips of young shoots, especially where they are particularly soft and succulent; they may be liquidated with derris, malathion, nicotine (which is very poisonous) or by squashing with the finger and thumb.
Leafminers sometimes attack shrubs, particularly holly. These can be difficult to deal with but rarely occur in great numbers; use a systemic insecticide containing dimethoate which is absorbed through the leaf, or taken up by the roots.
Caterpillars, if there are enough of them, warrant the use of malathion.
Capsids sometimes produce pinholes in the leaves, particularly on the tips of young shoots; treat as aphids.
Rhododendrons, azaleas, and occasionally other shrubs may be attacked by the vine weevil adult, which bites semicircular holes in the margins of the leaves. Attacks mostly occur on leaves near the ground. Control is difficult, but dusting with BHC may help, and spraying the ground to kill the larvae again with BHC may also help.
There are one or two diseases which may cause trouble. Where a shrub starts to die for no apparent reason and is not old, the fungus disease known as honey fungus should be suspected. This has toadstools the same colour as honey, with round, flattish caps, and they can be found growing at the base of trees or shrubs. They do not appear, unfortunately, until the plant attacked is dead, and the only other way to diagnose the trouble is to dig in the soil around the base of the shrub and at some distance away from it. Thick black threads like bootlaces will be found running close to the roots of the shrub concerned.
Even when diagnosed, it does not help a great deal since it is unlikely that the plant can be saved, but at least one will know that the ground is contaminated, and no more woody plants should be grown in that spot. Control by the amateur, other than wholesale removal of the soil from the spot, is not possible yet. Leaving dead stumps of shrubs and trees in the soil encourages this fungus, and they should be removed.
Shrubs are occasionally attacked by silver-leaf disease, a fungus which more commonly occurs ontrees. The name aptly describes the greyish-silver effect which overlies the natural green of the leaves. In some cases the disease dies out of its own accord, in other cases it is necessary to cut off the attacked branch. Any wood killed by silver leaf must be cut out and burned.
Canker sometimes occurs — the stems become swollen and knobbly and later the bark cracks in a circle round the stem, which then dies from the canker to the tip of the shoot. Cut back to a bud in healthy wood and burn the diseased part. This disease often occurs where theis damp.
Apple scab may occur on shrubs related to the apple family, the Rosaceae, but spraying regularly with captan as instructed by the manufacturer will control it.
Mildew may occur on leaves; this is a white powdery meal and can show on stems as well as leaves; dinocap will control it.