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Plant diseases and controlAmong common diseases attacking shrubs are
lilac mildew, rhododendron blight, juniper blight, hydrangea blight and
dieback, privet blight, box leaf cast, and twig blight. The dying back of hydrangea shoots at the
tips is caused by late spring frosts, sudden growth check resulting from
drought and heat following a period of fast growth, and injury to young
shoots by aphids; a fungus dieback sometimes spreads from old flowers.
Remove wilted flowers and provide plenty of sunlight, for shady, damp
locations tend to bring on the disease. The fungus of lilac mildew winters over
in the fallen leaves. A common control method is to rake the leaves from
around the bush at summer's end and burn them.
The branches of privet hedges, and sometimes
the entire plant may be killed by a twig-blighting fungus. Control of
this disease consists of fertilizing to maintain vigor, pruning away injured
parts, and dormant lime-sulphur spraying. Certain fungi may produce abnormal growth, cankers, discoloration, death of the bark and the wood below on twigs, trunks, and branches. Pruning well back of the infected areas can often eliminate diseases of twigs and branches, and even cankers may be cut out of trunks, but wounds made by such surgery should be treated with protective dressings and shellac. Severe leaf diseases may be controlled by spraying with Bordeaux mixture or lime-sulphur, or dusting with sulphur. These are expensive measures when applied to trees and require knowledge as to the correct strength for the specific tree. Information about diseased trees may be requested from the United States Department of Agriculture, Division of Forest Pathology, Washington, D. C. If it is feasible, send along with your inquiries a workable specimen of the diseased part and of the living parts adjoining it, and a description of the general conditions in which the tree grows. Insect pests and controlInsects which attack trees and shrubs may be classed according to their mode of attack. There are leaf-eating insects, insects which bore into the bark or wood, leaf-mining insects, sucking insects and mites which extract vital juices from the plants, and gall-making insects. Some of the sawfly larvae, beetle grubs, caterpillars, and fly maggots mine the leaves, eating into the inner tissue while leaving the surfaces intact. Protected as they are against sprays and dusts by the skin of the leaves, they are difficult to control. Picking off and burning the infested leaves is a sound attack. Nicotine sulphate sprays or tobacco dusts may also be used against several of the mining insects. Beetles and their grubs comprise the principal borers, but they are supplemented by some of the caterpillars, ants, carpenter bees, and wood wasps. Borers usually attack ornamental trees and shrubs which have been weakened or injured. Naturally a primary means of combatting them is to keep the plants growing vigorously, treat injuries promptly, and keep wounds clean and shellacked until healed. If the borers have gotten into the wood, a few drops of carbon disulphide injected into their tunnels will usually kill them. Seal the openings they have made with putty or moist clay to keep the fumes in. Carbon disulphide can be bought at seed stores and drug stores. Be careful how you use it. It is poisonous and also inflammable and explosive when mixed with the air in certain proportions. Do not inhale the fumes. Among the sucking insects are plant bugs, lacebugs, aphids, and mites, against which sprays of nicotine sulphate, pyrethrum, or derris, in strong solution, are effective. The suckers, however, include a group of scale insects, protected from sprays by their scale covering or by some waxy body secretion. There are numerous dependable sprays on the market for use against them, each with instructions for its correct use. Galls, or abnormal swellings, are formed on plants by several types of insects, including aphids, moths, mites, flies, wasps, even beetles. Remove and burn fresh galls which contain larvae before they can complete their development.
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