Pests and Diseases Affecting Pear Trees
Pears suffer from the same trouble as apples, but generally to a lesser degree. Natural predators and good cultivation, including regular feeding, should reduce the risk of damage considerably. If you have had serious trouble in the past, an annual spraying programme should be carried out, to control the damage done by a particular pest or disease; otherwise follow a system of ‘spot’ spraying. A tar oil wash applied in mid to late winter will control the eggs of aphids and leaf suckers, but is best applied only once every three or four years, as it also kills many beneficial insect predators. In early spring, at the green cluster stage, spray with captan against scab. In mid-spring, at the early white bud stage, spray with malathion to control aphids, caterpillars and pear midge and apply captan again for scab. Lastly, in mid to late spring, at petal fall when nearly all the blossom is off, apply captan against scab. Never spray fully open flowers, because of the danger to bees and other pollinating insects. Not all pesticides and fungicides are compatible, so follow manufacturer’s instructions before applying them together. And remember that these substances can also be dangerous to pets and children, so be sure they are properly stored.
Aphids: these sometimes infest young growth, causing the leaves and shoot tips to curl and the new shoots to be stunted, sometimes severely. The leaves may also become sticky. The insects are small, green, grey or dark brown in colour, and are usually found on the underside of the leaves. The most serious is the pear bedstraw species, which is a mealy-covered, pink aphid; in severe attacks the whole tree may be smothered. Remove and destroy aphid-infested shoots as soon as they appear and spray the tree with derris or malathion; the winter tar-oil wash will kill most of the overwintering eggs.
Pear sucker: in recent years this has been causing a good deal of trouble. It is a small, flat, pale green, sucking insect which feeds on the undersides of the leaves, and the flower trusses, in bud and open. Three generations can occur in a season. Leaves have pale green patches on them, flowers do not develop, and sticky ‘honeydew’ with black mould on it, covers the leaves and shoots. The winter tar-oil wash will deal with the eggs, or derris or malathion can be used in spring.
Pear midge: if the young fruitlets on your tree do not develop, but become badly mis-shapen, turn black and fall off the tree, then pear midge is the probable cause. The tiny, white maggots, live in the young fruit, and later move to thewhere they overwinter.
Thorough cultivation under the trees will expose the maggots to insect-eating birds, the weather and physical damage from hoeing. Remove and burn all infested fruitlets, before the larvae have a chance to get to the soil. If this pest has caused considerable damage in the past, spray with gamma HCH at the white bud stage, but not during flowering. Pear leaf blister mite: this microscopic insect spends winters beneath the bud scales and during the summer lives inside the leaves. Infested leaves will have numerous yellowish green or red dish pustules on them, from mid-spring onwards. The pustules eventually turn brown to black by mid-summer, and the leaves fall early. If the attack is a mild one, pick off and burn all infested leaves. In general, this is usually all that is needed in the private garden. However, if you have had serious trouble in the past, spray with lime sulphur in early spring as the buds start to open. Do not spray Doyenne du Cornice with lime sulphur, as it is sulphur shy.
Caterpillars: there are several varieties of caterpillar which may attack pear trees, including the fruit tree tortrix moth, the vapourer, lackey and winter moths. The treatment is the same for all types of caterpillars: hand-pick the caterpillars in the case of mild infestations, or spray with trichlorphon in severe attacks.
Fireblight: this is a very serious bacterial infection which enters the tree through the flowers, and moves from the spurs into the main branches. Dieback occurs and leaves turn brown and black but remain on the tree. It is seen mostly on new shoots which look as though they have been scorched by flame. Cankers develop beneath infected tissues, which ooze a sticky liquid in spring. This liquid contains bacteria, which are then carried by rain or insects to other trees.
If you suspect fireblight, you must notify your local representative of the Ministry of Agriculture, who will then give you instructions about treatment.
Pear scab: this fungal infection appears as blackish scabs on the fruit, or dark brown blotches on the leaves. Occasionally shoots are infected and they will appear blistery and scabby. Remove and destroy all diseased leaves; do not leave them on the ground, or the infection will spread rapidly. Spray with captan (except for fruit to be preserved) as indicated. For fruit for preserving, use benomyi at bud burst, and at three-weekly intervals as long as necessary. In winter, be very careful to prune off all infested shoots, as the spores overwinter on them and can remain viable for at least three years. The scab lesions also provide a means of entry for the fungus disease canker.
Pear stony pit virus: if the pears are mis-shapen and pitted, and have small, hard areas in the flesh, then the tree is infected with stony pit virus. In severe cases, the fruit will be completely inedible. At first, fruit on single branches will be affected, but eventually the virus spreads through the whole tree, and the whole harvest from that tree becomes useless. Old trees are most susceptible. There is no cure; dig up and burn any infected tree.
Canker: this fungus disease also attacks apple trees, and the symptoms and damage are the same. The bark of branches becomes sunken and cracked; if the canker girdles a stem, it will die above the infection. Cut out all infected parts and paint large wounds with a tree wound-sealing compound.