Pests and Diseases Affecting Vines
Do not be put off by the length of this list. Well-grown grapes are not particularly prone to. Rather they suffer from a wide range of troubles, many of which are extremely rare and which you are very unlikely to meet, especially if you correctly spray with a tar oil wash during the winter.
Red spider mite: This pest occurs most commonly in greenhouses which are too dry and the best precaution is frequent damping down and watering. The leaves of infested vines become speckled greyish green and are covered, particularly on the undersides, with tiny pale red insects only just visible to the naked eye but which can be seen clearly with a hand lens. Seriously attacked leaves turn yellow and die.
The most effective control is to fumigate the greenhouse with azobenzene. Alternatively, you can spray with malathion.
Greenfly: pale green leaves with curled tips are a symptom of the attack of greenfly. The small green insects can be seen on the leaves and stems. You are unlikely to be bothered with this pest if the vines have been sprayed with tar oil during the winter, as the oil destroys the wintering eggs. If you do suffer from greenfly during the summer, however, pinch off all infected leaves and burn them. Then spray the vines with bioresmethrin or malathion.
Scale insects: scale insects are flat, dark brown and oval-shaped insects, about 2 mm long, which feed by sucking the sap, mainly from the bark of the vine. The insects themselves are difficult to spot against the bark, as they are the same colour, but they often exude a sticky substance, called honeydew, which is usually accompanied by a black mould. The bark of infected plants is thus black and sticky. Sponge off the sooty mould and then scrape off as many insects as possible with the back of knife. Scrape the insects onto a piece of paper and then burn them. Kill any remaining on the vine by spraying with malathion; repeat at ten day intervals if necessary.
Mealy bug: these small white insects feed in a similar way to scale insects, living on the bark and leaves and sucking the sap of the plants. The most obvious symptom of an infestation is a white or grey cotton wool-like fluff on the shoots. During the winter the bugsbeneath the bark and a useful precaution is to scrape the bark with the back of a knife to dislodge them. A winter wash of tar oil also gives valuable protection. In addition they can be controlled by spraying with malathion; but if you use a spray, make sure to add a wetting agent. The fluff which surrounds the insects-is waxy and repels the spray if no wetting agent is added.
Botrytis (Grey mould, Noble rot): this fungus is only likely to occur in cool, wet conditions and is most common in unheated greenhouses in the autumn. It appears as a furry, greyish growth on the berries and foliage. The disease concentrates the sugar content of the grapes without rotting them completely and if wine is made from the diseased grapes it has a sweet, luscious flavour. In the Sauternes district of France, the disease is encouraged by commercial wine growers to enable them to produce sweet Sauternes wines. It is important, however, that the grapes are nearly ripe before they become diseased, if they are not the bunches may drop prematurely. If your grapes become diseased when they are nearly ripe and you intend to make wine from them, then count yourself lucky and leave the disease alone. If the berries are unripe, or you want dessert grapes, however, spray them with benomyl.
Honey fungus: this is serious, although, luckily, it is only common on badly drained. Honey fungus infects the roots, particularly where they join the stem just below the soil surface. If the bark is removed at, or just below, soil level, fungus can be seen as a white, flat, fan-like growth on the surface of the exposed wood. The fungus sends out creeping, black bootlace-like threads below the soil surface to infect other plants and it also produces honey-coloured toadstools on the ground around the stems of infected plants.
Infected plants die within a year or two. There is no successful cure for honey fungus and the best you can do is to prevent the disease spreading. Dig up infected plants, with as much root as possible, and burn them. Then fork the soil and sterilize it with a 2% formaldehyde solution. If there are other plants growing in the greenhouse, remove them before treatment.
Crown gall: crown gall is a bacterial disease which causes gnarled tumour-like growths at the base of the stem. Infected plants are not seriously harmed, however, and the disease can be safely ignored.
Dead arm: this disease can be recognized by elongated black spots on the lower parts of young shoots and on the leaves. It is most serious in spring when it attacks the base of the shoots near to the main stem—and can effectively kill the whole shoot. Since the disease over-winters as a sporing body on the vine, spraying the vines with tar oil in the winter is the best precaution. If, despite spraying, the disease is noticed in the spring, spray with captan.
Spray damage: spirally twisted shoots and small, cupped leaves are probably the effects of spray damage by hormone weedkiller. Vines are extremely sensitive to these weedkillers and if any spraying is to be done in the vicinity of your greenhouse close all the ventilators and doors. Do not store weedkillers in the greenhouse.
Splitting: split berries are usually caused by irregular watering. Water the vines regularly during the season.