Pests and Diseases Affecting Soft Fruit Plants
PESTS AND DISEASES
I’ve already mentioned the importance of viruses (and the even greater importance of avoiding them) in soft fruit growing, but as a group, these plants are prone to a considerable number of other problems, too. Some general principles about pests and diseases are appropriately dealt with here.
Diseases in plants are caused by viruses, by fungi or by bacteria, although bacterial problems are relatively insignificant in soft fruit. The importance of viruses stems from the fact that they are total parasites — they are unable to live and multiply away from the tissues of their hosts. And being totally dependent on the host means that they impose a major drain on its resources. Once a virus is present within the tissues, the plant gradually, almost imperceptibly, declines in vigour and fruit production. Although there can be obviously misshapen leaves with mottling or other patterns, virus contamination may not be revealed at all until the cropping is impaired.
Diseases caused by fungi are much less insidious and the two most important types on soft fruit are powdery mildews and botrytis grey mould, which may attack both the fruit, causing decay and the stems, causing a dieback. By and large, different types ofaffect different types of plant so the commonest species on gooseberries, American , won’t transfer to raspberries, although it will affect currants which are more closely related. The most important pests on soft fruit are aphids, different species of which attack all types of plant. Apart from their role in bringing virus contamination, their sap-sucking activities can seriously weaken plants too.
It’s an old adage in gardening that avoidance is better than control. And avoidance of pest and disease problems is best achieved by selecting varieties that are resistant to attack. Unfortunately, while resistance has been a major goal of plant breeders for many years, we still have relatively few soft fruit varieties for which it offers a complete answer although I have, of course, referred to these where appropriate. Among some of the greatest successes are aphid-resistant raspberries and mildew-resistant gooseberries.
Viruses are a special case, however, because initial avoidance is possible through the use of virus-free plants but even these won’t stay virus-free forever because virus-carrying aphids and, sometimes, eel-worms in the, will gradually introduce them into the plants with their feeding activities. Clearly, the only way that this can be minimized is by paying careful attention to aphid control, but it’s an inescapable fact that soft fruit plants must be replaced fairly regularly. Strawberries will be renewed frequently in any event, but among the longer term conventional soft fruit crops, only gooseberries are relatively free from virus problems and may, therefore, crop for twenty or more years.
Although the impact of diseases can be lessened by good husbandry — not overcrowding the plants and not overfeeding them with nitrogen for example — you will probably be faced from time to time with the need for a fungicide spray. Most soft fruit diseases, and certainly mildew and grey mould, can be controlled with the organically acceptable sulphur, or with a synthetic systemic fungicide such as benomyl, thiophanate-methyl or carbendazim. Care should be taken when using sulphur on gooseberries, however, as some varieties are damaged by it and are said to be sulphur-shy. ‘Careless’ and ‘Leveller’ are the commonest of those still widely grown on which sulphur shouldn’t be used. Red and white currants can also be damaged by sulphur under some circumstances, for example, such as during very hot weather.
Because of their twin-pronged nuisance value as virus vectors and sap suckers, aphids really must be controlled and although I would like to be able to say that encouraging beneficial, predator insects into the fruit garden by planting nearby will do the job, I can’t be totally enthusiastic. Nonetheless, such well known insect attractants as marigolds and will look extremely pretty when planted around the fruit garden and will certainly help with the cause.
When choosing an insecticide for aphid control, I try to limit myself to three very different ones. In the winter, when the plants are dormant, I always apply a tar oil spray to the bare stems. This is invaluable in killing the overwintering eggs and adults on the bark. But with the best of wills, this may not be the complete answer and a contact spray will also be needed at the first signs of attack in the summer. Then, I use either a soap-based spray, applying it in the evening, when harmless and beneficial insects are least likely to be affected, or a product containing the synthetic pesticide pirimicarb which is largely specific, in its action, to aphids.
Of course, other insect pests will occur from time to time but similar treatments may be used against them. The biggest difficulties arise with non-insect pests, mites especially. Big bud mites on blackcurrants are considered on p.68 but red spider mites are a real annoyance too. In common with other mites, they are little affected by insecticides and tend to thrive in hot, dry conditions. Ensuring that the plants are well mulched and not allowed to dry out will help to create an environment in which red spider mites are discouraged but if indivibecome consistently and severely affected, it might be sensible to destroy them and begin again with fresh stock of a different variety.
Sadly, though a number of biological control systems are fairly effective in, most notably for greenfly and whitefly control, their value in the outdoor fruit garden is almost nil. Interestingly however, two recent developments may offer some hope for the future. A biological control for vine weevil is now available and may be useful in established strawberry beds where the pest is troublesome, while a biological control for slugs is not far from final development. Both use species of predatory eel-worm as the controlling agents.