Growing Fruit and Vegetables: Pests and Diseases
A great deal of wasted effort can be avoided if you take the correct preventative measures against plant pests and diseases. If such problems are tackled when they are first noticed, it will save the extra time and trouble involved in getting rid of them once they have become firmly established. Prevention is always better than cure.
It is often possible to avoid the time, trouble, expense and hazards of using pesticides by following these common-sense rules. Firstly, you should ensure that you give your plants the best conditions so that they grow strong and healthy and thus have more chance of resisting attacks by pests and diseases.
Secondly, you should choose disease-resistant plants whenever possible – for example, there are tomato varieties that are resistant to cladosporium disease, and wilt-resistant asters – although this applies to relatively few plants only.
Thirdly, if you buy in any plants from nurseries or garden centres, make sure that they are guaranteed free from pests or diseases. It is pointless taking precautions if you then introduce trouble from outside.
Finally, it is essential to practise good garden hygiene. Prunings, for example, should always be burnt. Never leave them lying about under trees and bushes and do not put them on the compost heap. Diseased material is a common cause of infection in the garden.
Any garden rubbish, in fact, that is not suitable for composting should be burned as soon as possible. Burning kills disease spores and the resulting ash is a valuable source of potash for your plants.
The bottoms of hedges should be cleared out regularly and dead leaves swept up and composted as soon as possible after they have fallen. These are both common sources of plant and pest infestation. Patches of long grass and weeds, too, provide ideal breeding places for slugs and snails.
In recent years there has been a strong reaction against the use of many pesticides on environmental grounds, and it is very important to avoid the indiscriminate use of these chemical poisons and to confine their use as far as possible to those that break down after use to harmless substances and which do not harm wildlife.
Great care should be taken when any garden chemicals are used. The makers’ instructions should be strictly adhered to and careful heed paid to their warnings. Above all, keep them out of the reach of children in a locked cupboard and in their original containers – not in soft drinks bottles.
The safest pesticides are manufactured from natural ingredients. They are pyrethrum, obtained from the pyrethrum plant, derris (or rote-none), extracted from the roots of a tropical plant, and quassia, obtained from the wood of a South American tree. Quassia was formerly supplied in the form of chips for soaking in water to produce a liquid insecticide. Nowadays, it is obtainable in liquid form for making up into a spray. Pyrethrum is obtainable as a spray concentrate, while derris can be bought either as a dust in handy ‘puffer packs’ or as a spray concentrate. A mixture of pyrethrum and derris is also available, which is more effective at destroying pests than either pesticide alone.
Between them, these organic materials will control a large number of garden pests, including aphids (greenfly and blackfly), caterpillars, thrips, flea beetles and the grubs of the raspberry beetle. A small hand sprayer will be adequate for applying them in the average garden. They can be safely used on any garden or greenhouse plant and leave no long-lasting toxic residues to harm wildlife, pets or humans.
However, it is important to remember that derris will kill fish, so on no account spray anywhere near a pond or stream. You should avoid killing bees and other beneficial insects when using derris or pyrethrum by spraying after sunset when these creatures are no longer on the wing.
Another natural way of coping with pests, in which a good deal of interest has been shown in recent years, is by growing special plants in association with vegetable crops to repel the particular pests to which they are susceptible. The best-known example of this is the planting of French or African marigolds in the greenhouse to prevent infestations of whitefly, one of the most trouble- some greenhouse pests. I must admit that I have always been a trifle sceptical of such remedies, but I have given this method a trial for the past two seasons and, whatever the reason, there has been no sign of whitefly, whereas in previous years, my tomatoes and cucumbers have been plagued by these pests, which were impossible to control without constant recourse to chemical sprays.
Other similar plant associations include the growing of summer savory with broad beans to discourage the attacks of blackfly, to which they are usually so prone; chervil with radish as a protection against borers and weevils; and sage or thyme with carrots and cabbage. Both the cabbage moth and the cabbage white butterfly are also repelled.
If you have a serious attack of a pest, and the safer alternatives given above are not working, then as a last resort you could consider using one of the less harmful modern pesticides. Systemic insecticides and fungicides penetrate into the sap of plants and remain active for a week or more. They are extremely effective against sap-sucking insects, such as aphids (greenfly and blackfly), thrips and leafhoppers.
Systemic fertilizers can be mixed with foliar feeds to combine two spraying operations in one. Application is best made by some form of pressure spray but for the very small garden, a syringe with a fine nozzle should be adequate.
Benlate is a systemic fungicide that controls many disease conditions, including grey mould, tomato leaf mould and powdery mildew, as well as black spot on roses.
For potato and other blight diseases, the chemical Dithane 945, for many years used in agriculture, is very effective. It also controls other diseases such as rust, leaf spot, downy mildew, apple scab and peach leaf curl. However, potato blight should be easily controlled by spraying with Bordeaux mixture, a safer alternative to Dithane 945. Safer still, grow a blight-resistant variety such as ‘Majestic’ or ‘Pentland Crown’.
Particular care is needed when treating crops in the greenhouse, and an insecticide based on the chemical resmethrin can be safely used against green and whitefly. In fact the manufacturers claim that crops can be gathered and eaten on the same day they are sprayed.