Chemicals Used as Sprays to Prevent Plant Disease
Chemicals used to protect plants from disease parasites are called fungicides. They must, without harming the plant, kill the fungus parasite or stop its spores from germinating. For many years the principal substances used as fungicides have been copper and sulphur, and occasionally mercury. They are still important, but there are now many modern organic sprays. These are very good but they are more specific in their action and so do not have such a widespread application as the older chemicals. Hundreds of new synthetic organic compounds-are being examined and tested each year and there is still much to learn in this field of study. Even the substances known as ‘antibiotics’ in human therapeutic practice, penicillin, streptomycin, griseofulvin, etc. have been and are being tested against various plant diseases. In special cases they have proved successful.
The plant — including the under-surface of the leaves — is covered by spraying with a protective film of the fungicide. Attacked plants cannot always be cured but those not yet affected can be protected. Hence the importance of early detection.
With most fungicidal sprays it is best to include a ‘wetter’ or ‘spreader’. This has the effect of causing the fluid to spread and stick more closely to the leaves. Soft soap can be used in this capacity with some sprays, but it cannot be used if lime is present.
Many modern insecticides can be mixed with these fungicides so that both can be applied at the same time.
The ‘systemic’ type of fungicide enters the plant tissues and by its presence in the cells prevents, or at the very least minimizes, any attack.
Many different names are in use for the same or similar fungicides. Some of these names are proprietary, others are based on the name of the chemical composing the fungicide. This is sometimes confusing, but reference can, if desired, be made to the list of approved products which is issued by the Ministry of Agriculture, Fisheries and Food with the express purpose of helping the purchaser to select the best and most efficient products. This booklet is frequently revised and kept up to date, and it contains lists of approved fungicides, insecticides and weedkillers and the names of their manufacturers.
METHOD OF APPLICATION
There is a wide choice of machines-suitable for applying the sprays and dusts used as fungicides. The difficulty is to select the best type for the work required to be done. The nozzle should deliver the spray as a fine mist in the shape of a cone so that the fungicide falls on the plant as a fog and a film of fluid covers the foliage. Spray on a fine day but not in bright sunshine.
The importance of keeping sprayers clean cannot be over-emphasized. Wash them well after use, clean the nozzle or nozzles, and place the empty machine upside down to drain. Sprayers are not cheap, and if put away with fluid still in them they can be ruined during the winter by rust or corrosion. ‘Dusters’ forare available in plastic containers which, when squeezed, produce a fine cloud of dust.
Some plants are slightly resistant to disease, while others are completely immune. A plant may be immune to one disease but very susceptible to another. For example,that are immune to wart disease (Synchytrium endobioticum) can be easily attacked by blight (Phytophthora infeslans). Despite this, the production of resistant varieties marks an important step forward in checking disease and is probably the simplest method of doing so.
The work of producing resistant varieties is not easy, because they must stand comparison with susceptible varieties for quality of flower, or for flavour in fruits and vegetables. Breeding for quality and flavour, combined with a high degree of resistance, goes on continuously but there is always the chance that resistant plants will suddenly succumb to a new strain of the disease parasite. Then the search for new resistant plants starts all over again. This is why the study of crop protective measures such as spraying must always continue.