Plant Pest and Disease Prevention
Pest and Disease Prevention
Preventingfrom attacking our plants is clearly better than having to join them in battle once they are established. By that time they will have built up their strength and caused damage to a greater or lesser extent. They will be far harder to get rid of and, if chemicals are to be used, a greater quantity will be needed to overcome them.
Prevention should therefore be the watchword, and top of the list of priorities is that all plants must be grown as well as is humanly possible. A strong plant is much better able to withstand, and even in some cases repulse, attacks from its enemies.
Keeping plants well watered and fed is an obvious way of helping them to grow better, but it is not generally appreciated that besides the normal suffering that a lack of water inflicts, wilting plants give off a stronger scent than turgid plants (those fully charged with water). This is especially important with a vegetable like the carrot, because its main pest, the carrot fly, is attracted largely by smell. If wilting is allowed to take place during and straight after theare thinned, pests are much more likely.
Always, therefore,thoroughly after thinning or moving so that they recover as quickly as possible. Not only will this mean that they receive a minimal check but also they will be less susceptible to certain scent-attracted pests.
Likewise, all root crops should be kept as evenly moist as possible throughout their growing season. Sudden soakings after long periods of drought encourages splitting. This will happen with tomatoes, also gooseberries and currants, and besides the actual splitting, the wounds will be the entry point and breeding grounds for fungus diseases such as parsnip canker.
Along with growing plants under conditions in which they will thrive, we must also do everything we can to avoid those conditions that encourage the presence, development and spread of pests and diseases.
Although this is usually more applicable to a greenhouse than to the open garden, in that adequate ventilation is the best disease preventer, it can also apply outside. Never, for example, allow seedlings to become overcrowded – it leads to them becoming drawn and weak and can frequently create a micro-climate that allows pests and diseases to flourish. Overcrowding can easily be avoided by Sowing thinly and by transplanting or thinning out the seedlings as soon as is reasonably possible.
Not only will overcrowded seedlings and young plants create an environment favouring the nasties but, by being close together and touching, trouble, when it strikes, will spread like wild fire. Fungus diseases, such as Botrytis (Grey Mould), thrive in the close and cool atmosphere created by overcrowding. You only have to think of strawberries in a wet summer to appreciate that.
When hoeing amongst plants, be careful to avoid nicking them with the hoe. This not only scars root crops likeand beet, it also provides a perfect point of entry for fungus diseases and, frequently, for slugs.
Garden hygiene in general is extremely important. If bits and pieces of dead vegetation are allowed to lie around the garden they will quickly attract disease and can often provide top quality homes for slugs and snails. In particular, badly diseased and dead plants must be removed as soon as possible. If they are left where they are,they simply act as a source of further infection. Plants infected with serious disease should then be consigned to the dustbin or, better still, burnt.
A rather more specialised form of preventing the spread of infection concerns virus diseases. Although this is more applicable to raspberries and the like, a number of vegetables, principally, are also notorious for catching viruses. They’re usually spread by infected seed but can also be carried from one plant to another by contaminated tools as well as by greenfly and other , passing on the organism by feeding on the plant’s sap. Therefore, never knowingly use the same tool for dealing with healthy plants after handling infected ones.
Tobacco Mosaic virus of tomatoes is a very common example of this mistake. Cutting off side shoots from an infected plant and then using the same knife straight afterwards on a healthy plant is asking for trouble. Pruning tools, such as saws and secateurs, can also carry viruses from infected to previously healthy stock.
Something often ignored by gardeners is the fact that most pests and diseases attack more than one crop. Putting it a bit more technically, they have alternative hosts. Clearly, there is very little to be gained in protecting a crop from this or that pest or disease if another plant is the source of infection.
These other hosts are normally other plants in the vicinity of the crop but they could equally well be over the fence in the next door garden. This is a distinct possibility that should not be ignored.
Normally, however, it is just a question of, for example, club root disease of brassicas also being perfectly happy on the roots of ornamentals, like wallflowers, orlike shepherd’s purse. Nor is it only diseases; certain pests are also guilty of maintaining more than one home. Most greenfly are just as happy eating weeds as they are lettuces, for instance. Never ignore or treat this aspect of pest and disease control lightly; it is just as important as any other.
With crop rotation, which is the practice of not growing the same crop, or even type of crop, on the same bit of land every year, can help as a means of reducing the number of unwelcome invaders. In the present context it is a way of preventing a build up of specific diseases. These are not diseases like the mildews that attack the tops of plants but are essentially soil-borne ones like white rot ofand club root of brassicas, which go for the underground parts.
If susceptible crops are grown on the same land the year after an attack, there are no prizes for guessing that the disease will turn up again, and worse. Nor is it the least bit of good practising crop rotation if alternative hosts are allowed to survive to carry over an infection to the next year.
There is also a less obvious side to rotation. Some of the soil-borne diseases have only a limited life in the soil in the absence of a suitable host. Unfortunately, though, club root is not one of them.
It used to be thought that the number of soil pests was also reduced by crop rotation. Whilst this might happen to a small extent with largely immobile creatures like wire-worm, the vast majority are active enough to move on and find another home when the need arises. In many cases this will be the same kind of vegetable which is now growing in a different part of the plot.
The last method of what we call ‘cultural’ pest and disease control concerns the varieties of vegetables that we grow. Despite what cynics might think and say, modern plant breeders are putting all their resources into the creation of varieties that are either resistant or immune to attack from as many pests and diseases as is humanly possible. Clearly other considerations have to be borne in mind as well but that feature is very high on the list of priorities. This is achieved to a large extent by delving back into the mists of time and finding any wild species of plants that exhibit a tendency to throw off attacks by nasties.
One of the most exciting discoveries some years ago was in Central Europe where a wild brassica was found to be resistant to club root. This has given rise to a number of new and resistant cultivated brassicas, the first of which was the‘Trixie’. Undoubtedly there will be more and better varieties.
Most gardeners also know about disease-resistant varieties of tomato. These have all but done away with many of the more serious soil borne diseases as well as disorders like greenback, blotchy ripening, and bronzing. Another example is parsnip canker and the parsnip varieties ‘Avon-resister’ and ‘Gladiator’ both show good resistance to it. Fruit crops are also coming into this picture a lot more now.
Wherever possible, therefore, always use varieties that are resistant or even immune to the worst pests and diseases. There are extremely few varieties of vegetable which are actually immune to a disease, but some vegetables are definitely more resistant.
At the present time, no matter how many precautions you take or how seriously you take them, it is inevitable that every garden will get its fair share of pests and diseases. The problem then arises as to what should be done to combat them.
Even without using chemicals, there are several things that we can do but, above all, we must do them quickly. The top priority, therefore, is to keep a sharp eye open for any trouble and to act the moment you see it. The idea that you’ll wait until you can be bothered or can find the time is useless; every hour that action is put off will allow whatever it is to get worse. In the case of a fungus disease, this means it spreading further on the original victim and probably getting on to other plants. With pests, a delay in action will mean this as well but it is also possible that breeding or egg laying will take place.
Always remember that the more deep seated a problem becomes, the harder it is to control. The simple and obvious thing to do when a disease is seen is to pick off and burn the infected leaf, flower, fruit or whatever it is. This will prevent it spreading.
If the disease is really well established and has infected the stem of the plant, the only sensible course is to pull the thing up and destroy it. It may seem unnecessarily harsh but, if left, it is simply going to make matters worse by spreading to others, without any hope of saving the original plant.