Ten Methods for Reducing Garden Pests and Diseases

Reducing Garden Pests and Diseases

The ten lines of defence against pests and diseases:

There are many different approaches for reducing pests and diseases. Few problems are solved with just one measure but by combinations of several. They are presented here in order of minimum intervention, with the passive methods first and the less natural – or more unecological – last.


1. Good husbandry

A good method is 95 per cent of successful organic gardening. So organic gardeners choose suitable plants, avoiding those that are inevitably prone to problems. It is pretty daft to try and grow watermelons in the UK (I know, I keep trying), rhododendrons if you live on chalk, or cauliflowers in hot, dry, sandy soil. Grow plants adapted to your conditions and they succeed. Keep plants healthy by making sure they are growing steadily and they will overcome minor problems. Never let your plants become checked by being pot bound, crowded, dried out, cooked, frozen or choked. Any check reduces yield and may lead to disease, so crops grown in season always do better than extra early or late sowings. It is also daft trying to have strawberries in winter – another of my follies.

Of all the checks to growth, water imbalance is probably the most common. Obviously desiccated plants will die, but if they even start to wilt they have already been severely checked. Waterlogging can be equally serious, especially in cold, low-light conditions. Plants in containers are as often sickened by over-watering in winter and spring as they are by drought in summer. Dry roots and damp or stagnant air inevitably lead to mildews and botrytis rots, especially for roses and climbers on walls.

Fertility imbalance is rarely a problem for organic gardeners, but conventional growers may overstimulate their plants into lush growth with excess nitrogen, making them more susceptible to pests and diseases. Air and light are also very important factors when preventing infection, so give each plant sufficient space. Never crowd plants, grow a few well rather than many poorly. Vigorous and woody plants need pruning or tying not only to allow air and light, but also to give predators access.


2. Hygiene

Regular inspection of all parts of the garden and prompt action to remove infected material will significantly reduce further pest and disease attacks. For example, the removal of any infested tips, controls aphid attacks on broad beans. Similarly, coral spot is a common disease of woody plants prompt removal prevents its spread. All diseased and infested material should be immediately burnt, well composted or deeply buried. Secateurs, saws and knives should be sterilised with alcohol between operations to prevent cross-infection. Care needs taking not to introduce any problems with bought-in plants, manures or dirty tools. Never bring in soil-grown brassica plants because of the danger of clubroot. A good idea is to keep new plants in isolation for a while before putting them with others. Do not become over fastidious though, a small level of pest infestation is necessary to maintain predator populations.


3. Resistant varieties

In many ways the very plants we grow have been selected because they are inherently trouble free, but few cultivars are immune to all pests and diseases. The more important crops are grown most frequently and thus have acquired the most pests and diseases to bother them. For these important crops much research has been done to find varieties resistant to the commonest ailments. Most success has been against diseases – for example, there are blackspot-resistant roses, scab-resistant apples and canker-resistant parsnips. Pests are harder to discourage, but there are lettuces which are resistant to root aphids and carrots that do not attract their rootfly. However, choosing resistant varieties may entail some loss of flavour or quality compared to other varieties. Red and oddly coloured varieties often suffer less pest attacks because they are naturally camouflaged.


4. Cunning cultivation methods

Rotation is the most important of these methods. Move the plant each season and pests and diseases over wintering in the soil emerge to find their target has gone. (The same applies to replant disease – never replace a dead plant with one of the same.) Rotation also changes and modifies the conditions in the soil, ensuring that few pests or disease spores survive until the crop returns.

Timing can be important. Although plants grown in season are the healthiest and survive most diseases, there can be advantage in early or late sowing so that the crop misses the worst pest attacks. For instance, early potatoes are out of the ground before blight becomes a problem most years. Overwintered broad beans usually are too tough too early for the black aphid to bother them, while early peas miss mildew and early carrots miss the rootfly which is most preponderant when the weed cow parsley is flowering.

Raising plants in pots under cover can help to get better early or late starts and thus miss attacks, but can also prevent them entirely by isolating the crop during its more vulnerable stage. Beetroot sown in the open are razed off by birds, but survive if planted out when bigger.

Ten Methods for Reducing Garden Pests and Diseases - aphids Accurate sowing and indoor propagation also avoid the need to thin, which can attract pests to the scent as with carrot root fly or onion fly. Most shrubby plants benefit from summer pruning which controls growth, so encouraging flowering and fruiting. This can simultaneously remove a burgeoning pest population, especially aphids, as these cluster on tips.

Raking heavy mulches aside in winter disturbs many hibernating pests, killing them either directly or through exposure to hungry birds. This is useful against gooseberry sawfly and raspberry beetle. A heavy mulch before growth starts in spring seals spores and infectious material underneath, where they cannot be splashed up on to the buds. It is recommended for most plants, particularly blackspot-prone roses.

Seaweed and occasionally herbal sprays, applied to supplement fertility, are also good for reducing pest and disease problems. They do, not act directly as pesticides, but aid the plants to make resilient, vigorous growth that throws off attacks — much the same as us taking vitamin supplements. The smell of the sprays may also help by confusing pests.


5. Encouraging predators and parasites

Building up self-regulating ecosystems means good provision for predators. You need plants for nectar, fruits and pollen, shelter on the large and small scale, sites for nests and hibernation quarters, water and sacrificial plants to maintain colonies of pests on which the predators and parasites can feed. Thus for effective predator and parasite encouragement it is essential to provide a continuity of companion plants throughout the seasons, and particularly so early in the year. Not all flowers are equally useful; the deep-throated ones are only accessed by moths and butterflies, and double flowers wrought by man for their beauty may have no nectar or pollen for insects.


6. Hide and seek companion plants

The great variety of plants favoured by good gardeners creates diverse ecosystems which prevents the build up of pests and diseases. In a mono crop not only is there a vast amount of susceptible material, but it is all in contact so any problems can easily spread. Once plants are mixed and inter-cropped, it is harder for an initial infection to occur and much harder for it to spread. Growing several varieties of a crop is also useful, because each will mature at a different time and each may be more or less susceptible to pests and diseases. This greatly reduces the risk of total loss and also spreads the workload at harvest.

Not only do we find that mixtures of plants are less attacked than monocultures, but we can also use companions, such as the aromatic herbs, deliberately to camouflage or disguise the scent of the crop. French marigolds are particularly effective at this, their presence prevents whitefly coming into a greenhouse and they also kill soil nematodes. Many gardeners believe some companion plants, such as nettles and alliums, can help prevent fungal and bacterial attacks to other plants. Chives and garlic are often grown under roses and fruit for this purpose.

Some plants act as sacrificials, grown to attract pests away from the main planting. They may be the same plant grown around the perimeter or a more attractive lure. For example, redcurrants will keep birds off the blackcurrants. In a similar manner surplus leaves or seedlings shredded when transplanting and spread around transplants will fob off the slugs. Trap plants work in a similar way. For example sweet tobaccos, especially Nicotiana sylvestris, have sticky stems and leaves and are very attractive to whitefly and thrips. Growing these amongst other plants concentrates the pests so they can be further stuck on with a spray of sugar solution and removed with the plant.


7. Barriers and traps

There are many simple mechanical methods to exclude pests and thin out their numbers. These cause little harm to the environment as many can be made from recycled materials. Nets are the best way of protecting crops from many pests. A complete cage is best and makes economic sense, but any pieces of net held in place with clothes pegs can be used to protect a branch or two. The netting bags fruit and nuts come in are good for individual fruits and bunches, nylon stocking legs do as well. Whole stockings or tights can be pulled over long branches of fruit, such as redcurrants or cherries. Fine mesh bags can exclude wasps as well as birds, but may encourage mould or botrytis. Fine netting, woven fleeces and punctured plastic sheets can all be used to keep pests off vegetables as well as fruits and are very effective at preventing carrot root fly attacks. (This fly is about the size of a housefly and has to lay its eggs next to the seedling.) A barrier gives 100 per cent control.

The same materials can also be used to protect cabbages from root flies and butterfly caterpillars and to keep birds off beetroot and saladings. Cabbage rootflies need to lay their eggs in the soil next to the stem so a barrier made of squares of old carpet, tarred roofing felt or cardboard fitting snugly around the stem will seal the soil underneath.

Carpet can be used to seal larger areas trapping insect pests underneath when they emerge from hibernation or their pupae. This can considerably reduce infestations of gooseberry sawfly, raspberry beetle and pear midge. Carpet laid on a wet lawn will bring up leather jackets and other soil pests to the surface overnight and can be swept up or left to the birds in the morning. Cloth bands, carpet bands and corrugated cardboard bands tied round trunks and stems simulate shredding bark and attract many insects. The beneficial ladybirds can be retained on inspection and the pests evicted.

Ten Methods for Reducing Garden Pests and Diseases - barriers and traps Many creatures, especially earwigs, are attracted inside hollow tubes and can be blown out into a bucket. Earwigs are especially attracted to straw filled flowerpots on sticks. Sticky bands stop pests climbing up trunks, while few predators need to. They are especially effective against the female winter moth, which cannot fly, earwigs and ants. Ants ‘farm’ aphids by moving them to tender shoots and milking them for honeydew. A sticky band reduces the aphid populations as the ants cannot tend and protect them.

Sticky bands can be applied to the bark of old trees, but are better on top of a foil strip on young bark as it may soak in. Tie-on sticky bands on paper are even more expensive than the paint-on mixture which can be made to go further on rough surfaces if applied on top of cheaper pruning compound.

Sticky boards and flypapers are especially good in the greenhouse where they trap many pests, especially whitefly and thrips. More pests are lured to them if they are bright yellow. Sticky traps are even more effective if they are given a pheromone scent. When hung in fruit trees they are a very good way of reducing codling moth and plum fruit moth attacks.

Mouse traps are too well known to need describing. They are needed near peas and beans when sown or stored, and near crocuses, which mice love. Mole traps are similar and there are humane traps for both, so you can release them elsewhere.

Lures can be made for many pests. Tins or yoghurt cartons buried in the ground with bits of potato or carrot will attract mostly millipedes and wood lice. Slugs and snails will come to rotting fruit, and wireworms to bran or germinating grain. Dead-fall traps are just the same without the lure. Isolation trenches of about a spade’s depth all around vegetable beds present mice, slugs and other small creatures with a barrier, and so exclude them more than the conventional short drop.

Bird scarers are psychological barriers. They all work for a short time, but garden birds rapidly learn not to fear them. Use several and change them daily. Scarecrows, glitter bangs (coffee packaging bags are good), flashing or humming tape (video or cassette works, but remove it before the coating degrades). Black cotton gives them a fright when they touch it. Use an artificial spider’s web to give them arachnaphobia.

Hosepipe snakes, paper hawks and fur-hat cats all disturb them for a day, but they will be back.


8. Direct action

Ten Methods for Reducing Garden Pests and Diseases - slugs Hand picking is effective against many pests — gooseberry sawfly or cabbage caterpillars, for example. Many minor infestations can be terminated with finger and thumb before they become a plague. Slugs and snails come out on warm, wet evenings and tend to return to the same site so can be spotted with a torch and picked off — scissors are nastier, but effective. Battery-operated vacuum-cleaners are excellent for rounding up flying insects like whitefly or flea beetles which jump when disturbed. Using a water hose to produce a strong jet can knock aphids and other pests off plants especially rosebuds; some may return, but many will not.

Combining this with sticky bands works well.


9. Bought-in predators and parasites

These have been used commercially and now many are available to gardeners through the post. They are most suitable for greenhouse pests which are difficult to control because of the absence of natural predators, and because they can’t escape. Once the predator has been introduced, almost all pesticide use has to stop. They are most effective if introduced early in the season, but not before the pest has appeared or they will starve. If pest populations get large before the predators arrive, thin out the numbers with traps, trap plants and safe sprays before introducing them. Full instructions come in the packets, which may be just opened or emptied on to the plants. Often a large packet is enough to be shared amongst friends and much cheaper than buying a small one each.

Aphididoletes aphidmyza is a parasitic midge for controlling aphids. Cryptolaemus montrouzieri is a ladybird, its white shaggy larvae rapidly control mealy bugs. Cabbage white and other similar caterpillars can be safely killed with the naturally occurring disease Bacillus thuriengiensis: this is sprayed on to the crop and as the caterpillars eat they pick it up and very soon die. Trichoderma virides is a predatory fungus used to prevent other fungi attacking pruning wounds. It is applied before or instead of a sealing compound. It can also be used to cure silver leaf disease in plums and Dutch elm disease if these have not progressed too far. Pellets are inserted into holes drilled in the trunk and the predatory fungus permeates the tree. Trichoderma apparently prevents posts rotting and has been used in watering systems to prevent wilt in seedlings, but unfortunately is not available to amateurs in the UK.


10. Organic pesticides

Organic gardeners prefer not to use poisonous substances unless they are needed to save a valuable crop. However, they are there as last resort and can be used to kill pests, but you must take great care with them and not disrupt ecosystems that have slowly built up. Take special care not to harm bees and use poisons only after the bees have retired. Follow the instructions on the packaging as to their uses, application rates and precautions. Keep them in a safe, secure place.

Soft soap is just that. Traditionally used to kill aphids, spider mite, white fly and other pests, it has been reformulated to be more effective. Extremely safe to use and made from natural products it is the preferable pesticide, but cannot deal with the larger insect pests. Quassia solution is made from a tree bark and kills aphids, but few beneficial insects. I believe it is no longer available in the UK on its own. Pyrethrum is also no longer available to UK gardeners in pure form, as required by organic purists, though both it and quassia are legal in the US; in the UK pyrethrum is commonly supplied with a synthetic synergist. However, if it becomes available again, it is a useful powder or liquid for killing many insect pests including small caterpillars. It also kills beneficial insects and fish, but is very safe for mammals. It is made from the flower heads of a chrysanthemum and breaks down in half a day once exposed to air and light. Derris (rotenone) liquid or powder is extracted from various tropical plants and kills most insects and caterpillars, friends and foes, indiscriminately. It is particularly effective against mites, but is also lethal to fish, pigs and tortoises. It breaks down in sunlight and is slower to act than pyrethrum. Ryania, sabadilla, nicotine and false hellebore are similar plant-based insect killers but are unavailable to UK gardeners.

Bordeaux mixture is a fungicidal suspension of copper sulphate and slaked lime. Although a chemical, it is allowed under organic standards because it is not very harmful to us or to soil life. It is effective against potato blight, peach leaf curl, raspberry cane spot and many other fungal diseases. It is a preventative not a cure and must be applied thoroughly and in good time.

Sulphur is the pure element and is allowed under organic standards as a control for powdery mildews on fruit, flowers and vegetables, and for preventing rots in overwintering bulbs and tubers. Take care with fruit trees and bushes as a few varieties are allergic to sulphur, so read the label carefully.

Sodium bicarbonate solution was once used as a fungicide and was particularly useful against gooseberry mildew, but is not allowed under UK pesticide legislation.

07. January 2011 by admin
Categories: Featured, Pest and Disease Control | Tags: | Comments Off on Ten Methods for Reducing Garden Pests and Diseases

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