Pests and Diseases - Unwanted visitors

Your house is obviously man-made and except in rare instances nature doesn't intrude. Your garden on the other hand is actually a piece of nature that you are trying to mould to your own desires. Mainly your desires are not natural (for the garden that is, I seek not to cast aspersions!) and nature will try to reclaim the garden for her own.

To this end she will throw all kinds of plants (that we regard as weeds) and animals (that we regard as pests) into the fray, add to that a whole host of bacteria, fungi and viruses (that we regard as diseases) and we sometimes have what can seem to be an uphill struggle.

All is not lost however as it is more beneficial to have modest level of pests and diseases in your garden than to have none at all as they will support a range of beneficial insects and other creatures that will keep them in check without any intervention from you.

Clear all of the pests out of the way with over zealous chemical spraying and you clear away the "cavalry" too, so when the next lot of pests arrive - and they surely will - they will enjoy their meals completely unmolested.

Some feel that chemical sprays are the one and only answer, but you will need to keep applying them again and again for the reasons described above. You may prefer to be completely organic or use a combination.

My own preference is to be as organic as possible, but to use a limited number of chemicals sparingly as what I think of as a "smart bomb" for particularly artificial situations such as when the greenfly or whitefly get too numerous in the greenhouse, or when my favourite Victoria plum tree gets far more than its fair share of black fly that begin to cause the leaves to curl and whole small twigs to lose their leaves and die.

Avoiding Problems in the First Place

    Choose healthy plants that like growing in your garden. Plants that struggle to cope with your particular weather, soil etc. conditions will always be more susceptible to pests and diseases.

   Place plants in their preferred conditions. An excellent way of causing death-by-aphid for instance is to put a sun-loving (read that as sun-requiring) plant in the shade. They just won't be able to grow strongly enough or produce their own chemical retaliation (a sort of plant immune system) in enough quantities to fight back.

   Use pest and disease resistant varieties wherever possible. For instance species roses and floribundas are have much better resistance than hybrid teas. This is particularly important for fruit and vegetables.

   Make sure all plants get the best possible start in your garden and don't receive a check in growth when they are planted.

   Keep your plants growing strongly so they shrug off pest and disease attacks on their own. Clear them of weeds, remove dead growth, give them compost and feed for instance.

   Grow a wide variety of different plant types and mix them together. Pests and diseases love monocultures, also if anything does get a hold, it has less chance of spreading to nearby similar plants.

   Encourage beneficial pest predators to help control any potential problems.

   Know your plants. There used to be something mentioned every now and then about how plants responded if you talk to them. Now while this is certainly complete nonsense in itself, what does happen is that building familiarity with your plants means that you spot early on when one is starting to look a bit ill.

Slugs and snails

Signs - Irregular holes are eaten from leaves, slime trails often left around the plants on the soil and the leaves

Damage - Young plants and seedlings are particularly vulnerable and can be stripped bare or eaten down to the ground. Particularly fond of Hostas and Delphiniums.
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Aphids

Signs - Usually thought of as greenfly, but can be black, yellow, pink, greyish-white and brown. About 2mm long when fully grown, roundish.

Damage - Heavy infestations can reduce the vigour of a plant and leave it vulnerable to attack by other pests and diseases. Frequently spread virus diseases as they move from one infected plant to another. Virus disease usually characterised by irregular yellow patches on the leaves which may be wrinkled but otherwise alive.
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Caterpillars

Signs - Leaves stripped, or eaten from the outside edge inwards. May not always be easily visible as they can be well camouflaged. "Frass" may be seen, tiny black balls of excrement. More cunning and smaller types "mine" leaves between the upper and lower surfaces.

Damage - Young leaves can be eaten away and more mature plants are left with unsightly holes.
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Japanese beetle larvae

Signs - Not specific to chafer beetle larvae and can seem in the early stages like drought or stress. Patches of affected lawn wilt and turn yellow or brown. On investigation, the grass can be lifted easily from the soil not held down by roots to any degree. 

Damage - dead patches of lawn, may be made worse by animals such as birds foxes or badgers ripping up the grass to get at the larvae. Damage usually occurs during the autumn and is more common on light sandy soils where grass growth is already poor.
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Leatherjackets

Signs - affected plants may turn yellow, wither and die. Small plants and seedlings may be severed at ground level. The grey - brown leatherjackets are up to 3.5cm (1 1/2inches) long, smooth, tubular and legless, rarely seen above ground, usually found just below the surface particularly in grass where a small population that causes no visible damage is more common than not.

Damage - feed on roots particularly affecting young annuals, bulbs and vegetables, predominantly affects land that has been recently brought into cultivation recently and usually subsides after a couple of years. On lawns, more damage is usually done by birds and small mammals digging the leatherjackets up than by the leatherjackets themselves.
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Mealybugs

Signs - small fluffy white lumps about 5mm (1/4") appear on plants, often in the axil - where the leaf meets the stem. Leaves turn yellow and may wilt and die.  Usually affect house and greenhouse plants, but also Pyracantha and fruit trees. Stick honeydew and black sooty moulds may accompany. Despite being insects, they don't look like insects, just a shapeless piece of cotton wool.

Damage - plants are rarely killed unless very heavily infested over a long period. Commonly weakened, a heavy infestation is very unsightly from the pests themselves and from the sticky honeydew that they secrete and possibly even black moulds that grow on the sticky honeydew. Root mealy bugs damage the roots.
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Red spider mite

Signs - Leaves become speckled, as though covered with hundreds or thousands of pale dots giving a rather ill-looking and "dusty" appearance. Heavily  infested plants may become covered with fine webs.

Damage - Sap feeders which weakens the plant. Usually a problem in house and greenhouse plants as they like a dry atmosphere, can be a problem outside in hot, dry summers.
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Vine weevils

Signs - Irregular notches taken from the margins of the leaves are made by the adult weevil. If a plant suddenly dies and appears to have no roots, this is the work of the larvae. The way you find out is by looking at a poorly looking plant, when you go to pick off a dead leaf, the whole lot lifts from the roots in your hand - it's a bit late by then though. Adults may be seen around plants, look for cream / white larvae in compost when re-potting plants.

Damage - Depends on the size of the root system and the number of grubs eating it. Growth will slow, the plant may then begin to wilt and finally die as it no longer has sufficient roots to sustain it. Unfortunately by the time it has been discovered it is usually to late to take steps for that plant, though others not so far gone can be protected.
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Whitefly

Signs - Most commonly found on houseplants and in greenhouses, when disturbing the leaves, the tiny white flies that hide under the leaves will fly up.

Damage - Sap suckers, will weaken the plant and make it susceptible to further attacks of pests and disease.
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Gray mould

Signs - Stems and flowers become covered with a velvety grayish, fungal growth (that looks particularly unpleasant.)

Damage - A very common fungus which produces spores that are always around in the air. They attack plants through areas of damage. At first, small pale brown or white patches appear as the flowers start to rot.
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Rust

Signs - Very descriptive these fungal diseases - looks like the pant is going rusty. Orange-brown pustules develop initially on the undersides of leaves. These may be unnoticed for some time, so by the time the pustules appear on the upper leaf surface and are noticed, the disease has taken hold.

Damage - Badly affected plants can be so weakened they are unable to withstand winter frosts.
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Blackspot

Signs - Purplish-black usually rounded blotches appear on leaves, which then start to yellow and fall early. Stems may be affected in severe cases.

Damage - Badly affected plants may die as they are so seriously weakened they cannot withstand the winter frosts. It is very unsightly.
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Powdery mildew

Signs - White powdery mould appears on the leaves, stems and buds. Young growth in particular is affected. The disease is encouraged by the plant being dry at the roots with damp stagnant air around the top.

Damage - Looks unsightly and causes leaves to drop early. If the plant is well established this is a relatively harmless disease. However, care should be taken with younger plants since these may be drastically weakened.
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