I have seen many apparently healthy garden destroyed almost overnight by pests and disease, I say apparently healthy because if the garden owner had known what to look for they would have seen that the garden was indeed unhealthy. There are a number of signals that gardeners, especially organic gardeners should watch out for, that are early indicators of insects and pest attacking the garden. Knowing these signs will give the organic gardener a much better chance of control over the insects and pests before they become established in the garden.
When most gardeners see a plant that has deformed leaves with a mottling effect they immediately go for a fungicide, when in fact the problem is caused by any number of plant viruses and the fungicide will have no effect on the virus. Now many of these viruses are spread by insects via their mouthpieces as they bite into the plant to feed. Such insects include but not limited to; leafhoppers, aphids and mites.
So to prevent the spread of these plant viruses it is necessary to prevent the spread of the insects, by using floating row covers (in the vegetable garden) or organic insecticides in the formal garden, or even the introduction of predatory insects that feed on the leafhoppers etc.
If you do find a leaf that is showing signs of deformity and or mottling have a good look round to see which insect has introduced the problem and remove them if possible along with the damaged leaf.
Whiteflies and mites are tiny insects that are quite hard to spots as individuals but can be clearly seen with the aid of a magnifying glass. Wherever possible rinse leaves regularly to remove mites. Regularly disturb the leaves of your plants and if there are any whiteflies they will fly off and trap them by using sticky traps.
Another area where mistaken problem diagnosis may take place is with wilted plants, most gardeners will assume that the plant is thirst and will provide additional water, when the problem is being caused by wireworms, eating into the roots. In the flower garden, these wireworms are easy enough to catch without having to resort to chemicals, as much to the dismay of vegetable gardeners the favourite food of wireworms is raw potato. So to trap them bury chunks of raw potato near the affected plant, removing and replacing every two days until either the plant shows signs of full recovery or the weather warms up sufficiently to prevent the wireworms from being active.