I also explained that plants must have the right environment to thrive: suitable soil, the correct amount of food and water, and protection from the wind. They also require good air circulation around them, adequate light levels and a rich diversity of wildlife for a healthy ecosystem. If they are small or weak, your plants will readily succumb to pests and diseases.
Slug or aphid damage may pass unnoticed in the "wild", but because gardens are small, artificial places, every plant must look good. You therefore need to check regularly for any problems. Look carefully at the leaves. Are they distorted - swollen or curled? Have they been nibbled or stripped to the veins? Is the damage on the top or underneath? Can you see any stickiness, sooty moulds, white insect-like casts or deposits on the surface? Are the leaves discoloured or withered? Are there tiny larvae on or under the leaves? Do stems look misshapen, have distorted barks or are rotten at the base? Are they long and spindly? If the whole plant is dying, can you pull it out of the ground easily, find virtually no roots but several grubs?
This is a long list and there's only space to highlight some but by identifying them, you'll be halfway to solving the problem. Publications, such as Pests, Diseases And Disorders Of Garden Plants, published by Collins, and websites such as the Royal Horticultural Society can help.
Aphids are often the culprits. See if there are still any tiny insects on the damaged plant. They come in a range of colours, green being the most common. All species damage and may kill plants by sucking sap from leaves, stems or roots. This leads to curled, contorted or swollen leaves, with excess sugars - honeydew - on surfaces.
White body casts are often wrongly identified as whitefly. This pest needs constantly high greenhouse temperatures and is rarely found in domestic greenhouses in Scotland. As an added problem, aphids may also transfer viruses to plants such as strawberries and potatoes.
Like some aphids, vine weevil, cabbage rootfly and leatherjackets concentrate on a plant's roots, while aptly named cutworms specialise in stems. All may cause a plant to wilt and almost certainly die. You'll see the tiny maggots around the remnants of the roots. Once you've identified the small, creamy larvae of rootfly, or the white vine weevil caterpillar with its distinctive brown head, you can take appropriate action.
We all know the symptoms of slug or snail attack - the half-eaten lettuce or cabbage heart, the tunnel-riddled potato or the row of seedlings chomped off. But, if seedlings do emerge and quickly collapse or produce a stunted stem but no cotyledon leaves, the seedlings have a damping-off disease.
You can easily identify other fungal diseases, such as powdery mildew or downy mildew, by examining a plant's leaves. Powdery mildew needs dry, warm weather and attacks a wide range of plants that have been stressed through lack of water. The leaves, buds and stems are coated with what looks like a powdery, white or off-white substance. The leaves can be misshapen.
Downy mildew thrives during damp weather. At first sight, with its white coating on leaves, downy mildew looks similar to the powdery fungus. A simple test will show which is your troublemaker. Rub off the white substance. If you expose a yellow bit of leaf beneath the powdery coating, you're dealing with downy mildew.
A careful study of symptoms also helps identify the many other blights, cankers and pests.