Step 1: Check for Plant Disorders
Plants that look as if they are troubled by pests or have a disease, but are not visibly under attack, may be suffering from a “disorder.” Disorders happen when a plant lacks sufficient nutrients or water, or is growing under adverse conditions, such as excessive cold or warmth. They can also occur if plants are subjected to more wind or sun than they can handle, are growing in shade when they prefer sun, or have a poor root environment.
Disorders are very common, and if the underlying problems are not rectified quickly, they often lead to an outbreak of disease or attack by pests, which take hold of the weakened plant. Plant disorders are not diseases, although the symptoms can look similar. Adverse weather, such as drought or unusually low temperatures, can affect plant health, although mature, hardy types often tend to recover quickly when more favorable conditions return. Young plants, or those that have recently been planted, are more vulnerable; you will need to keep a close watch on them, and protect them when necessary. Increase their natural levels of resistance by making sure you are growing them in a suitable site and soil.
Step 2: Check for Lack of Water or Nutrients
Lack of nutrients is a less common problem than you’d think; most tended garden soil has been fed over many years, topping off nutrient levels. Nitrogen, however, may be lacking, since it is easily washed out of the soil. A deficiency will lead to pale leaves and sickly growth, and can be remedied using a nitrogen-rich fertilizer, such as dried chicken manure pellets or sulfate of ammonia, and watering well. Organic matter, such as compost or manure, dug into the soil before planting or used as a mulch, will also boost nitrogen levels.
Lack of water causes poor growth and flowering, as well as powdery mildew attacks. Without an adequate water supply, plants cannot take up enough nutrients from the soil. During prolonged dry spells, plants may turn brown and lose their foliage. Well-established woody plants have enough roots to ride out these dry spells and recover when rain returns. Lawns turn brown, but the living buds at the base of the grass remain alive, so they regrow when rehydrated. But plants with limited root systems, such as annuals and herbaceous perennials, are more vulnerable; cut back wilted stems, remove dead leaves, then water the plant well regularly. You should soon see new buds and stems emerge. Newly planted plants, especially trees, shrubs and climbers, are particularly vulnerable to drought and a lack of nutrients. To prevent problems, water them regularly, and keep the area around the roots free of weeds.
Step 3: Check for Lack of Light
Growing plants in shade when they prefer a sunny site will lead to pale or yellow leaves and stems, and poor growth. Plants and seedlings may grow tall and spindly, and lack vigor. The only cure is to move the plants to a location where they will receive the right amount of light.
Step 4: Check for Root Problems
When soils are waterlogged, dry, or airless, roots are more likely to contract fungal diseases. The first signs of root problems may be in the foliage; when roots are too dry, the leaves farthest from the roots turn brown, especially at the edges. Drought often causes the bark at the base of the stem to rot and die, too, although this is also a symptom of the root disease honey fungus. If the base of the stem is sound, check for dead roots that appear red or brown and brittle. If the root system is dead or damaged, the plant’s chances of survival are slim, but if roots are healthy, watering may save the plant.
All gardens, however carefully nurtured, are likely to harbor some pests and diseases. If you see these bugs around your plants, there's a good chance they are damaging them.