If there are serious diseases present in a garden, the sooner you know about them the better. It will help you plan your planting to avoid furthering the problem.
Inspect any wholly dead trees or shrubs and look for any obvious signs of disease. Death may have been just a case of old age, but be suspicious of anything dead that is still holding its brown leaves, of plants dying a branch at a time, of plants that die suddenly during the growing season, and of clusters of lifeless plants in one area of the garden.
These are symptoms of honey mushroom (species of Armillana), which comes in several forms. One can kill healthy plants outright; another will attack and kill already stressed plants; and a third, less dangerous form can leave plants almost unaffected. Simply seeing any of the symptoms of the disease is no cause for panic. Take it as it comes, but be careful and be hygienic.
Look out, too, for plants with bracket fungi either on the base or near old major wounds. These can mean the tree has a fatal infection of the trunk or roots, which could lead to collapse in a few years. The most common forms include Heterobasidion on conifers and Ganoderma, Fistulina, and Inonotus on broadleaved trees. Get a professional to inspect the tree before you plan the garden around it, or it falls over. If needed, has it made safe or have it removed.
Phytophthora (root rot) is another problem disease, killing woody plants suddenly, similar to honey fungus. It likes to run through coniferous hedges, especially on poorly drained soil. Look for dark, discolored roots and stems. Burn the plant and its roots and replace the soil, if it seems practical.
Think hard about badly cankered fruit trees or trees with severe silverleaf or fire blight. Unless you are anxious to keep them for a few more years as a screen or a shelter, it may be better to take them out at the start, while you are uprooting and moving other plants, too.
More minor diseases such as mildew and rusts need not be the cause for concern in an overgrown garden. Forget them for now. Once you have cut back overgrowth and let air and moisture back into the garden, you can consider how to deal with any remaining diseases.
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