Certain fungi that cause root or lower trunk decays are often associated with the decline of many tree species. When roots or trunks are injured, they provide access to decay fungi. When trees are under stress, physiological changes occur in the roots, allowing some fungi to infect and kill them. In the autumn, clusters of mushrooms may form at the base of trees, indicating that root and trunk infections are present. Symptoms of decline may develop quickly or they may not be noticeable for years. Early symptoms include premature fall coloration, late spring leaf development, decreased twig and stem growth, leaf scorch, death of tissues between the leaf veins, and premature leaf drop. Later symptoms include dieback of larger limbs and branches, sprouting from the trunk of the tree, heavy seed crops, and foliage noticeably smaller, lighter green, and sometimes produced in "tufts" or "clumps" on sprout-origin tissues. The foliage over the entire tree may also look thinner in decline conditions. Decline symptoms may also occur due to drought, flooding, construction activities that cut or injure roots, bacterial leaf scorch disease, and excess de-icing salts.

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