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lection, in the individual leaves bearing infections, “pseudohormones” trigger the leaves to fall as they do in autumn. Even if all the leaves should fall (which is rare), a single attack is usually not fatal to most broad-leaved trees. In a severe case, a fungicide should be used while leaves open next spring to make sure the tree is not weakened by repeated fungal attacks. Leafspot diseases are most abundant after a rainy spring.
Tree leaves may drop off after a rapid attack of a fatal disease like Dutch elm disease, or a sudden injury like girdling the trunk of cutting . through many roots for example, when installing a new underground line.
A slow attack (over several years), even by a fatal disease like root rot, or a slow-to-appear but fatal injury like root smothering from new fill or pavement, or excess soil compaction, usually leads to many dead branches in the tree top and smaller leaves, but not to a sudden leaf fall.
Chemical injury cannot be overlooked. The effects on a tree depend on the chemical, season, temperature, and species among other things. Some chemicals cause discoloration of leaves but not leaf drop. Some cause leaves to drop but do not hurt the rest of the tree, and new leaves come out.
Other chemicals can kill trees. Improper use of pesticides and weed control chemicals is one of the major contributors to chemical injury. Air pollution can also cause premature leaf drop in certain species.
Clusters of fallen leaves, attached to short twigs, result from a twig-girdling insect or squirrel activity. If an insect is responsible, a shallow depression encircles the broken twig end just inside the bark. If it’s squirrels, the break if diagonal and may have two or three small “steps.” Squirrel activity and most twig-girdling insects do no permanent damage to trees.
(Editor's note: Dr. Diane Relf is a horticulture specialist with Virginia Cooperative Extension and is a member of the faculty of Virginia Tech.
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