Leaf infections called "leafspots" are caused by a variety of fungi and some bacteria on many trees. An especially harmful version of this disease is called anthracnose which attacks many tree species including dogwood and sycamore. Positive identification usually requires laboratory diagnosis.
A leaf spot disease creates spots on foliage. The spots will vary in size and color depending on the plant, the organism involved and the stage of development. Spots are most often brownish, but may be tan or black. Concentric rings or a dark margin around the spot may be present. Over time the spots may combine to enlarge and form blotches. Spots or blotches that are angular and located around the veins are generally referred to as anthracnose. Leaves may yellow and drop prematurely.
Good tree care is sufficient for prevention. Avoid crowding plantings too closely. Thin out branches to open up the tree crown, but don't top or dehorn. Rake up leaves in fall and bury or compost them. Plant different kinds of trees in mixture. Fertilize trees in the spring with a complete fertilizer. Water trees deeply during dry spells. Do you know how to keep your tree healthy? Take This Test.
Use fungicides only when necessary. They MUST be applied prior to buildup of disease to effectively control leafspotting fungi. If severe leafspotting and/or defoliation occurs for several years, chemical control is probably necessary, but the leaf spot variety should be identified first. You can submit samples to your county agent for identification. Timing of protectant fungicide sprays is critical and varies for different fungi. Correct timing is a key to effective chemical control.