Answer: A leaf and stem disease called anthracnose has weakened and/or killed many dogwoods in your region. The symptoms include small, purple-rimmed spots or large tan blotches on the leaf, with the entire leaf eventually turning brown. Twigs may die back several inches, or all the way to the main stem. Trees are often killed 2 or 3 years after symptoms first appear. The fungus thrives in cool, moist weather, and since dogwoods prefer to grow in the dappled shade of larger trees, this can create cooler, more humid conditions than in full sun, increasing the disease problems.
This might be your trouble--or it could be related to water or other factors. Dogwoods prefer dappled shade and moist, rich, cool soil--the trees are native to open woodland areas. When watering, be sure to water deeply, soaking the soil to a depth of at least a foot. Then let the area dry out a little before watering again. Dogwood won't endure saturated soils, or dry soils either. In fact, anything that causes the plant stress will increase the likelihood that it will succumb to disease.
There are a few varieties that appear to be resistant to the anthracnose disease, so if you think this has been the problem, you might try one of these: 'Cherokee Princess', and 'First Lady' and 'Plena/Pluribracteata' appear somewhat resistant.
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