The symptoms and residue pictured look like powdery mildew, a very common fungal infection on dogwoods and lots of other plant species. When minor, the disease can be ignored, but occasionally the weather supports outbreak levels that might hamper bloom or weaken trees if they persist throughout the season or for several years in a row. If needed, a fungicide labeled for use on trees that controls powdery mildew could be used, but it won't necessarily cure existing infections, just prevent new ones on uninfected foliage. Be aware that fungicide use might put pollinators or other insects at risk, and they do not always work effectively, which is why we seldom recommend them. However, for plants struggling to tolerate a heavy infection, they can be useful. If used, follow all product label directions, and avoid spraying while temperatures are above 85 degrees to minimize the risk of foliage damage from the chemical itself.

As far as watering goes, regular monitoring is a good approach, as dogwoods do not have a high drought tolerance, and much of Maryland is currently in drought or near-drought conditions, which is stressing many plants. (We had drought for much of last year too.) Make sure the soil is checked before watering, in order to avoid accidentally over-watering by irrigating when it's not needed. (Feel the soil about six inches deep. If somewhat dry to the touch at that depth, watering is probably needed. If damp, watering can probably wait.) Even if you decide not to treat the mildew, then at least minimizing any drought stress for the tree (especially if it's growing in lots of direct sun instead of semi-shade) will help it remain vigorous enough to tolerate the mildew and hopefully flower normally next spring. (Flower buds on spring bloomers like dogwood are developed in summer/autumn, and if drought stress is too great during that time, a plant will either not form them or they will die while they sit dormant waiting for spring.)

Miri