Mildew may look horrible on your trees but it isn't likely to cause any serious damage. While it may seem obvious, keeping your tree clean and healthy are your best defenses against mildew infections. Beyond that, simple natural home remedies hold the key to effectively bringing the problem under control---or at least to a point that adheres to your level of tolerance.
Examine the leaves of your infected tree for mildew, particularly during the late summer and early fall. This is the time of year that mildew growth is most likely to take place due to warm days followed by cool nights. As the fungal infection progresses, leaf surfaces gradually take on a grayish or whitish cast. Late in the season, little black specks may appear to be scattered on the leaves, too. The tree can be expected to lose leaves, but you shouldn't worry unless a tree loses more than 25 percent of its leaves.
Treat your tree with safe and effective homemade fungicide at the first signs of mildew infection. Dissolve 2 tbsp. baking soda in a gallon of warm water. Stir in 1 tsp. horticultural oil or vegetable oil and blend thoroughly. Pour the solution into a repurposed plastic spray bottle and apply directly to the mildew on the tree. Coat all affected surfaces generously. Repeat weekly throughout the growing season thereafter as a maintenance measure. Reapply following rainfall.
Feed your tree appropriately during the growing season. Don't overfertilize, which produces succulent new growth that attracts mildew.
Water the tree only when necessary, and make sure that the area remains well-drained. Overwatering causes root rot, which will stress the mildewed specimen even further.
Prune and thin your tree correctly as necessary to ensure adequate air circulation.
Keep the area under and around your tree clean. Rake fallen leaves often. If the plant has an active mildew condition, pick up fallen leaves or raked debris immediately and destroy it. Don't add any of it to your compost heap.