Gardeners often plant dogwood trees or shrubs (Cornus spp.) for their showy spring flowers, vibrant fall foliage and bright berries. Commonly planted species include flowering dogwood trees (Cornus florida), North American natives that thrive in U.S. Department of Agriculture plant hardiness zones 5 through 9, and cornelian cherry dogwoods (Cornus mas), Eurasian trees that prefer USDA zones 4 through 8. Dogwoods are susceptible to powdery mildew, but several treatments exist that can help prevent and control this common fungal disease.
Powdery Mildew Disease
Dogwood powdery mildew fungi (Erysiphe pulchra, formerly Microsphaera pulchra) cause a fine, white to light gray powdery growth to coat the surfaces of leaves, buds, flowers and shoots. Severely infected foliage might look curled, distorted, discolored or dwarfed and often falls from the tree prematurely. Although the damage is primarily cosmetic, powdery mildew might also stunt plant growth or even kill young dogwood seedlings. Powdery mildew pathogens need high humidity levels to spread and infect new plant tissue, and the fungi typically becomes most active during periods of damp, cool nights followed by dry, warm days.
Taking care of the landscape area around your dogwood can help control the presence and spread of powdery mildew pathogens. Because the disease loves high humidity levels, it most commonly occurs in dense, heavily shaded areas with poor air circulation. Selectively pruning out crowded interiors helps increase airflow throughout the entire canopy. Removing other vegetation that crowds your tree or shrub also helps to increase air movement throughout the plant. Prune out and destroy all infected branches, twigs and leaves as soon as you spot them. Raking up and destroying fallen leaves and other plant debris near your dogwood in the autumn helps prevent the fungi from overwintering near your plant.
Severe powdery mildew infections sometimes require stronger control measures. Using a neem oil spray can effectively treat powdery mildew disease if you start spraying at the first signs of infection, suggests the University of Delaware Cooperative Extension. Following the manufacturer's directions for proper dosing and application procedures, mix about 2 tablespoons of neem oil product for every 1 gallon of water. Mix the solution thoroughly and spray the buds, twigs and upper and lower surfaces of leaves until the foliage glistens with wetness. Repeat neem oil applications every two to three weeks for optimal results.
Proper Dogwood Care
Although different dogwood species thrive in various lighting and soil conditions, flowering dogwoods and Cornelian cherry dogwoods both prefer partially shady to fully sunny locations. These trees also grow best in fertile, rich soils and require good drainage. Dogwoods generally need about 1 inch of water every week during the growing season. Irrigate only when the soil feels dry to the touch and allow the water to saturate the top 6 inches of soil. Placing a 4-inch layer of organic mulching material on the surrounding soil helps keep the root zone moist and cool during hot summer weather, but keep the mulch about 4 inches away from the trunk itself.