Red dogwoods (Cornus florida 'Rubra') are flowering trees that reach about 25 feet in height and spread, growing best in partially-shaded spots with moist, acidic and well-draining soils. Blooming in spring with bright-red flowers, the red dogwood is a popular landscape tree for its reddish-purple fall foliage as well. Red dogwood trees are susceptible to several problems, but most are usually rare in a home landscape setting.
Caused by the fungus Discula destructiva, dogwood anthracnose is a disease that has devastated dogwood tree populations across large regions of the United States. Dogwood anthracnose causes spots on the red dogwood tree's leaves that begin as small spots with purple borders and grow into larger, tan-colored blotches. The infected leaves remain on the red dogwood tree through the winter and until spring. When the fungal disease spreads to the twigs, branches and trunk of the red dogwood, it causes brown elliptical cankers that can grow large enough to girdle the branches or trunk and even kill the tree. Prune away and discard all diseased growth and fallen leaves. Treat the red dogwood with an appropriate fungicide, such as chlorothalonil, propiconazole or mancozeb and thiophanate methyl. Spray three to four applications of the fungicide in spring, spaced about 10 to 14 days apart.
The dogwood borer tunnels beneath the bark of the red dogwood tree and into the sapwood, eating the healthy tree tissue and leaving behind reddish-tinted sawdust mixed with excrement. The wasp-like adults are slender, bluish-black with yellow stripes on their abdomens and have clear wings that are up to 1-inch long. The adults live for only about one week, laying tiny light-brown, oval eggs in the dogwood tree's trunk during June or July. The dogwood borer larvae are cream-colored with reddish heads and are about ½-inch long at maturity. Red dogwoods with burr knots or other wounds in their trunks are especially susceptible to dogwood borer infestations. Apply an appropriate insecticide to the red dogwood tree, such as a pesticide that's injected into the borer holes.
Caused by the fungus Microsphaera pennicillata, powdery mildew infects the young shoots, leaves and buds of the red dogwood tree, covering these parts with white, powdery fungal spores. Powdery mildew is most common during moist times in spring, summer and early autumn, often causing the leaves to curl or deform. Treat powdery mildew by spraying the red dogwood with an appropriate fungicide, such as one containing propiconazole, azoxystrobin or myclobutanil. Spray the red dogwood's foliage with the fungicide in early spring as the flower buds are starting to open, again when the leaves first emerge, again in late July and a fourth time right before the leaves drop in the fall.
Spot anthracnose is a disease caused by the fungus Elsinoe cornii, which creates tiny circular or elongated reddish-purple spots on the flower petals and sometimes on the leaves and young shoots. The spots emerge in early spring and can merge to cover the flower bracts. The spots on the leaves are typically dark-purple with the centers turning pale-yellow and dropping out of the leaf. Septoria leaf spot (Septoria cornicola) also causes purplish spots on the leaves, but the spots are larger, angular and bordered with veins. Although the centers of the spots will turn grayish in color, they rarely drop out of the leaves. Control spot anthracnose and septoria leaf spot by spraying the red dogwood with a fungicide containing mancozeb. Perform the same spray schedule as the one recommended for treating powdery mildew.