Dogwoods can be attractive additions to home landscapes, but certain insects and diseases — spittlebugs, Septoria leaf spot or golden canker — can cause problems for these shrubs.
Here are some questions about dogwoods with answers from the horticulturists at Iowa State University, Ames. To have additional questions answered, contact Hortline at firstname.lastname@example.org or 515-294-3108.
Q: There are small masses of a foam-like material in my red twig dogwoods. What are they?
A: These masses probably were created by the dogwood spittlebug nymph that leaves a frothy, wet mass of “spittle” around itself as it feeds on sap from its host plant. The froth provides protection from natural enemies and desiccation.
Spittlebugs cause little harm to plants, so control is usually not necessary. If their presence is unacceptable, they can be washed off with a forceful stream of water from a garden hose.
Q: The leaves on my red twig dogwoods are spotted and beginning to fall off. Why?
A: This problem is probably due to Septoria leaf spot, a common fungal disease of dogwoods in the Midwest. The disease typically appears as irregularly shaped grayish spots with dark purple or reddish borders. Heavily infected leaves drop prematurely. Wet spring weather favors Septoria leaf spot development.
Fortunately, Septoria leaf spot does not seriously harm dogwoods. The damage is mainly aesthetic. The spotted foliage and partially defoliated shrubs are not very attractive.
The severity of Septoria leaf spot in future years can be reduced by raking up and disposing of infected leaves on the ground. It is possible to control Septoria leaf spot with two or three fungicide applications in spring. However, most home gardeners choose not to apply fungicides, as the damage to the shrubs is mainly aesthetic.
Q: Several branches on my pagoda dogwood have wilted and died. Why?
A: Golden canker — caused by the fungus Cryptodiaporthe corni — may be the culprit. Initial symptoms are wilting and death of leaves on infected branches. Infected branches turn bright yellow and die. Small, orange, raised spots develop on the surface of the yellow bark. These spots are the reproductive structures of the fungus.
Golden canker is best managed by pruning out infected branches. Make the pruning cut 4 to 6 inches below the yellow tissue. To prevent the accidental spread of golden canker, disinfect the pruning shears between cuts by dipping them in household beach or rubbing alcohol. Infected branches should be removed from the area and burned or buried. Fungicides do not control golden canker.
To discourage golden canker infections, place pagoda dogwoods in favorable locations in the landscape and provide good plant care. The pagoda dogwood performs best in moist, well-drained soils in partial shade.
Protected locations and eastern exposures are generally the best planting sites. Apply 2 to 3 inches of wood chips or shredded bark around plants and water during prolonged dry periods.