Michelle Grabowski, University of Minnesota Extension Educator
The word golden brings about images of wealth, vitality and prosperity. Unfortunately when this word is used to describe the branches of a pagoda dogwood, the connotation is quite the opposite. Golden canker is one of the most common diseases of pagoda dogwood small trees/large shrubs in Minnesota and it can be disfiguring and even deadly.
Photo 1: Yellow infected branch areas contrast sharply with purplish red healthy bark. Michelle Grabowski
In the early spring sunlight the infected branches are bright yellow to orange compared to the dark almost purplish red healthy bark. The striking contrast is almost pretty. Sadly any branch that has completely turned yellow is already dead and will not leaf out this spring. Close examination of these yellow branches will reveal that the branch is covered in tiny blister-like orange spots. A sharp line marks the border between healthy and diseased branch tissue.
Photo 2: Close examination of infected branches reveals yellow to orange blister like spots. Michelle Grabowski
Golden canker is caused by the fungus Cryptodiaporthe corni. Not much is known about the biology of this fungal pathogen. We do know that C. corni only attacks pagoda dogwoods (Cornus alternifolia), especially trees that are stressed from heat and drought. We also know that the fungi advances when the tree is dormant, killing any branch that it completely encircles. If the main trunk of the tree is infected, the entire canopy can be killed. In North Dakota, spores of the fungi were observed in May, when the new pagoda dogwood shoots were just beginning to grow. When these spores infect and how they start new infections remains unknown.
Luckily there are several things a gardener can do to reduce the risk of encountering golden canker and several steps to help control the disease in infected trees. Any gardeners with pagoda dogwoods should thoroughly examine the trees for golden cankers before new growth begins this spring. Infected branches should be pruned out, making the cut several inches below the line where the golden infected bark meets with purplish red healthy bark. These branches should be removed from the area and burned, buried, or otherwise disposed of.
Photo 3: If golden canker girdles the main trunk of the tree, the canopy above will be killed. Michelle Grabowski
Since golden canker seems to be most severe in stressed trees, gardeners should try to provide optimal growing conditions for any pagoda dogwoods in their landscape. These small trees typically grow in the understory of larger trees. They prefer shaded to partially shaded areas, with cool moist soils. This should be taken into consideration when choosing a site for new pagoda dogwood trees. In addition, mulching the soil around the base of the pagoda dogwood with 2 inches of woodchips or organic mulch will help keep the tree roots cool and moist. Water should be provided to the trees if less than 1 inch of rain has fallen each week during the summer months. With proper placement and care pagoda dogwoods can thrive in Minnesota’s landscapes, serving as a unique feature plant that will hopefully be golden in every way but one.