Q. Have you heard anything from folks with dogwood issues? We’ve had dogwoods in our yard for over 30 years. For some reason, several of ours seemed to be struggling. This past fall, the leaves were dying, but not dropping from the branches as normally happens. Does that make any sense? — C. Fincke, Virginia Beach
A. While I have not received any reports of dogwood (Cornus florida) issues, this question should resonate with quite a few readers. Unfortunately, this iconic harbinger of spring is in big trouble in many places. Many of the older dogwoods in my own neighborhood are in decline, dying back limb by limb.
Leaf senescence (death) and abscission (drop) is a natural and complex process that deciduous trees evolved to enable them to survive all seasons. It is a highly choreographed sequence that lets the tree capture remaining resources (metabolites and nutrients) from the leaves before they fall, and then seal off and protect its living tissues from the cold. Here is a simplified version of what happens.
After the tree has sequestered the resources, an abscission layer forms at the base of the leaf petiole (stem connecting leaf to branch). Cells within the abscission zone differentiate, forming separation and protective cells. (The phytohormones auxin and ethylene are those most directly associated with leaf retention and separation. The latter triggers the formation of the separation and protective cells within the abscission zone, as well as the enzymes that degrade cell wall materials.) Having completed their job, the leaves fall to the ground. (Here they play another role in the tree’s life, but this is another story, for another time.) This entire process is set into motion as trees perceive the environmental clues — lower temperatures and the shortening days of fall and winter.
The phenomenon you’ve noted is marcescence, the retention of senesced (dead) plant organs that would normally shed from the plant. It seems that leaf marcescence is more often observed in juvenile or newly planted trees. The potential for marcescence tends to diminish as the tree matures. Often it is seen on the lower parts of the tree, and sometimes scattered, unevenly on branches throughout. Some species are more prone to the issue, in both adolescent and mature plants, including beech, oak, hornbeam and witch hazel.
What causes this malady? While theories are numerous, the most widely accepted seems to be the delay, or interruption, of abscission layer development. While a number of stressors may be responsible for this, I’d guess that the extended drought, or early cold weather, might be the most plausible explanations for what you observed last fall.
Luckily, this is not necessarily something you need be concerned with. I asked if you were seeing any limb dieback, and you said you weren’t. Seeing it might have signaled a more sinister threat: anthracnose canker. This disease, likely imported, was first noticed in New York in the 1970s. Discula destructiva, the pathogen — a new fungal species — was identified and named in the early 1990s. In less than a decade, it had rapidly advanced throughout the Eastern and Southern United States. Today it is common throughout those areas, particularly in the cooler, higher elevations of the Appalachians.
Dogwood’s list of fungal adversaries does not end there. Others include Erysiphe pulchra (powdery mildew), Botryosphaeria sp. (leaf spot) and Elsinoe corni (anthracnose leaf spot). While fungicide treatments may offer some control, applying them may be impractical for the homeowner. Specialized spray equipment is needed to treat large trees, and fungicides can be expensive. Reaching the higher parts of trees by ladder is dangerous for nonprofessionals. So for homeowners, more realistic measures are pruning diseased branches and raking up and destroying fallen leaves.
Keeping your dogwoods healthy is key to helping them withstand bouts with these funguses. Increased sunlight, avoiding over-fertilization, using mulch to conserve moisture, and deep watering during drought can help minimize stressors that make them more vulnerable. If disease is found in your area, you might try a different dogwood variety. Kousa dogwood (Cornus kousa) and hybrids resulting from Cornus kousa X Cornus florida may offer some resistance.
A while back, I spoke with Tim Nuckols, of Nuckols Tree Care, about the issue. He has noted a definite and steady decline of dogwoods in our area over the past 30 years or so. He believes that rising temperatures and anthracnose bear major responsibility, worsened by improper cultural practices such as poor planting (too deep) and mechanical damage from lawn mowers and weed eaters.
Now, with the long, hot summer approaching, probably the best thing you can do is to provide supplemental water, as needed, to help minimize heat stress.
And one more thing (or two)
I hope gardens have finally started to catch up after the less than ideal growing conditions of the first two weeks of May. Daily average temperatures were almost 4 degrees lower than normal, and gardens were pummeled by winds from the lingering coastal low. Let me know how your gardens are looking out there.
Send questions to wkspen@gmail.com
