Dealing with Winter Dieback
This has been “The Year of Boxwood Winter Injury.” In my 30+ years with OSU Extension, I’ve never seen such widespread winter injury to boxwoods. It’s generally believed the damage is connected to the deep-diving temperatures experienced across most of Ohio during the week of Christmas. A horticultural version of coal in our Christmas stockings.
Winter injury symptoms have ranged from sections of the plants turning brown (= sectional dieback) to entire plants becoming brown. Of course, the latter is a more serious issue. If no new growth has sprouted from the base of browned-out boxwoods, the entire boxwood plant is most likely dead.
Boxwoods suffering from sectional dieback can be salvaged, but allowing the dead brown foliage to remain will interfere with the process. The key is to prune out the affected stems to allow light to penetrate deeper into the plant.
This stimulates buds to “break” and ultimately produce new stems and foliage as shown in the picture below. The recovery can progress surprisingly fast even if the pruning is done at this time of the year.
Unfortunately, there may be another problem afoot in the form of the fungal disease commonly called Volutella Blight/Canker. The disease is caused by two fungi, Pseudonectria buxi, and P. foliicola. P. buxi was originally described in 1815 and has been given 25 different scientific names in the intervening years including Volutella buxi, thus the common name for the disease.
P. foliicola was described more recently in 2015. So, its connection to the Volutella blight/canker disease was only recently sorted. You can read more about this in the Yang et al. 2021 publication listed under “Selected References” below.
The publication’s authors assert that we should focus on the stem cankering aspect of the disease rather than leaf blight because the cankers are the most impactful symptom. Indeed, even though large numbers of brown, blighted leaves may collectively affect plant health, stem cankers wreak havoc by destroying vascular flow.
The author of the 2022 Purdue publication, listed under “Selected References,” describes the fungi as “weak pathogens.” Others describe the fungi as “opportunistic.” Boxwoods that are in poor health or stem tissue that’s been weakened by such things as winter injury are fodder for infections.
Thus, the possibility that the winter-damaged stems are infected by the Volutella canker fungi adds another layer of complexity in dealing with winter injury. If the stems were only damaged by winter temperatures, the browning of the foliage should have slowed or stopped progressing as this growing season advanced.
However, if fungal cankering is afoot, additional browning may be occurring. This has been observed with the foliage on some boxwoods continuing to become chlorotic and eventually turning brown throughout June and into July.
Fortunately, Volutella canker can be halted through selective pruning, but recovery won’t occur if the pruning cuts are made above the stem cankers. Thus, the ends of the pruned stems should be examined. If the pruning cuts are made across the canker, as in the picture below, nothing will be gained. The infected stem tissue was not removed.
The Critically Important Confirmation
A “field” evaluation should never be your sole diagnostic guide when dealing with a suspected fungal plant infection; particularly those that may mimic other more serious disease issues. The pruned boxwood stems should be sent to a non-biased, university-based diagnostic clinic to confirm a suspected plant disease before developing a final management strategy.
If the dieback is associated with a more serious fungal disease, pruning may not halt disease progression. This is especially important with large collections of boxwoods in landscapes or nurseries.
Remember, that the Volutella cankering fungi are considered weak plant pathogens. Other more aggressive disease-causing fungi can infect boxwoods and rapidly spread to other nearby boxwood plants. Again, never develop a final management strategy before confirming your diagnosis. The nominal fee for learning whether you’re dealing with a plant killer or just a plant disfigurer is well worth it!
In Ohio, samples should be sent to our C. Wayne Ellett Plant and Pest Diagnostic Clinic (PPDC). Instructions on taking and sending samples are found on the PPDC website:
On a side note, no matter what kind of pruning you are doing, it’s a good practice to sanitize pruners, saws, etc., before moving to another plant. Certainly, before moving to a new landscape or another block in a nursery.
Box Tree Moth (BTM) Reminder
Box Tree Moth (BTM) (Cydalima perspectalis (family Crambidae) has only been confirmed in southwest Ohio. However, its discovery so far removed from all other known infestations in North America means everyone in our state should be aware of BTM and remain vigilant.
The most obvious and dramatic difference in symptoms between BTM and winter injury is that BTM consumes leaves creating see-through “stick shrubs.” Boxwoods that suffered winter injury are retaining their browned leaves; you can’t see through the shrubs.
You can learn more about BTM by clicking on these previous BYGL Alerts:
July 3, 2023: “What to Look for with Box Tree (Boxwood) Moth”
July 11, 2023: “Box Tree (Boxwood) Moth: Latest Observations”