Hello,
I wanted to confirm that what we are dealing with is boxwood blight, and if so if you might have recommendations as to if removing all the shrubs/plants and replanting with perhaps a less susceptible boxwood plant (green velvet?) would be advised for this spring, or a different plant (and if so, any recommendations)?
Height: 3 feet.
Where: Sun, south-facing hedge, approx. 20 plants, 30 feet of plants (more than half have similar issues to photo).
Location: North York
Looking at your ailing boxwood, I feel there are two possible causes of their struggle. Let’s have a look at both.
Firstly, as you mentioned it could very well be boxwood blight. This fungal disease usually starts at the base of the plant and works its way up, so the fact that the little leaf cover remaining is at the top leads me to think this is a good guess. According to Purdue University’s Plant and Pest Diagnostic Laboratory, the main sign that will indicate you are dealing with boxwood blight is the presence of thin black streaks or “cankers” on the green stems. You can see a good photo of this here on the second page: Boxwood Blight
When I zoom in on your photo, I do believe these cankers could be present, so again I think your diagnosis of boxwood blight is probably correct.
The other possibility that I cannot discount having seen it so many times in my work as a garden designer is the boxwood moth. From my experience, the key indicator for this (other than the presence of the caterpillars) is what is referred to as “window paning” of the leaves. The caterpillars eat the soft green interior of the leaf leaving the veins and a translucent layer so that when you lift it up to the light you can see through the leaf as if looking through a window. It is difficult for me to see from your photo if this is present on your boxwood. This alone does not usually cause the leaves to drop, so the fact that so many of the leaves have fallen off make me lean towards boxwood blight being the culprit.
Because you mentioned you have many plants in a similar state, I would be tempted to remove them outright and replace them with a native hardier alternative. I recently did this for a client of mine and we chose a plant that has a similar look known as inkberry. You can read a bit more about this option here https://plants.ces.ncsu.edu/plants/ilex-glabra/ I have observed that inkberry does best with fairly consistent watering. In the attached article it indicates it grows naturally next to bogs which makes sense as it is in the holly family which likes acidic conditions. Although not native, other evergreen options would be yews (which are very tolerant and take both sun and shade), euonymus or even lavender since you are south facing. The last option made me wonder if you would you consider doing away with the hedge altogether and planting some pollinator friendly perennials? Here is our gardening guide should you want to explore this option more https://www.torontomastergardeners.ca/gardeningguides/pollinator-garden-a-toronto-master-gardeners-guide/
I hope this gives you some information and ideas for a fresh start.