Cut leaf weeping Japanese Maple’s (Acer palmatum dissectum ‘Garnet’) leaves are drying up and falling off. The twigs also look like they are drying out. Whole branches have lost leaves and all parts of the shrub seem to be affected. The shrub is about 8 years old and is in part shade. It is getting a bit more shade each year as the Honey Locust trees shading it are giving more shade, so do not think it is in too much sun. Could this be verticillum wilt? How can I distinguish verticillum wilt from simple leaf scorch? The shrub is about 5 ft diameter.
I am in Toronto, part shade, clay soil but planted with peat moss and triple mix 8 years ago, gets watered when there has not been sufficient rain.
The photo shows what appears to be leaf scorch. Leaves tend to brown at the margins and, especially with dissected leaves of the Japanese maple, crisp and curl up. This can be a result of wind, heat, lack of water, or even too much water. Branch die back is sometimes a symptom.
Verticillium wilt is a fungus, and some of the following symptoms includes yellowing and shriveling of lower leaves, some parts or all of the plant suddenly wilts in the heat, yet may recover in cooler or wetter conditions. Unlike scorch, there may be brownish or black streaks under the bark.
The key differences are brown margins in leaf scorch, as opposed to yellowing prior to shriveling, and the fungus shows the blackish streaks under the bark, and usually starts from lower branches.
Judging from your description, I think scorch may be your problem. Even though the shrub is 8 years old and likely well established, perhaps exposure to wind is an issue, or as the honey locust is getting bigger, it may be taking more of the moisture in the soil. After 8 years, the organics are long gone, so extra compost would help with nutrients and opening up the clay soil. Even a 15cm layer of mulch around the shrub (not touching the bark) and more regular watering will help. Between now and mid-winter, pruning of the dead branches with sharp secateurs also is recommended.