How to save an ailing coral bark maple

Gardening expert Conway Lum reveals how to keep your your coral bark maple healthy all year long.

Q: Could you tell me what is wrong with my maple? Toward the end of summer the leaves on some of the branches dried up and turned brown, there is also black areas on a few of the branches. The tree has been growing quite well; it has been in the ground for about four years. There is also an irrigation system, so it does get watered regularly. I am writing to you from Nanaimo, B.C. —Ingrid

“Bacterial blight” (Pseudomonas) is common on “coral bark maple” (Acer palmatum ‘Sango Kaku’). Symptoms could include dead buds that fail to open in spring; new shoots wilt and turn black in the spring and early summer; and one-year-old or current-year shoots turn black. Most maples are susceptible, especially Japanese maple cultivars ‘Sango Kaku’ and ‘Oshio Beni’. Infections occurs during cool, wet weather in spring and fall.

To control the problem, prune out diseased tissue during dry weather (January or February) or mid-summer to clean green tissue. Sterilize tools between cuts with 10 percent Lysol (1 part Lysol to 9 parts water). Provide optimum growing conditions (nutrition, drainage and soil pH), thereby preventing stress to the plant. Avoid fertilizing tree after July to prevent susceptible new growth in the fall. Copper spray could be used as a fall application; user 5 grams (level teaspoon) per litre of water prior to leaf drop and again after leaf drop. I only recommend copper spray as a last resource since it could damage the plant depending on weather conditions. Copper-resistant strains of Pseudomonas have been reported.