How do I treat maple tar spot disease?

Maple tar spot is more of a cosmetic issue than a serious health threat to your tree — and in New Brunswick's climate, management is straightforward once you understand the disease cycle.

Tar spot is caused by fungi in the Rhytisma genus, and it's extremely common across New Brunswick. You'll recognize it by the raised, shiny black spots (1–2 cm across) that appear on maple leaves in mid-to-late summer. Silver and Norway maples are the most commonly affected here, though native sugar maples can get it too.

Understanding the Disease Cycle

The fungus overwinters in infected fallen leaves on the ground. In spring, it releases spores that infect new leaves — but symptoms don't appear until July or August, so many homeowners don't notice the problem until late summer. The good news is that tar spot rarely causes significant harm to an otherwise healthy, established tree. You may see some early leaf drop in August or September, but the tree's long-term health is generally not at risk.

The Most Effective Treatment: Leaf Removal

The single best thing you can do is remove and dispose of infected leaves in fall — do not compost them. Bag them for municipal pickup or burn them if permitted in your area. This breaks the overwintering cycle and meaningfully reduces spore load for the following spring. Rake thoroughly, as even a small number of infected leaves left on the ground can re-infect the tree. In NB, aim to complete this cleanup by late October before snow arrives.

NB-Specific Context

New Brunswick's high Maritime humidity and cool, wet springs create ideal conditions for fungal diseases like tar spot. If your spring was particularly rainy (as many NB springs are), expect heavier infection pressure that summer. Trees in shaded, low-airflow areas — common in older Fredericton and Saint John neighbourhoods — tend to show worse symptoms than trees in open, sunny spots.

Practical Tips

Keeping your maple in good overall health is your best defence. Water deeply during dry July and August spells (1 inch per week), avoid over-fertilizing with nitrogen (which promotes lush, susceptible new growth), and mulch around the base with 2–3 inches of wood chips to retain moisture and improve soil health. Avoid piling mulch against the trunk.

Fungicide sprays are rarely recommended for tar spot — they require precise timing at bud break in spring, need repeated applications, and the benefit rarely justifies the cost or effort for a disease that's primarily cosmetic. In NB, cosmetic pesticide use is also regulated, so chemical treatment should be a last resort.

When to Call a Pro

If your maple is showing tar spot along with other symptoms — thinning canopy, dieback, unusual bark damage, or significant decline over multiple seasons — it's worth having a certified arborist assess the tree. What looks like tar spot stress may be masking a more serious issue like verticillium wilt or root problems. Tree assessment and large-scale pruning are best left to professionals, especially for mature trees.

For most NB homeowners, consistent fall cleanup over two or three seasons will noticeably reduce the severity of tar spot on their maples. It takes patience, but it works.