Black-spotted leaves are falling from trees in Halifax. An expert explains why

People in north-end Halifax have been noticing the poor state of Norway maple trees in their neighbourhood this summer, with many black-spotted leaves falling off prematurely. 

"The last two weeks, we've been noticing a lot of leaves on the ground, in our driveway and in our backyard," said Ryan Turner, who lives near the intersection of Duffus Street and Novalea Drive.

"On the ground, the leaves are dry and brown and kind of crumpled with black spots."

Although harmless to the tree in the short term, these black patches are what's called tar spot fungus — and it's showing up more frequently this year.

To explain why it's happening, Crispin Wood, the superintendent of urban forestry in Halifax, spoke Wednesday with host Jeff Douglas of CBC Radio's Mainstreet.

This interview has been edited for length and clarity.

We reported on this fungus back in 2017. Is what Ryan is talking about and what others are seeing, is this just the progress of that same fungus?

Yes, it's the same fungus. It's unfortunate that in 2017 we had conditions favourable to the fungus, and again this year we have, again, conditions favourable to the fungus.

The past two years, we happened to have very dry summers, which are not favourable conditions. So, of course, while the fungus was still present, we didn't have serious infections or infestations like we do this year.

But this year, because of the high humidity and the amount of moisture and heat that we've had, the conditions are just perfect. And of course, the last couple of weeks have just been so warm, so humid, foggy conditions at night, which are keeping the leaves moist.

The fungus has just exploded within the north end of Halifax in particular.

And is this a particular ailment to the Norway maple or one to which it is particularly susceptible?

Mostly maple seems to be the primary host for this fungus. 

Unfortunately, here in Halifax, roughly 30 or 35 per cent of the street tree inventory in Halifax and in the greater peninsula in general are Norway maples. So when the fungus gets a hold of one tree, it can run across the populations fairly quickly, just based on the sheer volume of trees and the number and proximity next to each other.

Is it fatal for the trees?

It's not fatal for the trees. Primarily, it would be considered esthetic as the infection progresses, and currently we do have some trees that are fairly infested. That is causing a fair degree of stress on the trees. But recognizing the past two years, there was a bit of a reprieve because of the dry conditions.

It certainly doesn't benefit the trees. Could it kill a tree if a tree is already stressed out or nearing the end of its life, which unfortunately ... many of our Norway maples are? It could be the nail in the coffin.

But from what I've seen in the North End, most of the trees are in relatively good shape. While they are losing many of their leaves, it will simply be sort of an early leaf drop.

The tree will then go into a bit of an early dormancy, and fingers crossed, next year we'll have conditions that won't be as conducive and the tree will be able to recover.

Is it treatable?

It's not really treatable, no. 

There are sort of management practices. The fungus overwinters under the snow. So when the leaves drop, it hits the ground, the fungus will overwinter underneath the snow.

If you rake the leaves up, pick the leaves up, don't mulch them into your lawn. Compost them or dispose of them, you can really reduce the amount of sort of fungal inoculum that could reinfect those trees in future years. 

We always like to say try to keep your trees in as prime condition as possible, make sure they're in good health. 

If someone does have a Norway maple out there that is coming to the end of its life and they're concerned about it, what is on offer from the city — provided that, of course, it is a city tree?

If it's a city tree, contact 311. We can come out and have a look at it. If you're looking up, you're seeing something you're concerned about, if you're looking up and you think there's a hazard, contact 311.

We can have that assessed right away and get some arborist out there to conduct some pruning if required.