Why do so many Maple trees in Lakewood have black spots on them? « The City of Lakewood, Ohio -
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Why do so many Maple trees in Lakewood have black spots on them?

Every Norway maple in Lakewood has what is called tar spotting (black dots) or Maple tar spot on leaves this year due in large part to the very wet and cool spring we had earlier this year. It is less prevalent on other maple tree species, but approximately 50% of Silver maples also appear to be impacted. 95% of the time the tar spotting is only an unsightly cosmetic issue that can cause early onset defoliation, premature browning or leaf curl – but it does not harm or kill the tree. It occurs more frequently with wet spring conditions and often does not appear again (or is diminished) the following year if the spring/early summer weather is closer to normal.

All the Norway maples throughout Lakewood and thru Ohio and the Midwest are under a lot of stress due to climate change impacts that has made them more prone to repeated fungi infestations such as Verticillium wilt which can lead to canopy dieback and tar spotting on the leaves that we see throughout the City of Lakewood. Real wet spring weather and the more frequent freeze-thaw periods in winter with temperature spikes over 50-60 degrees followed by freezing compounds the above and can increase the amount of fungi infestations in all maple trees – Norway’s in particular and all other maples to a lesser degree.

However, some Norway maple trees in Lakewood have been impacted with a more serious fungal infection – Verticillium wilt.  We have confirmed some infection present within several of the Norway maples we have had to remove that had been suffering significant canopy dieback the past few years.

Verticillium wilt is very difficult to control because it persists in the soil indefinitely, treatments/sprays are not highly effective. Sometimes infected trees can “outgrow” the fungus – which we have seen take place. We frequently prune off dead branches first to help the overall tree vigor. If a tree has lost more than a 1/3 of its canopy – the decline cannot be stopped at that point and the tree should be removed. Whenever we prune Norway maples, we sterilize our pruning tools/saws by spaying them with a diluted cleanser between trees.

In addition, trees already weakened by Verticillium can be more prone to tar spotting and these two fungi issues can overwhelm a tree to where it gets the tar spotting for several years in a row and the leaf canopy production decreases to the point where there is not enough leaf canopy to feed the tree root systems and the Verticillium fungi in the soil spreads more readily into decaying roots versus healthy root systems.

Those Maple trees only impacted with just the tar spotting alone should all be fine. Although a bit unsightly now, the fungal infection will not kill the tree unless the tree is also impacted by other more serious issues that are pushing it into decline.

Unfortunately, per research and data gathered by the United States Forest Service in relation to the possible impacts of climate change on various tree species – the Norway maple is one of the species identified to suffer the most mortality due to the impacts of climate change. All maple tree species are at a heightened risk of increased mortality rates, with the Norway maple already seeming to suffer from the changing climate conditions now present and not being able to adapt to the stresses being placed upon this species. In short, Maple trees need a longer dormant period to thrive – repeated winters with more frequent freeze-thaw periods with very warm temperature spikes in January and February followed by freezing compounds the above and can increase the amount of fungi infestations in all maple trees – Norway maples in particular.

This impact also underscores the need to plant for greater tree species diversity. Norway maples were over planted throughout Lakewood – it is our 2nd most common species (over 1,100 trees) – and many streets are a monoculture of mostly Norway maples with only one or two other species as not much tree species diversity was established on certain streets.

One of the primary goals of the City of Lakewood Tree Action Plan is to continue to introduce more tree species to all city streets and citywide. The city’s plan for tree species diversity = high reward.

To accomplish the goal of increasing tree diversity and reducing the likelihood of large tree losses across the city, the city has an established a benchmark that the total tree inventory contains no more than 30% of a single family, 20% of a single genus, and 10% of a single tree species.

If dying trees need to be removed, they will be replaced with tree species that are not as susceptible to Verticillium. It is cases like this that remind us that reaching our tree species diversity goals is more important going into the future.

We will continue to monitor all the Maple trees – in particular our Norway maple population – throughout the City of Lakewood and take any necessary actions accordingly.