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Rust on leaves Last year these rust spots appeared on one lower branch of this five year old tree. A fellow at the nursery were it was purchased suggested that I water more which I did. This year however the rust spots have appeared on almost all leaves on all branches. It seems systemic. In spring and early summer the tree appears healthy, but the spots develop mid season. Other plants growing in the same area (clumping bamboo, hostas and heuchera) do not develop these spots and are healthy. There are four other species of maples in the yard in close proximity that do not develop these spots. Is this some type of fungus? If so can it be dealt with, I am concerned that it may spread to the other trees.

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This is what's known as Tar Spot of Acer (or Maple) and yes, it's a fungal infection. It is not related to dryness at the root, so your nursery guy either didn't know what it was, or preferred not to say. In the UK, there's no effective fungicide available for treating this. It will not kill the tree, just makes it look unsightly. You may reduce the effects by rigorously sweeping up or collecting fallen leaves, leaving none on the ground over winter, to reduce overwintering spores. More information here https://www.rhs.org.uk/advice/profile?PID=750

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    Thank you. The link you provided exactly described the progression of symptoms that the tree goes through. It is good to know that it possible to contain as well. Although it looks a bit wretched by the end of fall the sparrows and finches do not mind. Again, thank you for identifying this.
    – Rhea
    Jul 19, 2017 at 11:58
  • You've also got leaf miner action; those little squiggly lines? Again, not damaging to the host. The best thing you can do is what Bamboo is saying; clean up those leaves before the fungus produces spores that get blown up into the canopy to infect the leaves. plantclinic.cornell.edu/factsheets/tarspotofmaple.pdf
    – stormy
    Jul 19, 2017 at 18:40
  • The tough thing is after you work so hard to clean up your leaves and your neighbors do not you'll still get this stuff. I'd get a neighborhood party going this summer and get everyone liquored up and happy and then do a show and tell...get a community 'gardener' or someone to come clean up those leaves and pay them out of a community fund? Where there is a will there is a way?
    – stormy
    Jul 19, 2017 at 18:45

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