Having seen more than an inch of rain fall in the last 24 hours it is not surprising that the time I had available today did not coincide with amenable photographic opportunities; playing catch up I can therefore introduce you to an example of ‘tar spot’ which I encountered in several locations while we were in Edinburgh. We had to use Google to find out what the highly distinctive black leaf spots were, spots that looked even more tar-like when seen on leaves that were still green.
Apparently, according to the RHS, it is caused by the fungus Rhytisma acerinum and is seen most commonly on sycamore, but can also affect a number of other acer species. The spots are unsightly, and the disease can cause slightly premature leaf fall but it has no long-term effect on the vigour of affected trees. It starts as yellow blotches on the leaves in spring and these turn jet black by summer before resulting in an earlier than usual leaf fall. When I first noticed them at the zoo I could see it was only sycamore leaves that were affected, and I meant to go back and check the piles of crunchy leaves outside our accommodation to see if they might be sycamores and if tar spot was the likely cause of their seemingly early crispness.
There are no known chemical or non-chemical means of control, although sweeping up infected leaves can reduce the likelihood of tiny fruiting bodies developing during winter on disease-affected fallen leaves at the base of the tree which would then release microscopic spores in spring and in turn infect new leaves . In retrospect we did not notice any affected leaves at the Botanical Gardens but whether this was down to good plant husbandry or purely an absence of sycamores and maples I cannot say.