I live in Cambridge, (Zone 5, I think.) Most of this area has a lot of clay in the soil. Each year our harlequin maple starts out looking great. Round about early July the leaves start looking brown and dry on the edges. Although we water it frequently throughout the year we just cannot seem to figure this out.
Thank-you for contacting Toronto Master Gardeners with your question.
While it is possible that your Harlequin Maple (most likely Acer platanoides ‘Drummondii’) is suffering from a fungal disease or a lack of specific nutrients, the most probable ailment is leaf scorch. Harlequins tolerate soils from clay to sand, so the problem is not likely to be the soil conditions in your garden. They are reasonably cold-hardy (to Zone 4), so again, this is not likely the issue.
Certain types of maples, such as Japanese maples and harlequins, are particularly prone to leaf scorch. The fact that this occurs each year as the summer heats up supports this diagnosis. Leaf scorch rarely causes long-term damage, but it can leave an affected tree unsightly for the remainder of the growing season. Scorch happens whenever water is lost from the leaves more quickly than the roots can take it up. The fact that the scorch develops in July suggests that under-watering, drying winds, or hot sun are the most likely culprits. Another possible contributor is fertilizer burn which can cause scorched edges (you have not mentioned fertilizing your tree, but the burn can occur even if an adjacent lawn is fertilized).
Harlequin maples do like sun but do best in a somewhat sheltered location, protected from the midday sun and drying winds. Since you can’t do much about the location of your tree at this point, the best way to avoid this problem in future is to water deeply at regular intervals. Watering frequently, but for short periods of time, does not provide much moisture to the roots. It is also possible that frequent watering that hits the foliage can lead to fungal issues which can exacerbate the leaf scorch problem. A good strategy to retain moisture is to apply a layer of mulch around the tree (but not touching the trunk).
You may want to have a look at the following website related to scorch:
The following link provides some background information on leaf scorch specifically on maples:
https://www.rhs.org.uk/advice/profile?PID=591
Best of luck!