I have a 7 foot Japanese Maple with leaves now fully emerged (mid-May and looking great). For the past 4 years the leaves have browned and curled in early-mid summer. I doubt it is weather variables (sun, water etc.) as each year has been different but the results the same.
Is it possible this is a reoccurring fungus?
You’ll find a guide to growing Japanese maples on the Toronto Master Gardener’s site, see link below.
The guide notes that Japanese maples are susceptible to leaf scorch that causes the leaves to turn brown around the margins. Leaf scorch is caused by a variety of circumstances – wind burn, exposure to extremely hot sun, salt runoff from roadways, excessively alkaline soils or short intense drought periods. Under these conditions the roots are unable to supply water to the foliage as rapidly as it is lost by transpiration from the leaves. Unfavourable locations such as sandy soil, proximity to pavement or other obstructions that restrict root growth or exposed windy sites will also promote leaf scorch.
You have not indicated that the leaves fell off suggesting that this is a mild case. However, when leaf scorch occurs annually, the stress will gradually weaken the plant making it more susceptible to insects and diseases.
Check the growing conditions of your Japanese maple for these issues. Some you may be able to correct. Adding a compost made from oak leaves will make an alkaline soil more acidic and adding organic matter such as compost or composted manure will help keep a sandy soil from draining quickly. You’ll need to repeat these amendments annually.
However, most of these conditions are specific to the location of your Japanese maple which I assume you don’t want to change. I suggest you ensure your Japanese maple is watered regularly with extra water during any periods of extreme weather. To quote our Guide again “Japanese maples should have a uniform supply of moisture. Proper water management is far more important than fertilizer or soil types”.
I’m also including a link to a page on the Missouri Botanical Garden site which has a good picture of leaf scorch on a Japanese maple.