I've noticed these spots on the leaves for a couple of months, but they seem to be getting worse, with some leaves dying. This tree is in it's fourth year, planted as a tiny seedling, it's in Burnaby in a well drained sunny location. I have three other garry oaks and none of them have this. This tree gets water almost every day and receives the most water of the four trees.
Maybe Phylloxera glabra, tiny bugs similar to aphids, that suck the sap out from the leaves? Check underside of those leaves with a magnifying glass.
The spots you see on your young Garry Oak are almost certainly caused by Jumping Gall Wasps (Neuroterus saltatorius). There is also a native gall wasp which is contolled by a native parasitoid and unlikely to be a problem. My 2 dozen or so Garry Oaks are infested with gall wasps almost every year and I can actually hear the sound of thousands of wasp larvae in the galls when I walk under the trees. It seems the trees can withstand heavy infestations and hang on to their leaves well into autumn. From: http://www.goert.ca/gardeners_restoration/garryoak_trees.php Jumping gall wasp: The jumping gall wasp lays its eggs on several ornamental oak species in British Columbia, but the Garry oak is the only tree species on which it can complete its life cycle and where it does the most damage. Tiny (1.5 mm, 1/16") yellow galls that look like mustard seeds on the undersides of the leaves house the wasp larvae. When the larva matures, the gall falls to the ground, and as the wasp moves around inside, the gall visibly and audibly “jumps”. Yellow-brown spots are left where the galls were attached to the leaves. Symptoms appear in mid-June and include anything from simple spotting of leaves on lightly infested trees to complete scorching and premature defoliation on the most severely infested trees. Fortunately, there are some parasitoids that feed on and kill the gall wasp larva, and earwigs and some ground beetles eat galls. Although there are commercially-available insecticides, these are not recommended. For more information and photos see this Canadian Forest Service Leaflet (PDF). It is so strange though that this year, the 14th we’ve lived here, that there are no Jumping Gall Wasps on any of the trees but instead, for the first time, a huge infestation of aphids which have been reigning honey dew down on everything for at least 8 weeks. Despite that, the leaves look beautiful. Some might say that you are giving the tree too much water since they are used to drought conditions in the wild but, as I say, even in the wild, Jumping Gall Wasps are common.
Thanks for the replies. I'll take a look at the underside of the leaves tomorrow. I don't actually directly water this tree any more, but I fill jugs with a hose to water other trees next to it, and there's always spill over. Two of the trees get watered fairly often, from overflowing jugs, the one in my yard gets watered during lawn sprinkling, and the smallest one at 18x18 inches never gets watered and looks super healthy, with large dark glossy leaves. I've concluded that even though the one that gets no watering is the smallest, watering after the second year is unnecessary and has no effect on growth. Of the three I planted in a park, the largest seedling at the time is still the largest at 38 inches (original stem),55 inches (secondary stem) and 36 inches(third stem)second biggest (the one with the spots) is 35 inches, and the one I planted in my yard in the fall of 2013 is 114 inches and 108 inches, all of the above ground growth starting in 2015.
I checked with a magnifying glass, and I didn't see any galls, aphids or other insects. Just one lady bug which may be a sign there are aphids, maybe they're just too small for me to see.
