I am a resident of Toronto area (Richmond Hill). This sapling appeared beside a sapling that previously died from critter stripping of its lower bark. It grows from the same root mass and is growing fast and from underneath looks very healthy, but all leaves are now red spotted on top (see photo). What to do?
Thank you for contacting the Toronto Master Gardeners about the spotted leaves on your oak sapling (Quercus rubra). These spots look to me like the results of Tubakia (formerly Actinopelte) Leaf spot which is caused by a fungus called Tubakia dryina. It thrives in wet weather and its spores spread in the wind and splashing rain. It can cause some loss of foliage but is usually not considered serious. Most trees tolerate this disease with little or no damage, unless they get this disease for several years in a row. However small trees that are defoliated are more at risk of damage until they become established. It sounds like your sapling is growing from an established root mass, so this is likely an advantage.
There are a couple of things you can do to manage this disease. You should remove and dispose of (not compost) all fallen leaves and twigs as they drop, since the organisms that cause this disease survive in fallen infected leaves and twigs. The spores can also overwinter there, so a thorough fall clean up is important. And you should keep your tree healthy and strong by fertilizing as needed based on the results of a soil test. If your tree is weak or under stress, it will be more susceptible to insects and disease.
Best of luck with your young oak!