This type of leaf spot infection is fairly common on several dogwood species (including Silky, Cornus amomum), and despite being an eyesore, fortunately they don't tend to cause serious long-term damage to the plant. Treatment is not needed, nor would use of a fungicide provide much benefit at this stage, since it can only protect healthy growth from infection (and sometimes fails, at that); it cannot cure existing disease. Additionally, we seldom recommend fungicides since they may risk harm to other organisms, including pollinators visiting flowers in a treated area. When they are warranted, treatment begins in the spring and several applications of fungicide tend to be needed well into the summer.

It's hard to say what pathogen is responsible since symptoms from one agent to another can overlap greatly; leaf spots can be fungal or bacterial in origin, though fungal is more common. Elsinoe Leaf Spot, for example, can be common on its relative Flowering Dogwood (Cornus florida), and while this spotting may be due to a different fungus, it can't be ruled-out here. In either case, as the leaves shed this autumn, you can rake them up and dispose of them (don't compost) in order to reduce the spore presence for next year's disease cycle. That is not a foolproof solution, though, and reinfection might occur anyway from spores blown-in on the wind. Years with wet spring weather, as we experienced this year before drought set in, tend to increase the likelihood of infection. Since weather can be highly influential in plant disease development, one year's outbreak won't necessarily result in as heavy an infection the next year.

For now, just monitor the plant for watering needs and irrigate as needed to minimize stress on the shrub, since Silky Dogwood naturally grows in habitats that tend to stay evenly moist and our ongoing drought is stressing lots of plants in the landscape.

Miri

Thanks so much.

You're welcome.