Septoria Leaf Spot on Variegated Red-Twigged Dogwood
This leaf spot disease is caused by the fungus Septoria species. It most commonly occurs during the wet summer months, and may become so numerous that they cover the leaves by the end of summer. Heavily spotted leaves may be shed early. The light spotting seen on leaves of dogwood usually has little impact on tree health, but repeated years of early leaf drop can weaken the tree.
Septoria leaf spots are small (1/8 to ¼ inch), angular to irregularly shaped and usually bordered by leaf veins. They are dark brown purple in color, later developing light brown or gray centers with dark borders.
To control Septoria leaf spot, clean up and dispose of infected leaves on the ground and on the tree, since this is where the fungus survives the winter. For severe infections, fungicides containing chlorothalonil, thiophanate-methyl, mancozeb or bordeaux 4-4-100 can be applied.