Frequent rain events in Kentucky this spring have promoted leaf spot diseases in many English ivy beds. In the landscape, there are two important English Ivy leaf spot diseases, one caused by a fungus and the other caused by a bacterium. The two diseases are sometimes difficult to tell apart, however, the bacterial leaf spot tends to occur more frequently in summer.
Fungal leaf spot (anthracnose) appears as large, irregularly shaped tan or brown spots. Within the dead spots numerous slightly raised dark blister-like fungal fruiting structures may be observed when examined with a hand lens. The dead areas may be surrounded by a yellow “halo.” There are other fungal leaf spot diseases of English Ivy which also produce fungal fruiting structures; this distinguishes them from bacterial spot, which produces none.
Gardeners should avoid planting diseased plant material and avoid sprinkler irrigation which splashes fungal spores from diseased to healthy plants. To prevent fungal leaf spot disease, treatments with fungicides containing fixed copper, thiophanate-methyl, or mancozeb can be tried. These fungicides will not “cure” already infected leaves.
Bacterial leaf spot is favored by periods of warm, wet weather typical of summer in Kentucky. This disease, caused by a bacteria, can be especially damaging to ivy growing in many landscapes. The bacteria invade leaves, shoots, and stems through stomata and wounds causing a greenish-brown angular leaf spot 1/4 to 1/2 inch or larger in size. The spots sometimes appear greasy and may have a yellow margin; as they age, spots turn dark brown and may crack as they dry. Rainstorms and overhead irrigation help spread the disease in the landscape by splashing bacteria from diseased to healthy plants.
Purchase and install only disease-free plants and avoid sprinkler irrigation. Treatment with copper-based fungicides is helpful in slowing the spread of bacterial leaf spot.