Wet weather can favor leaf spots in English Ivy. The following is more information.
Frequent rain events can promote leaf spot diseases in many English ivy beds. In the landscape, there are two important English Ivy (Hedera) 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.” The causal fungus is Glomerella cingulata, however, the imperfect fungal state, Colletotrichum trichellum is normally seen now. Close examination with a hand lens may show spine-like formations associated with the fungal fruiting structures though they are not always present. 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.
Growers 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. This disease, caused by the bacterium Xanthomonas hortorum (campestris) pv. hederae, 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.
Bacterial leaf spot disease is diagnosed in the laboratory by observing bacterial streaming under the microscope, however, the disease is often so active that county agents or landscape industry specialists can also diagnose the disease in the field in the same way. Cut through several leaf spots with a sharp knife and place small infected leaf pieces on a glass slide. Add a drop or two of clean water to the infected tissue and cover with a small glass cover slip. After a few minutes to an hour, bacterial streaming can be seen just by holding the glass slide up to the light and observing the milky color of the water near the dissected leaf spot. Fungal spots do not produce bacterial streaming as described above.
Avoid sprinkler irrigation. Treatment with copper-based fungicides is helpful in slowing the spread of bacterial leaf spot.
Information from "Leaf Spot Can Damage English Ivy" By John Hartman in the current edition of the Kentucky Pest News.