We have received reports at the Plant Clinic of several cases of anthracnose on trees and shrubs. Some re-ports are from commercial concerns and some from landscapers. If you remember from the spring (see issue no. 3, “Leaf Diseases of Deciduous Trees”). Anthracnose is a term used to refer to diseases caused by fungi that produce fruiting structures called acer-vuli. There are many fungi that may be involved causing symptoms that include leaf spotting, cankers, or a both. The anthracnose fungi generally thrive in cool, wet conditions. Tender foliage is most susceptible. For these reasons, we usually see anthracnose diseases in the spring. So why are we getting reports of anthracnose now? These fungi can produce several cycles of infection throughout the year. We don’t know a great deal about the annual cycles of the anthracnose fungi, but it is clear that they infect in cool, moist conditions throughout the growing season.
It does not appear that late-season anthracnose should be a threat to tree and shrub growth and development. Although leaf spots are common now, most plants have already formed buds for next year and can sustain significant leaf loss without jeopardizing plant health. There is no new leaf growth occurring now, so tender foliage is not present to host the fungus. Fungicides are sometimes used to fight anthracnose fungi but always in a protective mode of action. We may recommend spraying new foliage in the spring to protect that foliage from infection. Because there is no new leaf growth in the fall, fungicides are not recommended.
One concern with late-season leaf disease is that more fungal inoculum is present to overwinter on or near the plants. Many of the anthracnose fungi overwinter as mycelium in leaves or on twigs. Remove fallen leaves from the site this fall, and prune dead or dying wood. Burn, bury, or remove this plant material from the site. Consult Report on Plant Disease (RPD) No. 621 for details about anthracnose. This publication is available in Extension offices or on the Web at the VISTA site, http://www.ag.uiuc.edu/~vista/horticul.htm.