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Drive through town, and you might get confused about
the seasons. Under some trees, it looks like fall. Other trees
seem to be just getting started on spring. Most sycamores, for example,
have lost their mature leaves and are now producing new ones. The cause of this odd phenomenon is anthracnose, a
group of related fungal leaf and stem diseases that thrive with cool spring
temperatures and frequent rains. Spring 2008, unfortunately, has been
perfect for spreading fungal spores. Different fungi produce anthracnose on different host
plants. The fungi cause tan, brown, or black lesions on the leaves, stems,
flowers, and fruits of various plants. Infected leaves are marked with
blemishes ranging from tiny dead spots to large circular or irregular dead
blotches. Symptoms of anthracnose vary, depending on the tree species. On
sycamore and maple trees, infected areas often develop along the leaf veins
and midribs and expand outward to the leaf edge. This causes the leaf to
become distorted. Heavily infected leaves often curl and drop prematurely,
littering the ground. Most infections occur in the two weeks following bud
break. Anthracnose can affect most shade trees, but the ones most likely to
be hit in Illinois are ash, dogwood, elm, maple, oak, sycamore, and walnut.
In the early spring, newly emerged leaf tissue in
sycamores infected with cankers will suddenly wilt and turn brown. The
fungus grows from leaf tissue down the leaf petiole and into stem tissue.
(Do not confuse the natural fuzziness of a sycamore leaf with this
infection.) The fungus spores will overwinter within the cankers. In the
spring, the spores can reinfect the tree and spread the disease to other
trees. Cankers in branches can girdle and kill the branch. “Twig
blight” refers to the death of 1-year-old twigs. Repeated annual bud
or twig dieback stimulates the development of many short twigs at the base
of the dead twigs given the tree a “witches broom” appearance.
Although anthracnose is a common and unsightly
shade-tree disease, it is rarely fatal. However, repeated infections can
weaken a tree. Healthy, vigorous trees should quickly recover. One
exception is dogwood anthracnose, an aggressive disease that can cause
permanent damage and death in dogwoods. So what can be done for infected trees? The first step is to properly identify the problem. A
fact sheet on anthracnose diseases of shade trees can be found on the
University of Illinois Extension Web site at
www.aces.uiuc.edu/vista/abstracts/a621.html. The Ohio State University
Extension also has a fact sheet with good pictures at
ohioline.osu.edu/hyg-fact/3000/3048.html.
Fungicides are generally not recommended except for
dogwood anthracnose. They must be applied before damage appears, and the
whole tree must be treated. This is usually cost-prohibitive.
The fungus overwinters within fallen leaves and
twigs, so good sanitation is important in minimizing the severity of the
disease in the following year. Rake up and discard fallen leaves and twigs
and, to prevent reinfection, remove or bury all diseased plant parts. Maintain good tree health. Water trees during periods
of drought: Supply 1 to 2 inches of water weekly during dry periods, making
sure to apply water slowly and deeply. Mulch the root zone of the tree with
a 2- to 4-inch-deep layer organic mulch, such as wood chips. Fertilize trees in the late fall to maintain vigor.
Trees that contract severe anthracnose infection and defoliate early may
weaken. Defoliation depletes the energy reserve of the tree and increases
its susceptibility to other pests and diseases. Yearly infections can
reduce growth and may predispose the tree to other stresses.
Make sure trees have proper spacing for good air
circulation, and thin out excessive twig and branch growth. Sycamore
anthracnose can overwinter in cankers on stems within the tree canopy. If
possible, prune out infected twigs and branch cancers. In addition to these measures, consider planting
resistant tree varieties in your landscape.
Jennifer Fishburn is a horticulture educator with the
University of Illinois Extension Sangamon-Menard Unit. Contact her at
fishburn@uiuc.edu.
This article appears in May 29 – Jun 4, 2008.
