In the News - May 2009
Anthracnose affecting shade trees statewide
LITTLE ROCK - Recent cool temperatures and frequent rainfall have created
ideal conditions for anthracnose plant disease on several common shade trees,
says Dr. Steve Vann, assistant professor/extension urban plant pathologist with
the University of Arkansas Division of Agriculture.
"Anthracnose of our shade trees may be caused by several fungal organisms
that usually cause a variety of leaf spots and twig lesions," says Vann. Maple,
sycamore, dogwood and ash trees are especially susceptible. Trees usually react
to the disease by dropping infected leaves, much to the dismay of many
homeowners.
Maple anthracnose produces dark brown to black greasy-colored spots on the
leaves. These spots are usually block-shaped and may have a yellow area, or
halo, surrounding the spot. "Numerous spots may grow together and give the leaf
a tattered look," says Vann. "These spotted leaves readily fall to the ground."
Sycamore anthracnose is caused by a similar fungus that causes premature leaf
drop and may kill young twig tips. Diseased twigs may resemble frost injury,
says Vann. These young, newly expanded leaves will often die and droop on the
twigs, which may also die and remain on the tree. "On older sycamore trees, the
fungus tends to kill leaf tissues alongside the major veins of the leaf blade,
giving it a ‘netted’ look," he says.
Both fungal organisms produce microscopic seeds called spores. These may be
carried on wind currents or splashing water droplets to other leaves and twigs,
especially if there is an extended rainy period in the spring. Anthracnose fungi
typically over-winter on infected twigs and leaves. Dead leaves that don’t fall
from the tree during the fall and winter seasons can serve as sources of
infection the following spring.
"As drier weather returns, infections begin to diminish and trees will often
put on a crop of new leaves," says Vann.
Maple and sycamore anthracnose are usually a minor concern for homeowners
since occurrences of the disease are generally sporadic, he says. However, if
the diseases occur for several successive growing seasons, the tree’s health may
be compromised.
If a homeowner wishes to have a "specimen" tree in the landscape, it may be
necessary to spray with an appropriate protectant-type fungicide in the spring,
just before buds break, says Vann. Multiple applications may be required during
the growing season to keep the tree disease-free, so complete coverage and the
timing of applications are crucial to its effectiveness.
"Waiting until after leaf emergence is usually too late to protect the leaves
during the season," says Vann.
A stressed tree is often more susceptible to disease. Trees should be watered
and fertilized properly based on a recent university soil test to encourage
vigorous growth.
Spraying taller trees may not be practical or feasible for homeowners, thus a
professional applicator should be consulted in these cases. Always read and
follow the label directions for fungicides when considering their use.
Promoting good plant growth is an integral part of disease management, but a
misdiagnosis can still be deadly for any homeowner’s shade tree. "Correct
disease identification is the single most important first step in designing an
effective plant disease control program," says Vann.
For more information on anthracnose diseases found in trees and ornamentals,
contact your county extension office or visit www.uaex.edu.
The Cooperative Extension Service is a part of the University of Arkansas
Division of Agriculture.
May 1, 2009
Media Contact: Elizabeth Fortune
Extension Communications Specialist
U of A Division of Agriculture
Cooperative Extension Service
(501) 671-2120
efortune@uaex.edu
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