Reports on Plant Diseases
RPD No. 621 - Anthracnose Disease of Shade Trees
[ Symptoms ] [ Disease Cycle
] [ Control ] [ Table of Symptoms
Anthracnose is the name given to a group of diseases that are caused
by a number of morphologically similar fungi. The diseases cause tan to
brown or black lesions on the leaves, stems, flowers, and fruits of various
plants. The shade and forest trees most often affected in Illinois are
ash, elm, maple, oak, sycamore, and walnut. While individual leaf lesions
will cause minor damage, severely diseased trees are unsightly and defoliate
prematurely. Damage to buds and stems occur on such trees as sycamore
and white oak, sometimes causing disfiguration from the die-back of twigs
and branches. Economic loss from anthracnose is usually caused by repeated
annual defoliation and the dieback of twigs and branches, which weakens
the trees and makes them more susceptible to other diseases, frost injury,
and insect damage.
Click on image
for larger version
Distorted leaves caused
by Anthracnose of White Oak
(photo by Jim Kuntz).
on image for larger version
Figure 2. Anthracnose
(photo from NC State).
On trees other than sycamore, anthracnose symptoms are mostly confined
to the leaves. These symptoms vary from small, circular to irregular
spots that are tan, dark brown, or black, to larger blotches that are
usually associated with the midribs and veins (Table
1). Veinal necrosis is common on sycamore, ash, oak, and maple (Figure
2). When immature leaves are infected on trees such as oak, these leaves
may become severely distorted (Figure 1). Young leaves may die and fall
soon after a heavy infection. If a severe infection occurs early in
the growing season and the trees defoliate, a new set of leaves may
emerge, depending on the environment and the anthracnose fungus affecting
the tree. Not only do sycamores show symptoms, but also their buds,
shoots, and one-year-old twigs are commonly blighted.
Four stages, which often overlap, may be identified in years when optimum temperature
conditions exist for anthracnose development. Bud blight and twig blight occur
before the leaves emerge in the early spring and kill either individual buds
or the tips of one-year-old shoots. Later, fruiting bodies (pycnidia), the size
of a pinhead, rupture the bark.
A sunken girdling canker may form below the twig tip after leaf emergence and
cause the death of young shoots. This symptom is often mistaken for frost injury.
The lateral buds behind the dead twig will eventually produce new growth and
a new flush of leaves, giving the tree a bushy appearance. Twig infection is
not common but may occur in ash, elm, hickory, hophornbeam, linden, maple, oak,
poplar, and walnut.
Bud blight and twig blight occur in April or early May. The buds may be killed
before the end of dormancy because of a girdling canker that has formed below
the buds. Twigs may be killed when the canker encircles the one-year-old twig.
Shoot blight begins after the leaves emerge and resembles frost injury. Leaves
suddenly die and later drop from infected twigs. The severity of the blight
depends on the temperature during the two-week period following the emergence
of the first leaves. Damage is most prevalent when the average mean daily temperature
(the average of the maximum and minimum temperatures) during this period is
between 50° and 55° F (10° and 12° C). Above 60° F (15°
C), little or no shoot blight takes place.
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Anthracnose fungi that primarily attack the leaves commonly overwinter on the
ground. When cool, moist spring weather occurs, spores (conidia or ascospores)
produced on the dead leaves are spread by wind and splashing rains to newly
emerging leaves where infection begins. After a short time lesions form, and
a new generation of spores (summer spores or conidia) reproduce, again infecting
the leaves. Intensification of the disease from summer spores is especially
noticeable on walnut and ash, leading to pronounced early defoliation.
With fungi that attack both stems and leaves, overwintering commonly occurs
within infected buds, the bark of twigs, and branch cankers. In prolonged cool
spring weather, the conidia separate from the minute fruiting bodies (pycnidia)
that form at these sites. Rain splash disperses the spores to emerging leaves,
causing infection. Overwintering in twig cankers is advantageous for the fungus
since it never loses contact with its host. Sycamore and oak anthracnose fungi
operate in this manner, making them particularly difficult to control. From
infected leaves, the fungus grows into the veins, through the petioles, and
into the stem where it overwinters. In the spring, under proper environmental
conditions, the growth of the fungus to form girdling cankers may result in
the death of buds, twigs, and shoots . Later, summer spores form (in acervuli)
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Since anthracnose diseases are rarely fatal, control measures may not be warranted
depending on the value placed on the tree, the weather conditions when the leaves
and twigs are growing rapidly, and the amount of disease expected.
Practical control measures for trees growing under forest conditions are not
available; however, some cultural control methods may reduce annual losses.
- Since free water on plant surfaces is necessary for spore germination,
penetration, and infection, practices designed to hasten evaporation may reduce
- Prune branches and remove trees in a densely planted plantation to allow
better air movement, increase the sun's penetration, and to speed drying.
- Select a good planting site with good air flow.
- Anthracnose on young walnut trees is less severe in plantations fertilized
with nitrogen fertilizers.
Shade and Ornamental Trees
- Sanitation, the removal and destruction of fallen leaves (a spore overwintering
site and the source of some spores for early spring infections), reduces the
potential for infection.
- Infected twigs and branch cankers, a source of spores, should be pruned
- All diseased plant parts should be buried, burned, or removed from the
site to prevent reinfection.
Procedures 1 through 3 above are recommended for anthracnose diseases of
ash, birch, catalpa, dogwood, elm, hornbeam, hophornbeam, linden, poplar,
- Trees that have a severe anthracnose infection and defoliate early may weaken
and should be fertilized. Defoliation depletes the energy reserves of the
tree and increases its susceptibility to other pests and diseases. Surface
application of ammonium nitrate fertilizer at the rate of 18 pounds per 1,000
square feet, or of urea at 13 pounds per 1,000 square feet, is recommended.
If grass is growing under the canopy of the tree, do not apply fertilizer
until the grass is dry--the grass may "burn" otherwise. Apply the fertilizer
with a cyclone spreader followed by an inch of water (600 gallons per 1,000
square feet) every 7 days for several weeks.
- Resistant tree varieties should be used whenever possible. The London plane
tree is notably less susceptible to anthracnose than the American sycamore;
black and pin oaks are more disease resistant than white oaks.
- Various fungicides can be used to control anthracnose on valuable trees,
but chemicals rarely control this disease completely. Spraying can protect
leaves from infection but it will not prevent the development of cankers on
sycamores and oaks in which the fungus overwintered within the tree. A hand-held
sprayer may be satisfactory for smaller trees, however, high-pressure spray
equipment is required to treat larger trees. Apply a suggested fungicide two
or three times, at 14-day intervals, starting at leaf emergence. Thorough
coverage is required. The manufacturer's directions should be carefully followed.
Spraying after the disease is evident will only protect healthy new leaves.
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Table 1. Symptoms of Anthracnose on Various Shade
and Forest Trees
||Large, irregular, tan to brown lesions form on the expanded leaflets,
especially along the margins and veins; leaves may become distorted and
fall early. Trees weakened by repeated infection may become unsightly.
||Small to large, round to irregular, light brown to dark brown lesions
with yellow margins form on the leaves. The disease is seldom serious.
||Small, brown to black lesions form on the leaves. The center of the lesion
may drop out. The disease is rarely serious.
|| Small, round to angular, dark purple lesions form on the leaves. These
lesions often drop out, producing "shot-holes." Reddish purple lesions form
on the bracts; flowers are spotted and disfigured.
|| Small, round to irregular, grayish to black, shiny lesions form on the
leaves; cankers may form in the twigs. Infected leaves turn yellow and,
in wet years, premature leaf drop may be heavy.
|| Small to large, irregular, reddish brown lesions with yellowish, indefinite
margins form on the upper leaf surface and are dull brown on the lower surface.
A severe infection may cause premature yellowing and leaf cast.
||Small to large, reddish brown (hophornbeam) or brown to gray brown (American
hornbeam) lesions on the leaves that later merge causing leaf tips and margins
to scorch and curl. Diseased leaves, especially on the lower branches, may
drop prematurely. Reddish brown twig cankers on hophornbeam result in dead
shoots with bleached, withered leaves.
|| Small, round to elongated, light brown spots with dark margins may enlarge
along the veins. Petiole and twig cankers may form. In wet years, leaf loss
may be severe.
|| symptoms are variable. Entire young leaves of Japanese maple may become
blackened and shriveled. Purple to brown streaks may develop along the veins
on Norway maple. Small to large, round to irregular, green brown or red
brown areas develop along or between the veins of sugar, Norway, and Japanese
maples. If numerous, the lesions may merge, affecting the entire leaf.
||symptoms vary. Small, scattered brown lesions may form on red and black
oak leaves. On white oaks, small to large, irregular, expanding brown blotches
may form on the leaf. Infected leaves often appear scorched and distorted;
twig cankers may cause dieback. The disease is most common on the lower
branches of white oaks.
|Poplar, Cottonwood, Aspen
|| Dark brown to reddish brown spots and blotches on the leaves may result
in a general yellowing or browning of the foliage. Tiny, pustular cankers
form in green twigs which later die back. If severe, foliar browning and
nearly complete defoliation may occur by early August. Seedlings in nurseries
|Sycamore, London plane
|| Small to large, irregular, brown lesions form along the veins to the
leaf edges. Girdled twigs, bud blight, and shoot dieback may occur in early
|Tuliptree or Tulip-poplar
|| Irregular brown leaf blotches with dark brown borders may develop. The
symptoms usually appear late in the season.
|| Small to large, oval to round or irregular, dark brown to black lesions,
some with yellow margins, may develop on the leaflets. Black sunken areas
form on the nuts. Gray-brown lesions with reddish brown borders develop
on the shoots. In wet years, severe defoliation may occur by early to mid-August.
Nancy R. Pataky is Director of the Plant Clinic and Turf and Ornamentals
Specialist, Department of Crop Sciences, University of Illinois, Urbana-Champaign.
University of Illinois Extension provides equal
opportunities in programs and employment.
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