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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.
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Click on image for larger
version
Figure 1. Distorted Leaves Caused by Anthracnose of
White Oak (photo by Jim Kuntz)
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Click on image for larger
version
Figure 2. Anthracnose of Ash (photo from NC State)
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Symptoms
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.
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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.
Disease
Cycle
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) and
spread.
Control
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.
Forest
Trees
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 disease.
- 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 out.
- 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, and tuliptree.
- 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
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| Host
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Typical
Symptoms |
| Ash
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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. |
| Birch
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Small
to large, round to irregular, light brown to dark brown lesions
with yellow margins form on the leaves. The disease is seldom
serious. |
| Catalpa
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Small,
brown to black lesions form on the leaves. The center of the
lesion may drop out. The disease is rarely serious. |
| Dogwood |
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. |
| Elm |
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. |
| Hickory |
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. |
| Hornbeam,
Hophornbeam |
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. |
| Linden |
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. |
| Maple
Leaf |
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. |
| Oak
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 are killed. |
| 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 to mid-spring. |
| Tuliptree
or Tulip-poplar |
Irregular brown leaf blotches with dark brown borders may develop.
The symptoms usually appear late in the season. |
| Walnut,
Butternut |
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.
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