of Rhode Island GreenShare Factsheets
Diseases of Shade Trees
affects a wide variety of plants. Many popular shade trees, including
sycamore, ash, oak, maple and walnut, commonly exhibit symptoms
each spring. Although symptoms observed on different tree species
may be similar, the various anthracnose pathogens are host-specific,
infecting only one tree species.
fungi overwinter in twigs and small branches on the tree and on
fallen leaves. The fungi are active during periods of mild winter
weather, forming cankers and causing twig dieback. In spring, fruiting
bodies form on dead twigs and fallen leaves. Spores are released
and disseminated by wind and rain to the young expanding shoots
and leaves. Cool, rainy spring weather provides optimal conditions
for infection. Repeating cycles of the disease may occur throughout
the summer when weather conditions are favorable. Hot, dry weather
halts the progress of the disease.
Pathogen: Apiognomonia veneta
Sycamore anthracnose results in twig cankers, shoot blight and leaf
blight--in that order. Substantial defoliation often results. The
fungus actively colonizes buds and twigs when the plant is dormant
and susceptible during mild weather in autumn, winter and early
spring. Twig dieback occurs when a canker enlarges and girdles the
twig. Trees with high bud and twig mortality produce only thin crowns
by late spring. In early spring, spores are produced on dead one-year-old
twigs which infect expanding shoots and leaves. Infected shoots
suddenly wilt and appear scorched, commonly following spring rains.
Leaf blight of newly expanded leaves also occurs, especially on
the lower branches. Infected leaves develop tan to reddish-brown
lesions that typically center on and extend along the leaf veins.
This disease tends to be worse along the coast due to high moisture
plane tree has developed resistance to this disease and the London
plane tree (a cross between the highly susceptible American sycamore
and the Oriental sycamore) is partially resistant. Select a cultivar
of London plane tree (such as 'Bloodgood', 'Colombia', or 'Liberty')
rather than a seedling to ensure resistance is maintained through
is a common problem on white ash and, to a lesser extent, green
ash. Symptoms first appear as water-soaked spots on the expanding
shoots and leaves when cool, wet weather prevails in spring. The
spots enlarge and develop into brownish-green to dark brown lesions
along leaf veins and margins. Growth of the infected tissue slows
or stops as the rest of the leaf continues to expand. As a result,
the leaf tissue around the lesion becomes twisted and wrinkled.
As leaves mature, they tend to become more resistant to infection.
Considerable leaf drop may occur, particularly from lower areas
of the canopy. The fungus can also grow into twigs and develop as
cankers, where it will overwinter, but little damage occurs from
this phase of the disease. Green ash, which possesses some resistance
to anthracnose, is a better landscape selection than white ash.
Pathogen: Apiognomonia quercina
attacks many species of oaks including bur, white, swamp white,
English, pin, northern red and chestnut oak. Susceptibility varies
among species, with white and bur oaks usually most severely affected.
When outbreaks are promoted by rainy weather in spring, leaf and
shoot blight become prominent on low branches, then spread upward.
The first symptom is usually shoot blight, which can develop suddenly
following a rainstorm. Young leaves and shoots appear brown and
shriveled. Leaves that have already expanded may become cupped and
distorted with large areas of dead tissue. Small, brown, spore-bearing
structures may become visible on the lower surface of dead leaves.
Necrotic spots result from infection of mature leaves, which are
fairly resistant. The fungus also infects twigs, producing cankers
and causing twig dieback during the winter and early spring.
Pathogens: Gloeosporium spp.
of fungi cause maple anthracnose. K. apocrypta causes necrotic
spots or scorch-like blight on leaves of Japanese, Norway, red,
silver and sugar maples as well as boxelder. Symptoms, which develop
during wet weather in late spring and early summer, begin as discrete
reddish-brown lesions (tan on Japanese maples). Lesions soon coalesce
and kill large areas of the leaf. Young shoots and leaves may shrivel
and blacken following infection. Severe infection may lead to defoliation.
Discula sp. affects sugar and striped maples, causing brown
to reddish brown lesions along or between leaf veins. Spore masses
of the fungus can sometimes be found on lower leaf surfaces along
veins during extended moist conditions. The fungus spreads from
previously infected tissue in spring to new growth. This disease
can be serious in rainy seasons.
Pathogen: Gnomonia leptostyla
Black walnut is most severly affected by walnut anthracnose under
natural conditions. Symptoms first appear in late spring as small,
brown, circular lesions, typically surrounded by a yellow margin,
on the under side of leaves which have attained full size. Lesions
are soon apparent from both the lower and upper leaf surfaces. Extensive
leaf spotting causes yellowing, curling and premature defoliation.
Lesions may also appear as dark, sunken spots on the nut husks.
Affected nuts may have reduced meat quality and drop prematurely.
The severity of walnut anthracnose tends to increase as the summer
progresses, especially if rainfall is frequent. This disease usually
does not significantly affect tree growth, however, as most of the
year's growth is complete by the time symptoms become severe.
Rake and remove
fallen leaves and twigs from the site to reduce the overwintering
population of anthracnose. Prune out dead twigs and branches Wet
leaf surfaces promote establishment of this disease--improve air
circulation and sunlight penetration by thinning crowded stands
and pruning. Maintain health of trees by providing adequate water
and fertilizer. Healthy trees are more likely to recover from a
severe anthracnose infection than stressed trees. Chemical sprays
to control anthracnose are rarely justified except when the disease
occurs in stressed or recently transplanted trees, or when the disease
causes repeated defoliations.
from the University of Nebraska Cooperative Extension, 1999
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program