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University
of Rhode Island GreenShare Factsheets
Leaf
Spot Diseases of Ornamental Trees & Shrubs
Fungal leaf spots are perhaps the most prevalent plant
diseases in the Northeast. They are generally favored by cool,
wet weather. Although unsightly in appearance, most leaf spot
diseases result in little damage to the host plant. Following
are descriptions of some of the more common ones.
Pathogen: Botryosphaeria obtusa
This leaf spot disease is a minor
problem on most crabapple trees. Small brown spots with purple
borders develop on leaf
surfaces following moist spring weather. Some spots enlarge,
developing irregular brown lobes. The original spot turns gray,
causing the appearance of a "frog eye."
B. obtusa also causes black rot cankers on weakened
crabapple tree limbs, especially following winter injury. Infection
may be superficial (confined to the bark) or may extend down
into the wood and grow perennially. Prune out and destroy branches
with black rot cankers to prevent release of spores.
Pathogen: Taphrina caerulescens
Oak leaf blister is a common disease which primarily
affects red oaks, although it can be a problem on white oaks.
Symptoms appear as blister-like, circular bulges on upper leaf
surfaces (depressions when viewed from the lower leaf surface)
which turn from yellow to brown as they age. Spores are produced
on the upper leaf surface in midsummer, giving the leaves a
powdery appearance. Some of these spores become lodged in bud
scales where they overwinter. The following spring, the spores
germinate and infect leaves as they unfurl. The disease may
be severe when cool, wet weather prevails. As leaves mature,
they become more resistant to infection. Controls are generally
not warranted and fungicide recommendations are rarely made
except in some nursery/garden center situations.
Pathogen: Plasmopara viburni
This is one of the few significant downy mildew diseases
of ornamentals. Symptoms appear as light, greenish spots on
the upper leaf surface that coalesce, forming angular patches
bordered by veins. Infected leaf tissue often reddens, then
browns and shrivels. Corresponding areas on the lower leaf
surface are covered with downy grayish-white fungal mycelium.
The fungus reproduces rapidly and multiple infections
occur during periods of leaf wetness and cool to warm (although
not hot) conditions. This disease is diagnostically different
from powdery mildew of viburnum in that the fungus is present
on lower leaf surfaces. Promote dry leaf surfaces to control
this disease. When fungicides are required, apply a labeled
product containing mancozeb to the undersides of leaves.
Pathogen: Gunignardia aesculi
This disease creates a serious aesthetic problem on most
horse chestnut and buckeye species. However, since much of
the annual growth is completed before symptoms become severe,
damage to the plant is minimal. Leaf symptoms first appear
as water-soaked areas which turn reddish-brown to brown with
yellow borders. These lesions coalesce, causing large blotches
which curl the leaves. By August the whole plant appears scorched.
The fungus overwinters in fallen leaves, producing spores for
new infections in spring, so removal of leaves should be thorough.
As with other leaf spot diseases, infection is enhanced by
moist conditions. Improve air circulation to hasten leaf drying.
If pesticide treatment is desired, apply a labeled fungicide
containing mancozeb or chlorothalonil at bud break during wet
springs. Reapply at intervals specified on the label until
conditions are no longer moist. For new plantings, select plants
with resistance to Guignardia blotch such as bottlebrush buckeye
(Aesculus parvifolia).
Pathogens: Rhytisma acerinum, Rhytisma punctatum
These dramatic, but inconsequential, diseases of numerous
maple species cause tar-like spots on leaves which can reach
almost 24 mm (1 inch) in diameter. Symptoms first appear as
yellowish spots on the upper leaf surface. Later in the summer,
a black, tar-like mesh of fungal and leaf tissue develops within
the yellow spot. Some early leaf drop may occur but is not
considered serious. Fungicide treatment is usually not necessary.
Pathogen: Phyllosticta minima
This disease affects a number of maple species, particularly
silver, red, Japanese and Amur maple. Leaf spots are roughly
circular, tan in color with purple to red borders. Later in
the season, black fruiting bodies of the fungus arranged in
rings appear within the lesions. Since damage to the plant
is minimal, fungicides are rarely recommended.
Cultural management is generally sufficient to keep these
diseases in check. Leaf spot fungi overwinter in and produce
spores for new infections on the fallen leaves, which should
be gathered in the fall and removed from the site or destroyed.
Infection occurs when leaf surfaces are moist; improve air
circulation by thinning crowded plants and pruning overly dense
growth. Avoid overhead irrigation, or water only in the morning
to ensure leaves dry quickly.
Adapted from Stephen Nameth,
C. Wayne Ellett and Jim Chatfield, Ohio State University Extension,
1999
Pesticides
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.
For more
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.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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