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
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
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
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.
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,
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
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.
is generally sufficient to keep these diseases in check. Leaf spot
fungi overwinter in and produce spores for new infections on
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.
from Stephen Nameth, C. Wayne Ellett and Jim Chatfield, Ohio State
University Extension, 1999