Even the most conscientious and hardworking gardener
is likely to encounter leaf spot problems on trees and shrubs. The seemingly
sudden appearance of brown or black blotches on leaves and defoliation
are common occurrences. It is unlikely that most homeowners will make
it through a season without at least one problem with a leaf spot pathogen.
Symptoms
Symptoms of leaf spots vary depending upon the causal agent. Although
leaf spots can be caused by air pollutants, insects and bacteria et al.,
most are a result of infection by pathogenic fungi. Once into the leaf,
the fungi continue to grow and leaf tissue is destroyed. Resulting spots
vary in size from that of a pinhead to spots that encompass the entire
leaf. Dead areas on the leaves are usually brown, black, tan or reddish
in color. Occasionally the necrotic areas have a red or purple border.
Partial to complete defoliation may occur under favorable conditions for
the causal fungus.
Life Cycle
Many of the leaf spot fungi have a similar life cycle. The causal fungus
over-winters on fallen leaves. In the spring, during or following a rain,
spores produced by the fungus are discharged and carried by the wind and
splashing rain to newly emerging leaves. The spore germinates and penetrates
these young tender leaves causing infection. In a few days to several
weeks, depending on temperature, small spots appear on the leaves. As
the fungus grows, the spots enlarge. The fungus in the spots may produce
more spores. These spores are capable of causing secondary infections
on other leaves.
In general, the leaf spot fungi are favored by cool, wet weather early
in the growing season. Leaf spot diseases are seldom a problem following
warm, dry weather in the spring.
Hosts
All commonly grown trees and shrubs are subject to attack by one or more
leaf infecting fungi. Although coniferous trees (needled evergreens) can
be severely injured by leaf spot fungi, they are rarely attacked in successive
years. Therefore, control measures are rarely required. Many different
fungi cause a variety of symptoms on hardwood trees and shrubs. Oak, maple,
sycamore, ash, walnut, hickory and horse chestnut are some trees commonly
attached by the anthracnose fungi. Anthracnose is caused by several species
of closely related fungi that produce brown or black lesions on leaves.
Distortion of the leaves and defoliation usually result. Another leaf
spot fungus will often completely defoliate susceptible hawthorns such
as Paul's scarlet and English varieties by midsummer. Leaf blister of
oak is common following cool, wet spring weather. Many circular raised
blisters are scattered over individual leaves. Although unsightly, there
is little or no damage to affected trees. Symptoms of fungal leaf spots
on elms vary from small, black, pinhead lesions to brown blotches covering
an extensive portion of the leaf.
As many as ten different leaf spot fungi can be found on rhododendron.
Although unsightly, they rarely cause serious injury. The above are a
few of the hundreds of leaf spot problems likely to be observed by the
home gardener.
Control
In many cases, the home gardener becomes overly alarmed when encountering
a severe leaf spot problem. A common reaction is to run for the sprayer
and quickly apply a chemical to the ailing tree. Usually this is a waste
of time and money. The majority of trees and shrubs have learned to live
with leaf spot diseases. Even severe defoliation will not cause the death
of an otherwise healthy tree. Also, by the time symptoms of leaf spot
are obvious, it is often too late to apply a chemical for control. Trees,
which are subject to serious injury when attacked by a leaf spot fungus,
are those trees that are under stress. This might include recently transplanted
trees, trees growing under droughty conditions or trees weakened by continuous
insect attack. The additional stress of a leaf spot disease on an already
weak tree may cause permanent injury or death. In such cases, chemical
control of leaf spots is often recommended in the spring. In order to
be effective, the proper fungicide must be applied as a protectant before
the fungus spore is disseminated to the leaf. Most leaf spot fungi infect
trees early in the spring just as the leaves are unfolding.
Successful control usually requires two to three spray applications. In
general, the first spray is applied at bud break and the second seven
to fourteen days after that. A third spray might be necessary, particularly
during rainy periods. The more rain the more frequent the spray applications
must be. Since many of the leaf spot fungi over-winter on fallen leaves,
one cultural method of reducing the severity of leaf spots is to rake
and remove from your yard all old leaves under the tree. This will reduce
the number of fungal spores available to infect developing leaves in the
spring. Disposing of old leaves is not likely to be effective if leaves
from the same species of tree or shrub in your area are not disposed because
spores of most of the causal fungi can be wind disseminated for long distances.
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271.
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