Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001666
11/12/99
Fungal And Bacterial Leaf Diseases:
Symptoms: Leaf spot, tar spot, leaf blotch, leaf blister
and shot hole are symptoms of leaf diseases. Shot hole
occurs when dead leaf tissue falls out, leaving a hole.
Cause: A number of leaf diseases that occasionally damage
ornamental plants are caused by fungi and bacteria. Many
species of fungi cause leaf diseases. Leaf diseases cause
leaves to fall prematurely, leaving trees and shrubs
without the green factories that produce food for them.
Repeated defoliation of trees or shrubs makes them
progressively weaker. Most deciduous plants can withstand
several defoliations without serious damage, but broad-
leaved and narrow-leaved evergreens usually do not recover
from a defoliation.
Most fungi and bacteria that cause leaf diseases require a
wet leaf surface for an extended time, usually about 24
hours. The wet leaf surface allows the fungal spores to
swell, germinate and penetrate the plant and the bacteria
to swim to a natural opening in the leaf surface, such as
a stomate.
Leaf-infecting diseases are more severe if the fungus or
bacterium is present at bud break, when the leaves are
tender and new. If the weather is dry during bud break,
infection occurs later in wet weather after leaves are
expanded. Late infections may be unsightly but seldom
harm the plant.
Bacterial Leaf Spots
Symptoms: Bacterial leaf spots may appear similar to
fungal leaf spots, so it may be difficult to distinguish
the two. Leaf spots caused by bacteria are often
initially light green and look "water-soaked". Later,
these leaf spots usually turn brown or black and may have
definite margins.
Cause: Many species of bacteria may cause bacterial leaf
spots of landscape plants. These bacteria are often
splashed from the soil onto wet foliage, where they enter
a leaf through its breathing holes (stomates) or wounds.
Thereafter they spread from leaf to leaf when plants are
watered or during rainy periods. Bacterial leaf spot of
English Ivy is a good example of a common bacterial leaf
spot.
Control: Avoid using high nitrogen fertilizers on
susceptible cultivars. Only fungicides containing copper
may be effective against bacterial blights and bacterial
disease, but copper may burn foliage in some cultivars.
The antibiotic streptomycin is used in commercial
agriculture, but is not available to the homeowner.
Fungal Leaf Spots
Symptoms: Leaf spots are characterized by brown or black
spots randomly scattered across the leaf. The spots may
or may not have the appearance of concentric rings. The
margins of the spot may be a different color than the
center of the spot.
Cause: Leaf spots are caused by many different fungi that
attack landscape ornamentals. Rainy periods favor their
sporulation and spread to unaffected foliage. Infection
may occur at any time, but the disease is usually most
severe on new, emerging foliage during rainy periods.
Control: Fungi that cause leaf spots form spores on the
leaves. The new spring infections are commonly caused
by spores from last fall's leaves. Therefore, rake and
burn or compost last season's leaves before buds break in
the spring. Help foliage dry quickly after rain, heavy
dew, or sprinkler watering. In the case of deciduous
trees and shrubs, prune out overthick growth for better
penetration of solar radiation and better air circulation
to aid in drying foliage. In the case of conifers, it is
most important to control weeds that surround the lowest
branches. Weeds inhibit air circulation and increase
humidity, favoring fungal infection of needles.
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to commercial products or trade names does not imply
endorsement by MSU Extension or bias against those not
mentioned. This information becomes public property upon
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This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
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