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Plant Diseases
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Armillaria
Root Rot
Anthracnose
Pine Pitch Canker
Powdery Mildew
Rust
Sooty Mold
Verticillium Wilt
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Armillaria Root Rot
Description: Armillaria root rot, also called oak root fungus or
shoestring fungus, is a soil-borne parasitic or saprophytic
fungus that occurs naturally in forests and woodlands throughout
the United States. It can live for many years in the soil
on stumps or dead roots. During fall and winter during wet
weather, individual or clusters of honey-colored mushrooms
2 to 5 inches in diameter may appear on the lower trunk or
near infected roots. The fungal strands appear as white fan-like
fungal plaques between the bark and wood. The fungus spreads
through the soil by means of short (under a foot-long) shoe-string-like
fungal strands that can invade new wood when in close contact.
Damage: In addition to breaking down dead wood, armillaria causes
decay of living root and trunk tissue of susceptible hosts.
The symptoms are the stunting of the plant, sparse or yellow
foliage, branch dieback and the slow decline of the plant.
However, in some cases, the decline can be quite rapid, especially
for weakened plants and for plants in heavy, poorly drained
wet soil. Because armillaria can exist in the soil for many
years on dead wood, a tree or shrub may grow nearby and be
unaffected until its roots come into close contact to the
armillaria. The fungus may then spread rapidly through the
root system and kill the plant.
Management: Though about 700 species of plants are known to be
susceptible to armillaria, others are relatively resistant.
Where oak root fungus is known to be a problem, such as on
cleared woodland, plant resistant species if possible. Remove
infected stumps and old roots to eliminate sources of infection.
Because armillaria grows more slowly in dry soil than in wet
soil, native oaks and other plants tolerant of dry soil conditions
are killed more rapidly when they receive summer irrigation.
Avoid summer irrigation if possible. If the root crown is
found to be infected, excavate the soil from the base of the
plant and expose it to the air to dry it out. No fungicide
is effective against armillaria.
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Anthracnose
Description: Anthracnose is a fungal disease that over-winters
on old leaves or cankers on branches. The water-borne spores
are spread by splashing by rain or overhead irrigation. The
fungal spores infect young leaves and branches, causing dead
spots and dieback.
Damage: Dead spots develop where the anthracnose fungus enters
leaves. These spots tend to occur along veins. When spots
run together, they may result in death of the entire leaf.
The damage may resemble sunscorch but is distinguished by
the presence of dark spore-producing structures on the underside
of the leaf. Anthracnose may result in defoliation of some
trees, such as sycamores. If defoliation takes place early
in the season during wet weather, a new set of leaves may
be produced later in the season. However, when trees are repeatedly
infected over successive years, branches may be killed.
Management: Avoid overhead irrigation because this can spread anthracnose
spores. Use mulch to reduce splashing from rain or irrigation.
Prune out infected branches to eliminate a source of spores.
Rake fallen leaves. Water and fertilize trees that have been
defoliated to promote the production of new foliage.
Fungicides can be applied to help control anthracnose. These include
lime-sulfur, copper, chlorothalanil, zineb and maneb, among
others.
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Pine Pitch Canker
Description: Pine pitch canker is a fungal disease of pines that
has been introduced to California from the southeastern United
States. It is a serious disease of Monterey pines and other
pine species. Fungal spores are introduced into trees by bark
beetles or pruning wounds. The fungus grows through the vascular
tissue of the tree, blocking the water conducting tissue.
In addition to symptoms of dieback, the disease can be identified
by the pitch streaming down the trunk from the cankers.
Damage: Pitch canker causes dieback of branch tips and may kill
large branches or the entire tree. The symptoms often begin
on branches with cones because it is vectored by beetles that
attack cones. Thousands of Monterey pine trees have been killed
by pitch canker, including trees in native groves. A small
percentage of Monterey pines appear to be resistant. Also,
some pine species, such as Canary Island pine, Brutia pine
and Italian stone pine, appear to be less susceptible to pitch
canker and have little damage.
Management: Prune pines in the winter when insect that transmit
the fungus are not active. Avoid planting Monterey pine and
other highly susceptible species. No fungicide is effective
in controlling the disease.
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Powdery Mildew
Description: Powdery mildew is a disease caused by fungi that grow
on the surface of a plant. The fungal strands produce a grayish
white powdery coating on the leaf. Mildew is favored by warm
dry daytime conditions and cool humid nighttime conditions.
Damage: Infected leaves may become distorted and in severe cases
may turn yellowish or reddish brown and drop. New growth may
be stunted and flower bud may fail to develop. On oaks and
other plants it may produce a stunting and distortion of new
growth called withes' broom.
Management: Because mildew on some plants is favored by shady conditions,
avoid planting susceptible plants in the shade. Overhead watering
early in the day may wash off mildew spores and reduce infection.
Powdery mildew can be controlled by applying sulfur or a number
of other fungicides, such as triforine and chlorothalanil.
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Rust
Description: Rust is called by a number of types of fungi, which
tend to be specific to one or a few plant hosts. Often rusts
require two different types of host plants to complete the
life cycle. Among the plants susceptible to rust are turfgrasses,
roses, snapdragon, geranium hollyhock and mahonias. Infection
takes place during wet weather, and these develop into pustules
on the under sides of leaves. These pustules will produce
millions of rust-colored microscopic spores.
Damage: Leaves infected with rust may be spotted or mottled in
color and may become twisted and distorted. In severe cases,
leaves may fall off.
Management: Remove infected leaves to reduce production of spores.
Avoid overhead watering. Water early in the day so that foliage
has a chance to dry out. A number of fungicides help to control
rust.
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Sooty Mold
Description: Sooty mold is caused by several fungi that grow on
honeydew, a sugar excretion of aphids, scale, mealybugs and
other insects. It appears as a dark brown or black coating
or spots on leaves, stems and other plant parts.
Damage: Sooty mold does not attack the plant directly. It grows
on the honeydew produced by insects that are feeding on the
plant. However, the fungal coating can block light absorption
by the leaf and limit photosynthesis. In severe cases, leaves
may turn yellow and drop.
Management: The most effective way to control sooty mold is to
control the insects producing honeydew. An application of
insecticidal soap will help to control these insects and also
wash off some of the sooty mold fungus.
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Verticillium Wilt
Description: Verticillium is a soil-borne fungus that attacks a
wide variety of plants. It can persist for many years in the
soil. The fungus invades the plant through the roots and spreads
up the trunk and into the branches through the water-conducting
tissue. It can be seen as streaking or brown discoloration
of the wood under the bark.
Damage: Verticillium plugs the water-conducting tissue, causing
discoloration and wilting of the foliage. This may result
in branch dieback. Often branches will die on one side of
the plant.
Management: Plants stressed from too much or too little water are
more susceptible to verticillium wilt. Over-fertilization
may also increase susceptibility. If plants are given favorable
conditions, including proper watering and fertilization, they
may recover from infection by verticillium. Because the fungus
persists in the soil for many years, plants resistant to verticillium
should be planted if it has been a problem in the past.
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