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Plant Diseases

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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