All trees, both deciduous (hardwood) and coniferous (softwood) can be harmed by fungi and moulds. Many common tree diseases are caused by microscopic pathogens called fungi. Since they are unable to make food because of a lack of chlorophyll, fungi must derive food from nutrient-producing green plants.
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Deciduous Leaf Diseases
Powdery mildew and sooty mould are two fungi that affect the leaves of deciduous trees. Powdery mildew appears as a white, powdery substance on the leaves of infected trees. Sooty mould grows on the honeydew extracted by sucking insects, and it appears on the leaves as a brown mould.
Cankers are wounded, dead or blistered areas in the bark, branches and the trunk of infected trees. Oak wilt is an example of a canker-causing fungi disease that infects deciduous trees.
Deciduous Root Disease
Verticillium wilt is a common soil-borne fungal disease that enters deciduous trees through the roots. This disease clogs the vascular system, makes leaves wilt and causes whole branches to die. Dutch elm disease and armillaris root disease has killed millions of trees in North America.
Coniferous Needle Diseases
Needle cast tree disease and needle blight disease cause coniferous trees to lose their needles. Rust fungi sometimes develop as orange, gelatinous growths on infected juniper trees. These trees sometimes become infected by rust-diseased deciduous trees growing near them.
Coniferous Canker Disease
Canker diseases infect coniferous trees in much the same way they infect deciduous trees. Douglas-firs and Japanese larch are sometimes infected with papery bark syndrome when fungi cankers grow in a wounded area and the bark becomes paper thin and peels off.
Coniferous Root Disease
Some coniferous trees develop decay when fungus enters through wounds in the roots. These fungi often travel from one tree to another, using airborne and soil-borne infected stumps and root systems as their infection vehicles.
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