Rust Diseases of Woody Ornamentals
Rust Diseases of Woody Ornamentals
Rust diseases are caused by fungi that form orange-rust colored spores, thus giving this group its name. Many conifers and deciduous trees and shrubs are injured by rust diseases. Most rust fungi require two unrelated plants to complete their life cycle. The other host plant is usually called an alternate host. Some rust fungi such as western gall rust have only one host. Rust fungi cause cankers, witches brooms, galls, and leaf spots. Thus these diseases can kill trees if a canker or cause visual problems and stress trees if the disease causes leaf spots, brooms or galls.
Rust fungi spread from host to host by windblown spores. Infection almost always takes place through leaves. Local climate conditions and location of the alternate host affects how many infections will take place. High humidity and rain is needed for spore production and infection. The figure below describes a typical life cycle of a fungus that requires two hosts.
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