Diseases of Trees

Dutch elm disease 

Here in WA State: Yes 
Infects: American elms 
Symptoms: yellowing and wilting leaves which can be seen from May to October 
Description: This is a disease caused by an invasive fungus, but it is moved by both native and non-native elm bark beetles. The larvae, the life stage after egg, feed on the inner bark which creates tunnels called “galleries.” Adults then emerge carrying the fungus to other healthy trees. This disease acts quickly and can kill a tree within a few months to several years. 
Treatment: Trained arborists can use fungicide injections to prevent disease. If a tree is infected, removing branches with wilting leaves may prevent the fungus from moving to the rest of the tree. Planting trees resistant to Dutch Elm Disease, such as elm cultivars that are Dutch Elm Disease resistant, can reduce the spread of the fungus. 

Sooty Bark disease 

Here in WA State: Yes 
Infests: Maple species primarily, horse chestnut, pacific dogwood, cherry plum 
Symptoms: The disease is caused by a fungus, Cryptostroma corticale and doesn’t produce symptoms that are distinct and easy to identify. Wilting, dieback, root and trunk sprouting and more can occur. 
Description: Inside the bark, black mats of fungus spores form giving it the name “sooty bark”. The bark looks like it has charcoal, soot, or other dark gray colored substances on it. There are many lookalikes so you will need to send spores into a lab to confirm the presence of the fungus. 
Treatment: There is no current treatment for sooty bark disease. It is important to note that Maple Bark Disease is a human illness that can be caused by breathing in the Cryptostroma corticale spores. There is a low risk of breathing in wind-blown spores. “Exposure risk increases when handling or working with infected wood. In Europe, Sooty Bark Disease outbreaks follow hot summers. Trees in cities with high pollution levels and higher temperatures are at risk of greater infection rates. Based on information from Europe, Sooty Bark Disease may become more common in the Northwest as changes in climate cause warmer, drier summers.” - Washington Dept of Health.

Sooty bark disease Photo credit: JM Hulbert, WSU

Pear Trellis Rust 

Here in WA State: Yes
Infests: Pear trees both edible and flowering varieties, junipers 
Symptoms: Yellow to bright orange spots appear on leaves, fruit, twigs, and branches. 
Description: This fungus, Gymnosporangium sabinae, is the cause of the disease affecting pear and juniper. It is found throughout the PNW from British Columbia down to Northern California. This annual infection can cause fruits to mummify and die. The leaf lesions can affect the trees ability to make food and will cause the tree to decline. 
Treatment: Chemical control of pear trellis rust is possible but not recommended. Contact an ISA certified arborist to assist you in selecting and applying a fungicide if you choose this method. Due to Pear Trellis Rust, Trees for Seattle discourages planting pears and doesn’t distribute pear trees through its planting program, Trees for Neighborhoods. 

Other diseases:

Maple: Verticillium 
Apple Scab 
Cherry- Blossom Brown Rot 
White Pine Blister Rust