Tree Diseases

Diseases Prevention and Treatment

Dutch Elm Disease

Also known as DED, Dutch Elm Disease is an invasive fungus that can be spread by root grafts (roots connected between similar species near each other) or by Elm Bark Beetles. The beetles can feed on infected trees then fly to a healthy tree to feed, depositing the spores it carried from the infected tree. The fungus can spread from root grafts by exchanging of water from an infected tree to a healthy tree, then as the water spreads through the tree, so does the fungus. Elm's response to the invasion is to block off water supply in parts of the tree eventually leading to wilting and death. 

Prevention measures would include removing both infected trees and removing exposed trees, always confirm with a forester. Using a vibratory plow to break up the root grafts to minimize the continuous spread is another wonderful option. After removal any wood from infected trees should be buried or burned to prevent further spread. Fungicide injections are an option for trees where root grafts infections aren't a concern as they are not effective in root graft protection. Additionally Elm resistant tree species are not immune to DED and can still contract the fungus, they have just shown resistance. 

Dutch elm disease | UMN Extension

Oak Wilt

This disease is caused by an invasive fungal pathogen that is dispersed by root grafts and sap beetles. Fungal mats (exposed spores that jut out from the tree), produced by infected trees, emit a sweet smell to attract sap beetles who then pick up fungal spores. The beetles are then attracted to the smell of an injury (something as simple as a snapped branch) on a healthy tree, then transferring the spores into the healthy tree. Root grafting is a very common Oak practice and has been known to occur between different species of Oaks. Oak wilt is spread in a similar way as DED via root systems, through the vascular system from water and nutrients trading. 

Preventative matters for oak Wilt include being mindful of when to prune. Do NOT prune April-October. Of course, injuries and emergencies happen so if you notice an injury or you have to prune, immediately paint over the open wound (water-based paint is ideal). Breaking up root graft connections, with a contractor, is a great way to save the trees that haven't been exposed. The infected tree's wood should not be moved from the site during unsafe pruning months; they can also be burned. Fungicide is a method that works for above ground protections, but it does not prevent root graft infections or with Red Oaks in particular, that already have symptoms, fungicide unfortunately will not save your tree. 

Oak wilt in Minnesota | UMN Extension

Fire Blight

A disease that's caused by a native bacterium that travels through tree's vascular systems. Fire Blight is spread by Trees becoming damaged from natural disasters like storms, hail, strong winds, or heavy rain. The wounds create moist spots for the bacteria to enter the tree. The fungus creates cankers in the infected branches and these cankers then release a sweet smell that attracts insects. These insects who are carrying the spores then deposit them onto a healthy tree with a wound or even to its flowers. Once the bacteria enter the trunk of the tree, it is lethal. The trees that are frequently targeted are Apple, Crabapple, and Mountain Ash trees. However, Raspberries, Serviceberries, Hawthorns, and invasive Cotoneaster have also been known to be susceptible to Fire Blight.

Prevention methods for Fire Blight mostly surround tree care and observations. If the infection is caught early enough, you can remove the branch to prevent further spread of the disease. When it comes to pruning, do NOT prune before or after late winter to early spring, and always sterilize tools between uses. The diseased wood should not be removed from the site and should be burned or buried after removal. When pruning, make sure the tree canopy is open (as apple species can grow a condense canopy) to allow air to dry the canopy after storms to avoid infections. Pesticides are typically unnecessary and harmful to pollinators and birds. 

Dutch elm disease | UMN Extension

Pruning trees and shrubs | UMN Extension (check out their section on pruning apple trees)