Trees are integral to our environment, supplying considerable benefits such as clean air, shade, and aesthetic beauty. Nevertheless, like all living organisms, trees are susceptible to diseases that can threaten their health and vitality. These diseases can have detrimental effects on both urban and forested landscapes. We will investigate some of the most common tree diseases, their causes, symptoms, and management strategies. Comprehending these diseases is paramount for conserving and caring for our valuable tree resources. Are you looking for professional tree services in Portland, Oregon? Look no further, as our team of certified arborists is here to provide expert care for your trees and address any tree-related concerns you may have.

Most common tree diseases

1. Dutch Elm Disease

One of the most notorious tree diseases is Dutch Elm Disease (DED), which has ravaged elm trees across North America and Europe. DED is caused by the invasive fungal pathogen Ophiostoma ulmi, primarily spread by bark beetles. The disease clogs the tree’s water-conducting vessels, leading to wilting and dieback of branches. Infected trees often display yellowing and browning of leaves, starting at the tree’s upper branches and progressing downward. DED management involves removing and destroying infected trees to prevent further spread. Fungicides can be injected into healthy elms to protect them, and resistant elm cultivars have been developed to mitigate the disease’s impact. Vigilant monitoring and early intervention are essential for managing Dutch Elm Disease.

2. Oak Wilt

Oak Wilt is a deadly disease that affects oak trees, particularly species in the red oak group. Caused by the fungus Ceratocystis fagacearum, it is transmitted through root grafts and sap-feeding beetles. Oak Wilt disrupts the tree’s vascular system, causing leaves to wilt, turn brown, and fall off. Infected trees often die within a few months. Managing Oak Wilt involves trenching to break root connections between healthy and infected oaks and pruning and removing infected trees. Preventative measures include avoiding pruning during the growing season when beetles are active and using wound dressings on freshly cut oak wood to deter beetles. Using fungicide injections can also help protect high-value oak trees from infection.

3. Emerald Ash Borer (EAB)

The Emerald Ash Borer is not a disease but rather an invasive beetle that has caused significant damage to ash trees in North America. These metallic-green beetles lay their eggs in ash tree bark, and their larvae bore into the tree, disrupting its nutrient and water transport systems. Infested ash trees exhibit canopy thinning, bark splitting, and a distinctive “D”-shaped exit hole left by emerging adult beetles. Management of EAB includes the removal and disposal of infested ash trees, the use of insecticide treatments to protect valuable ash trees, and the promotion of biological control methods, such as releasing parasitic wasps that target EAB larvae. Additionally, efforts are made to monitor and regulate the movement of ash wood and products to prevent the spread of this destructive insect.

4. Apple Scab

Apple Scab is a fungal disease that affects apple and pear trees, causing cosmetic damage to fruit and leaves. The pathogen, Venturia inaequalis, overwinters in infected leaves and releases spores in the spring, which can infect new leaves and fruit. Infected leaves display dark, scaly lesions, and fruit may develop scab-like blemishes. Managing Apple Scab involves cultural practices, such as pruning to improve air circulation and removing infected leaves and fruit. Fungicide applications are also commonly used to protect trees from infection. Selecting disease-resistant apple and pear cultivars can be an effective long-term strategy for managing Apple Scab.

5. Sudden Oak Death

Sudden Oak Death (SOD) is caused by the water mold Phytophthora ramorum and primarily affects oak species, as well as other trees and shrubs. SOD has been responsible for significant tree mortality in California and Oregon. Symptoms include bleeding cankers on tree trunks, leaf discoloration, and dieback of branches. Management of Sudden Oak Death includes removing and disposing of infected trees and implementing quarantine measures to prevent the movement of infected plant material. Fungicides can be utilized as a preventive measure in high-risk areas. Additionally, efforts are made to study and understand the biology and spread of the pathogen to develop more effective control strategies.

6. Chestnut Blight

Chestnut Blight is a fungal disease caused by Cryphonectria parasitica and affects chestnut trees, including the American chestnut, once a dominant tree species in eastern North America. The disease causes cankers on the bark, leading to tree girdling, wilting, and death. The introduction of the chestnut blight fungus from Asia in the early 20th century devastated American chestnut populations. Management of Chestnut Blight has primarily focused on breeding programs to develop disease-resistant chestnut varieties. Efforts are also made to monitor and control the spread of the disease in remaining chestnut populations. The goal is to restore the American chestnut’s role in eastern forests.

Tree diseases threaten the health and vitality of our forests and urban trees. Understanding the causes, symptoms, and management strategies for common tree diseases is crucial for effective tree care and conservation efforts. Arborists and tree care professionals play a paramount role in diagnosing and managing tree diseases, helping to protect our valuable tree resources, and maintaining the ecological balance of our natural landscapes. By implementing preventive measures and early intervention, we can mitigate the impact of these diseases and work towards healthier and more resilient tree populations.


Northern girl Laura is the epitome of a true entrepreneur. Laura’s spirit for adventure and passion for people blaze through House of Coco. She founded House of Coco in 2014 and has grown it in to an internationally recognised brand whilst having a lot of fun along the way. Travel is in her DNA and she is a true visionary and a global citizen.

Comments are closed.