Sudden Oak Death: Causes, Treatment, and Prevention

Last modified on: August 27, 2021


Is your tree dying? Is the beautiful foliage you love falling off These may be signs your tree is suffering from Sudden Oak Death. 

According to recent estimates, Sudden Oak Death has killed more than 1 million oak and tanoak trees over the last ten years. Sudden Oak Death isn’t just limited to California forests, either– the syndrome often affects residential trees in the yards and gardens of homeowners in The Golden State. 

Your oak trees are precious, though, and you spend so much time planting and tending to them.

Can they be saved?

These are questions we will answer in this blog

Fortunately, the team here at Smith’s Pest Management offers a variety of tree disease prevention services designed to help your oak trees stay healthy and strong for years to come, and we know a thing or two about this deadly tree disease. 

Let’s dive in. 

sudden oak death management

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What is Sudden Oak Death?  

Sudden Oak Death is a disease that affects oak trees. It is caused by a pathogen known as phytophthora ramorum, which impacts trees and a variety of horticultural shrubs that are common in California.

Today, Sudden Oak Death extends throughout California’s coastal counties–including Humboldt and Monterey–and all the way up to southwest Oregon. 

Once the trees have been weakened by the disease, many suffer additional damage in the form of secondary bark beetle attacks.

The combined forces destroy trees in many of California’s most beautiful wild environments. Since California’s forests are an important ecosystem for nutrient cycling,  soil formation, water filtration, carbon storage, and the prevention of erosion, Sudden Oak Death can quickly wreak havoc on ecosystems. 

sudden oak death on tree

What Causes Sudden Oak Death?

The cause of Sudden Oak Death is an oomycete known as phytophthora ramorum. Sudden Oak Death is transmitted by this non-native pathogen, which was introduced into the US via nursery stock.

The pathogen is considered an invasive species and has resulted in mandatory quarantines for nursery plants in over 60 countries, with restrictions enforced on shipments of nursery stock and other host plant materials. 

First discovered in the mid-1990s, Sudden Oak Death impacts certain oak species more than others–specifically coastal live oak, Notholithocarpus densiflorus, tanoak and Quercus agrifolia, which is a close relative to the oak tree. When left unchecked, the disease can quickly devastate forests. These oak tree diseases can also spread to and damage plants like rhododendrons, camellias, and more. 

Phytophthora ramorum spreads when infested soil and plant materials move from place to place. As such, there are a variety of federal and state regulations in place to control the potential spread of the pathogen to uninfected areas. 

Currently, the California Department of Food and Agriculture (CDFA) and the US Department of Agriculture Animal and Plant Health Inspection Service (USDA–APHIS) carefully regulate and control the movement of known host species. 

How Does Sudden Oak Death Kill Trees?

dying tree due to sudden oak death

The plant pathogen that causes Sudden Oak Death thrives in cool, moist environments. When it affects shrubs, the shrub drops its leaves, but the plant typically survives. Meanwhile, the spores present on the shrub leaves are blown by wind or splashed by rain onto the trunks of nearby oak trees. 

When Sudden Oak Death infects trees like tanoak, coast live oak, Shreve’s oak, California black oak, and canyon live oak, it enters the trees through the trunk. From there, Sudden Oak Death appears to kill the trees by girdling their main stem or trunk

The fungus moves into the living bark layer, spreading around the tree’s circumference, cutting off the nutrients of the trees to the roots, and eventually killing the roots themselves.

Eventually, the upper tree dies from dehydration. While Sudden Oak Death does not always kill infected trees, its mortality rate is very high. 

What are the Signs and Symptoms of a Dying Oak Tree?

signs of dying tree

Symptoms of Sudden Oak Death include leaf spots and shoot dieback. To identify a tree affected by Sudden Oak Death, look for these symptoms:

  • Leaf spotting and twig dieback
  • Cankers and calluses on the bark or wood of the tree, which may seep a black or red ooze

Once these symptoms are visible, the tree will typically die in two years or less. The most severe symptom is cankers on the trunk of the tree.

Generally, the severity and concentration of the cankers on the trunk indicates how serious the Sudden Oak Death infection is, and whether the tree will die. 

How Do You Test for Sudden Oak Death?

While the impact of Sudden Oak Death is severe, testing for it can be challenging.

This is because many of the symptoms caused by the pathogen responsible for Sudden Oak Death are similar to conditions caused by other plant pathogens, insects, and even poor environmental conditions. 

As a result, the only way to confirm a Sudden Oak Death infection is to harvest a sample of the affected tree’s trunk or bark and have it analyzed in a lab

For more help diagnosing and identifying a possible Sudden Oak Death infection, we recommend consulting either the Wildland Diagnostic Guide or the Nursery Diagnostic Guide.

Both of these guides will help you identify possible infections and know when it’s time to send a sample to a lab. 

Sudden Oak Death Treatment, Management & Prevention

pruning diseased oak tree

While there is currently no cure for the Sudden Oak Death infection, you can stop the spread of the disease and take proactive steps to protect your trees. 

6 Practical Tips to Manage Trees Affected by Sudden Oak Death

If you have a landscaping oak tree affected by Sudden Oak Death or another disease, you can take steps to prolong the tree’s life. 

We recommend the following tactics to manage the disease:

  1. Prune and safely dispose of diseased branches (refer to the section above for more detail on how to dispose of affected clippings)
  2. Spray diseased areas of the tree with fungicide
  3. Consider injecting fungicide treatments into the trunk of the tree
  4. Fertilize the tree
  5. Add 2-3” of mulch to the area at the base of the tree
  6. If the tree is in boggy ground, dig drainage ditches to help the soil drain properly 

Here’s one more thing to keep in mind:

If you have a tree infected with Sudden Oak Death, it may stay reasonably healthy for months or even years. Still, it’s important to consider the long-term future of the tree.

If the tree’s location means it will eventually present a hazard to human safety or property, it’s wise to remove the tree now, to prevent it from falling on your roof later–once it becomes very sick.

How to Prevent Sudden Oak Death 

prevent sudden oak death

With Sudden Oak Death, as with all plant diseases, preventing the condition is easier than curing it. With that in mind, here are our top tips to keep Sudden Oak Death from impacting your trees in the first place:

Inspect all nursery plants carefully

Before you purchase a possible Sudden Oak Death host plant, inspect its leaves and trunk very carefully. Be aware that some nurseries use a fungicide that can mask symptoms of the infection, so even an infected tree may look healthy. 

Even if the plant doesn't show signs of infection, consider keeping it quarantined (unplanted) in an area of your yard that tends to be moist for eight weeks to see if symptoms develop.  

Remove host trees and plants

One of the best ways to prevent future Sudden Oak Death outbreaks in your garden is to remove host plants that are located near healthy yet susceptible oaks. 

For more information about which plants to remove, contact Smith’s Pest Management. Our team will evaluate your property and help create a customized management plan that protects your existing trees, plants, and other landscaping elements. 

Dispose of plant debris properly

While Sudden Oak Death has affected trees and horticultural plants in California for upwards of a decade, there are still areas of the state that are uninfected. Unfortunately, improperly transporting plant materials is one of the fastest ways to spread the disease to these uninfected areas. 

As such, it’s critical to dispose of plant debris and infested material correctly. The best option is to reuse plant matter on the property–grinding or chopping twigs and leaves for use as ground cover, or burning larger piles. 

Clean and disinfect all pruning equipment

After you’ve pruned the plants in your yard or garden, take care to clean and disinfect your pruning tools with a mixture of Lysol or bleach and water. This prevents pathogens from spreading to an uninfected area. 

You should also clean your truck or car and shoes of any dirt, leaves, mud, or woody debris before you leave an infested site. 

Apply preventative treatments

There are certain phosphonate fungicides approved as a preventative treatment for Sudden Oak Death on individual trees. 

While this treatment is effective, it’s not recommended for areas where the pathogen isn't already present. Additionally, the treatment will not cure an existing Sudden Oak Death infection. Instead, it helps protect nearby trees from infection and can suppress the disease in its early stages. 

The mixture is designed to coat the trunk of the tree or be injected into the wood, and requires the administration of annual booster treatments.

To determine which trees require treatment with the preventative mixture,  look for healthy tanoak trees within 150 feet of other infested plants. 

Apply insecticides

Applying insecticides won't help combat or prevent the onset of Sudden Oak Death, but they can help mitigate damage from secondary bark beetle attacks. If you choose to apply an insecticide, only apply it to trees that aren't in the advanced stages of the disease, and don't expect it to prolong the life of the tree for long.  

Want to Save your Dying Oak Tree in the San Francisco Bay Area? We’re Here to Help!

Sudden Oak Death has caused widespread damage to California oak trees over the last decade. Fortunately, you don’t have to stand by as the trees on your property succumb to the disease. 

Here at Smith’s Pest Management, we help residential and commercial property owners through Northern California–from Marin to Monterey–mitigate the spread of Sudden Oak Death.

If you think your tree may be suffering from Sudden Oak Death, the best thing you can do is consult with an expert quickly. The team of certified arborists here at Smith’s Pest Management will evaluate and diagnose your tree and determine which steps to take to prevent the spread of the infection. 

Ready to learn more? Contact us today.


Author Bio: Zach Smith

zach smith, owner of smith's pest management

Landscape Pro Turned Gopher Pro: Owner, Zach Smith, graduate of Cal Poly’s Horticulture program worked nine years as a landscape professional- dealing with gophers, moles, and ground squirrels and was quickly recruited by other local gardeners. Fast forward to the past 10+ years, where Zach and his team trap and remove burrowing pests from residential, municipal and commercial properties throughout the San Francisco Bay area, from Marin to Monterey.

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