How to keep oaks safe from sudden oak death, other diseases

First the bad news: In addition to wildfires, there are some gnarly diseases attacking the beautiful oaks in our home landscapes.

And now the good news: There’s a lot you can do about it.

As the old saying goes, “An ounce of prevention is worth a pound of cure.” That’s why Jess Running, district manager for Napa and Sonoma counties for Davey Tree Expert Company, says “The best thing that homeowners can do is to give their oaks the best possible cultural practices.”

Those practices include, first, identifying the varieties of oaks in their landscapes, and then applying the right irrigation, mulch, fertilizers, and pruning.

Be aware that different kinds of oaks - there are more than 400 species in the world - have different needs. You don’t have to know them, but you should know the types that grow on your property. Or, if you’re thinking of planting an oak sapling you might consider varieties that aren’t susceptible to the worst oak diseases.

“The worst disease is sudden oak death,” Running says. “That’s the number one bad guy. And it’s a pathogen that’s native to our region.” SOD, as it’s known, is the common name for a fatal tree disease caused by Phytophthora ramorum, a species of oomycetes similar to a fungus. (In case you’re wondering, oomycetes is pronounced oh-oo-my-seats.)

Their microscopic spores break dormancy and send out long filaments that are either male or female. When these filaments meet, they reproduce sexually. They can also reproduce asexually by splitting themselves apart. Once the disease gets going, first limbs and then a whole tree will simply die with its dead leaves still clutched to the branches.

Fortunately, the lovely valley oaks and blue oaks in the North Bay are not susceptible to the disease. Unfortunately, the susceptible varieties are black oak, California live oak, tanbark oak and red oak. A culprit in the development of sudden oak death, Running says, “is our California bay laurel tree. It’s the only tree that can build up enough of the pathogen to threaten the susceptible oaks. So the closer your oak tree is to a bay laurel, the higher the risk of infection.” If you value your oak more than the bracing fragrance of the bay laurel, consider removing the laurel.

But sudden oak death, which gets all the press, is not the only disease that will attack your oaks.

“Wet conditions also increase the risk of infection by SOD,” Running adds, “and those conditions also lead to the development of the second worst disease, armillaria root rot.” Wet conditions most often mean low spots in the landscape or yard where water drainage is poor and lawn sprinklers might keep the soil moist. “Also,” Running says, “over time, people put things around the base of a tree like flower beds that use a lot of water, or years of leaf fall will build up a layer around the crown (the base) of the tree.” Root rot typically shows up as slow or despondent growth in the oak, like its strength is being sapped - because it is, by the fungus. Treatment consists of removing the layer of soil surrounding the base of the tree, exposing the roots extending out a foot or more from the crown. Arborists use an air spade, but homeowners can just do the task with a shovel. This allows the roots to dry. An arborist can identify the cause of the armillaria infection, but usually wet conditions are a big factor. Another factor is the lack of biodiversity in the soil so that the fungus can spread unchecked. Counteract this by adding rich compost that adds a myriad of other microbes to the soil, thus overwhelming the fungus, and whose loose, crumbly texture aids quick moisture drainage.

Right now in mid-fall is the best time to give your oak trees a nutritional boost to help their immune systems grow strong and resist the worst diseases. Two treatments approved by the SOD Task Force at UC Berkeley are made with Reliance and Agriphos compounds. They are mostly a mixture of phosphorus and potassium nutrients that strengthen a tree’s defenses.

Deciduous red oak trees have shallow roots and suffer in our summer drought, so may need occasional irrigation. Just be careful not to water so often the soil is soggy. Water infrequently, but deeply. Black oaks are common throughout Sonoma County, especially in the east. These deciduous trees have a large tap root, and they thrive in poor, dry, sandy, or clay-based soils (although they prefer rich, well-drained soil). They’re native to North America east of the Rockies, but also our Mediterranean climate. Tanbark oaks are evergreen, with leathery, lustrous leaves 4 to ?10 inches long. A related species is native to Sonoma County. It has masses of dark green, toothed, evergreen leaves with silvery undersides and grows only about 20 feet tall and wide. It’s frequently seen in wild hillsides or mountain spaces.

The California live oak is a handsome tree - majestic when mature.

All these susceptible oaks need good drainage, so site them up on a slope if planting young ones. Most will do well in native soils, but if the soil is poor and growth is slow, top dress the soil away from the crown but under their drip line with well-made compost.

Proper pruning means removing dead or crossed branches, infected wood or branches with infected leaves. When dealing with infectious plant diseases, always dip your pruning tools in a bucket of one part bleach to nine parts water between cuts to avoid spreading the spores to healthy wood.

Jess Running from Davey, offered this cheering note: “Most of the other diseases of oaks, such as anthracnose blotches on leaves, leaf blister, diplodia canker, fusarium wilt, and downy mildew aren’t life-threatening.” They may disfigure the leaves or the tree growth, however. They are fungus infections, so treat them with Bordeaux mixture, copper drenches, elemental sulfur, or a tablespoon of baking soda dissolved in a gallon of water. The sulfur and baking soda cures are OK in organic culture.

For more Information: Suddenoakdeath.org; cnps.org/wp-content/uploads/2018/04/CNPS-fire-recovery-guide-LR-040618.pdf

Jeff Cox is a Kenwood-based garden and food writer who can be reached at jeffcox@sonic.net.