Got Problems with your Oaks or Cedars???

Robin Stanley

UCCE/El Dorado Master Gardener

Because of our cool and rainy spring, this summer has been short, but not so sweet. The endless hot days seem to have taken their toll on us and our native trees. Recently Master Gardeners received some updates that were very useful in helping us to answer questions we were getting in the office about defoliation and dieback of oaks and cedars. We sometimes refer questions about native trees, especially conifers, to the experts at the California Department of Forestry. But since we have gotten so many questions and the problems seem to be pretty widespread throughout the west slope, we thought our readers might also have some questions about what they are seeing in their yard or in oak woodlands or forest land throughout the county. It turns out that weather has been a factor in the problems for both oaks and cedars, although in very different ways.

Native California Oaks Losing Leaves Early

The first update was from the University of California Agriculture and Natural Resource News Agency and it concerned our local oaks. The e-mail stated that in spite of our rainy winter and spring, many Sierra Nevada and Coast Range native oaks, such as black oak and blue oak, are experiencing browning of leaves and early defoliation. Because of all the publicity about Sudden Oak Death, it was natural for tree owners to fear that Sudden Oak had struck.

The article quotes Dave Rizzo, a UC Davis Sudden Oak Death expert, who says he is “almost certain that what is being observed is not related to Phytophthora ramorum, the pathogen responsible for sudden oak death. Blue oak is not a host of SOD and on black oak, SOD infections are limited to stem cankers. The most likely culprits are fungal organisms, including Septoria leaf blight and oak anthracnose…”

Another UC expert, Doug McCreary, of the Integrated Hardwood Range Management Program, is also quoted in the article. “While these foliage diseases are fairly common and can be found on many oaks in normal rainfall years, they are much more prevalent and widespread this year, a direct result of the unusually wet late spring conditions.” It appears that our extended wet spring hit the oaks with a double whammy. During the cool wet weather, the period that leaves could be susceptible to fungal pathogens persisted. Because the rains continued, even as the weather turned warmer, the perfect conditions for infection were created.

But don’t despair. It seems that because unusual weather patterns create the conditions for leaf diseases, when the weather returns to a more normal pattern, the trees recover and there isn’t a major long-term effect on the health of the tree. In fact, even as the trees continue to lose their leaves, they might begin to grow new leaves in the fall.

McCreary does not recommend the use of fungicides at this point, because they are really only effective in the spring. So we are relieved to be able to reassure our public that their oak trees should “weather” this weather-related malady.

Incense Cedar Dieback

The second article, excerpted from a previous Placer County University of California Cooperative Extension newsletter, The Real Dirt, highlights the problems that are causing death and dieback in our local cedar trees. Homeowners and forest landowners are reporting such symptoms as “top kill, branch dieback and total mortality” within the last few years. Even though this article was written in 2003, we are still getting lots of questions about this problem this year.

There are several factors that seem to be contributing to these problems among incense cedar. The primary one is drought. Before our rainy and cool spring, Northern California had experienced some drought years. Although incense cedars are well adapted to our Mediterranean climate, the cumulative effect of repeated drought years can take its toll on the trees. The depletion of soil moisture causes water stress in the trees, which generally shows up in the tops and branches. This is particularly a problem in smaller incense cedars, which don’t have a very extensive root system.

A second factor that is weakening the trees may be frost injury. Early freezes in October 2002, particularly in northeastern California, increased stress on trees that were already struggling.

Once the dieback and top kill is excessive, there are few treatment options. As the dieback increases, the stressed trees are less able to withstand insect attacks. Cedar bark beetles infest the trees and wood boring beetles finish the job.

While you can’t do much about the weather (just ask those of us who sweltered through July without air conditioning), like the oaks, our cedars (at least the ones that are still alive) should begin to recover with some additional years of normal precipitation.

The article concluded with these general suggestions for preventing tree stress:

• Reduce tree density and competing vegetation.
• Prevent or avoid soil compaction in the root zone.
• Avoid root and trunk damage by animals or machinery.
• Avoid over-fertilizing and over-watering.

For those people who live in forest land, the dieback of these cedars, and their resultant removal, may actually help homeowners who are trying to keep their property fire-safe. It can be hard to cut living trees, but often the trees that are most likely to suffer dieback may be the ones that are too crowded and needed to be thinned anyway.