Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk with your question about your oak tree. First off, let me say that I am sorry about your oak tree. It does not look very healthy in the picture you provided. While it is common for live oaks to shed significant leaves in the spring, it sounds like you are seeing a more general decline over years which is likely due to other causes. I also should note that our advice is based solely on the picture, as opposed to the arborist who come to your house and personally reviewed the tree on-site. Their advice is likely to be more accurate. You mentioned that they want to treat the tree yearly with a fungicide. Did they actually diagnose a fungal disease?
These would be my concerns for your oak based on the information you provided:
You mention that your neighbor's driveway was put in a few years ago. I presume that is the driveway either in front or behind the tree in the pictures. Regardless, both driveways appear to be under the drip line of the tree. The drip line of a tree is the outermost circumference of the leaf canopy. This is where, in rain, water will drip from the leaves of the tree onto the ground. Oak trees are very susceptible to damage from root compaction and grading. Any construction that has occurred under the tree could result in the death of important roots and the subsequent impending death of the tree. The death of the tree under these circumstances may not be fast, it might take years. Below is a link to a graph with an algorithm to determine if your oak tree may have Sudden Oak Death. Notably, it mentions that you need to consider physical damage first. Depending on when this driveway was installed, this may be the most likely cause of the trees decline. http://ipm.ucanr.edu/PMG/PESTNOTES/pni7498-1c.html
- It is impossible to tell what kinds of plants were placed under the oak tree. The general rule for planting under oak trees is that plants should be quite a few feet from the trunk and that the understudy plants cannot require summer water. If these plants under the oak tree are being watered regularly, that can make the oak tree more susceptible to fungal diseases from the soil. Mature oak trees should not be watered during the summer at all. You should only water during the winter if there is insufficient rain (less than 20 inches/season).
- Lastly a fungal disease would be a possible cause for this kind of decline. There are a variety of fungal diseases that can affect oak trees, however, one of the most concerning is a fungal disease called Phytophthora ramorum. The common name for the disease it causes is Sudden Oak Death. This disease is typically spread from nearby infected California bay laurel trees, rhododendrons or camellias. The pathogen survives on the leaves of the bay laurel and is transmitted to nearby oaks by rain or wind. Once on the oak it infects the trunk and the tree can develop a canker where it bleeds a thick, sticky sap. There is a rapid browning (over 2-4 weeks) of the leaves, however, the tree has usually been infected for two years at that point. This disease is diagnosed by sampling the leaves of nearby host plants (usually a nearby bay laurel) or the bark. Bark sampling should be done by an expert. This disease does not have any cure. There is a fungicide (phosphonate, Agri-Fos) which is approved as a preventative treatment. This will not cure affected trees but will suppress disease progression in early cases. It is injected into the tree or sprayed on the trunk. While it appears you can purchase this product and apply it yourself, it requires special equipment to give the tree injections. You could spray the tree but would likely need special equipment to spray a tree of this size. Given those considerations, treatment is likely a job best left to the arborists. If you treat for fungal disease, and it is not a fungal disease, you may adversely affect the health of the soil under the tree. Fungicides are non-specific and using one may kill non-disease causing fungi that are normal components of soil. For this reason, it is best to get a diagnosis of this disease before starting expensive treatments. There is more information about this disease at the following link: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74151.html
I hope this information proves helpful to you as you make decisions about this tree. Below is a link to a very extensive guide to diseases of oak trees which you may want to look through. http://www.suddenoakdeath.org/pdf/psw_gtr197.pdf
Best of luck with your oak tree. Please contact the Help Desk again if you have more questions.
Help Desk of the UC Master Gardener Program of Contra Costa County (SES)
Note: Contra Costa's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, email: email@example.com, or on the web at http://ccmg.ucanr.edu/Ask_Us/. MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog.