bay

Can coast live oak be used in a large-scale Bay Area tree planting project without exacerbating SOD?

Question:
My organization is considering launching a large-scale urban tree planting project in the Bay Area, with the goal of adding one million trees to the urban forest inventory within 15 years. We are interested in coast live oak (CLO) as a major component of the effort for a number of reasons, including the ease with which seed can be collected and planted, drought tolerance, habitat value, high potential for carbon sequestration and rainfall interception, and status as a “signature tree” of the local landscape.

The plan is to gather seed locally from many locations, and plant with minimal site preparation and no supplemental irrigation or other artificial life support systems. Trees will be monitored and cared for over time, and replanting will take place as needed to ensure that we continue to make progress toward our goal. Obviously we don’t want to exacerbate the sudden oak death problem. Our hope is that, on the contrary, widespread and large-scale planting with subsequent monitoring might help identify resistant trees. What recommendations do you have?

Answer:
First of all, this is a really neat project and you are absolutely correct about the benefits of planting CLO. Based on the information provided, the following are recommendations for the project:

In addition to coast live oak, it would also be good to plant Oregon white oak in the northern greater Bay Area, valley oak in the middle portion of the Bay Area, and blue oak in the southern part of the Bay Area. The rationale for this is that diversity/biodiversity is always good and the three alternate oak species are immune to SOD. A good overall planting ratio would be 75% CLO and 25% other species.

The big issue is going to be when California bay laurel is present, as it helps to spread Phytophthora ramorum. CLO should never be planted within 20-25 yards of bay for this reason. Unfortunately, bay has become invasive and SOD is actually increasing its abundance, so where bay is at least 10% of the stand, plant the other oak species recommended above. In riparian areas, bay should be left untouched.

We have old camellias and a pittosporum on our property that are healthy, but growing very close to coast live oaks. Should they be removed to help protect the oaks from SOD?

When thinking about P. ramorum dispersal, it is important to think about spread at two scales. First, the pathogen establishes itself on transmissive host leaves (e.g. California bay laurel, tanoak, redwood, rhododendron, and camellia) by moving up to several hundred meters during conducive rain events. Then, as the pathogen builds up, infection can spread from a transmissive host to an oak if it is within 20 m from the oak. If incidence of transmissive hosts is low in your area, chances are that an oak will not become infected even if a camellia or pittospsorum is a few meters away from it, so there is no need to remove these hosts if there are few bay laurels or tanoaks in your neighborhood. The second thing to keep in mind is that SOD is not everywhere. You can use SODmap mobile to help determine current levels of risk for oak infection on your property. If risk is low or negligible, again, there is no need to remove these plants, but they should be monitored for symptoms. The last thing to keep in mind is that the maximum distance between a transmissive host and an oak depends on the size of the transmissive host and on how infectious it may be (i.e. how well does it sporulate). For both camellia and pittosposrum, if the distance between your oak and plants is 10 m or more, they probably represent a low risk. If they are 7 m or less, they may represent a risk if your neighborhood has plenty of transmissive hosts.

If a home/landowner suspects SOD is affecting their trees, how do they get them tested?

If a homeowner suspects SOD is affecting their trees, there are a few options to get them tested. SOD kills several oak species and tanoaks, but it is primarily spread via infected California bay laurel leaves. Parties interested in testing for the presence of SOD can contact their local UC Farm Advisor or county Department of Agriculture to find out if they accept symptomatic bay laurel leaves for testing. (Go to Hosts and Symptoms on the California Oak Mortality Task Force [COMTF] website to learn what symptomatic bay leaves look like.)

Another option is to participate in a spring SOD Blitz. These are community-based, volunteer sampling efforts that are organized in cooperation with the Garbelotto lab. Training dates and locations are posted February 1st of each year for the upcoming sampling season at www.sodblitz.org. Training and sampling take just over an hour each and happen on the same scheduled weekend.

For infected oaks, call an arborist. A list of arborists that have attended a SOD training within the last 3 years is available on the Arborists and Applicators page of the COMTF website. NOTE: This list is only intended to be an additional filter for those looking for an arborist. It is not a list of recommended professionals.

We have bays 100 feet downwind from a coast live oak that seems to be dying of SOD. How do we have our oak tested and how do we determine if the bays are carriers?

Oak testing is routinely done by arborists. It is a rather involved and tedious process that requires a fair amount of finesse and know how as it entails wounding the tree by shaving off the outer layer of bark to collect infected tissue. If you decide to have your oaks tested for SOD, consider going to the Trained Professionals List for arborists that have recently attended an official SOD seminar (Not intended to be a list of recommended professionals, this list does serve as an additional filter when trying to identify arborists that are up-to-date on the latest science-based SOD information.).

Alternatively, a less invasive method of determining the presence or absence of SOD on your property is to sample symptomatic bay leaves (See Hosts and Symptoms for SOD symptoms.). Keep in mind that any infested bay in the general vicinity (typically within 200 ft of an infected oak) may be the inoculum source for infection of your coast live oak. If bay sampling is your preferred method for pathogen detection, you can call your local UC Cooperative Extension office to find out if the California Department of Food and Agriculture is processing leaves from your county and, if so, what the protocol is for sample submissions. In lieu of submitting samples through your county, you can go to www.sodblitz.org to find out when a SOD Blitz will be held in your region (Dates for each year are posted by January 30th. Blitzes typically occur from March to June). Attending a blitz will afford you the opportunity to learn more about SOD, including the latest disease control strategies, during a 1-hour training session. At the end of the session you will be given all sampling materials and instructions for submissions. Blitz participants are free to submit as many leaves as necessary, meaning that you can send in all suspicious leaves from your property as well as from the neighborhood or nearby local parks. All leaves are processed at UC Berkeley and all sampling results are published in a database available at www.sodmap.org or through the free App SODmap mobile.

Is it okay to prune coast live oaks in the winter if there is no SOD inoculum in the area? Are there other pests that are active in the winter that are of concern?

Although there may be few insects flying in the winter, the wet winter season (actually fall through spring) is a prime time for production of fruiting bodies by many wood decay fungi. The combination of fresh pruning wounds (especially large wounds), high spore densities of wood decay fungi, and moist conditions (favoring spore germination) can favor new infections by these fungi. From the standpoint of reducing risk of infection by wood decay fungi, pruning in late spring or early summer is probably preferable.

In general, pruning live branches from mature oaks should be kept to a minimum. However, if it is necessary to prune coast live oak branches, as long as they are less than 4″ in diameter, the presence of SOD is not a concern. Though, it is important to prune the oak correctly.

• Make sure you prune right where the tapering of the branch ends from the main stem or even slightly closer to the stem (1/2 in. from the end of the tapering towards the stem).

• Make sure the cut is clean and allows for water to drip without accumulating.

I’m considering removal of bay with ramorum blight, but I have symptomatic and asymptomatic tanoak on my property. Will bay elimination protect my oaks and tanoaks?

Both bay laurels and tanoaks can spread SOD while most oaks are not infectious.  Therefore, bay removal is typically aimed at protecting oaks. In the absence of bays, you may actually be able to facilitate survival of asymptomatic tanoaks, but only if their density is low (about 20 per acre).  A possible plan of action may thus be to eliminate all medium-sized bay trees (up to 20 inch DBH) that are not in a riparian setting and to eliminate all infected tanoaks. You may then decide to leave a few well-spaced tanoaks (50 feet minimum from one another), ensuring none of them are within 30 feet of an oak.

After taking down a P. ramorum-infested bay tree, can the foliage be left onsite to decompose?

Once down, spores from bay foliage will not travel nearly as far as they do in the canopy. The concern on the ground is that infested leaves, while still fresh, could have spores picked up during a wet event from soil splash onto the trunk of the tree. So, spread bay foliage in a thin layer in a sunny area that will promote drying. Be sure to not have foliage near the base of oak or tanoak trunks where soil splash/infection could occur.

SOD: Bay Laurel Removal

Doug Schmidt All Posts, Features
SOD: Bay Laurel Removal

Topic:

Selective removal and pruning of California bay laurel (Umbellularia californica) trees as a strategy to protect oaks and tanoaks from SOD infection.

Recommendation:

 For oaks between 3″ (8cm) and 32″ (80cm) diameter at breast height (DBH), remove small and medium sized bay laurels for a distance of 30ft (10m) around the tree. If large bays are present it may be useful to prune large branches that fall within the 30ft (10m) buffer zone. For oaks larger then 32″ (80cm) DBH, increase the bay-free buffer zone to 50-65 ft (15-20m).  

Bays are persistent landscape plants that re-sprout quickly and produce lateral tillers in abundance. As a result they are difficult to remove permanently. Useful techniques include adding a spreader to the herbicide or applying the herbicide with a “hack and squirt” technique (see links below). For best results, apply the herbicide when the tree is alive, wait approximately a month for the chemicals to be distributed throughout the tree, killing the roots, and then cut it down

Research behind the Recommendation:

Managing the presence of bays, the primary carrier of P. ramorum, around high-value oaks and tanoaks can reduce the number of infectious spores in the environment. However, bays are important species in their own right and care must be taken not to disrupt their role in the environment,  particularly along streams where they are particularly important.

Links and References: