Q&A

What can I plant under my oaks?

As a general rule, avoid planting under the canopy of oak trees. It is recommended that you remove non-native plants from under oaks but avoid disturbing the soil or drainage patterns in the process. If you need to plant something, choose species that require similar soil types and watering. Avoid azaleas and rhododendrons as they typically require more water than is appropriate for oaks and may transmit diseases such as Sudden Oak Death.

California Native Oaks

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California Native Oaks

There are a number of common oak species in California including both tree species and shrub species.

Trees species:

blackoakBlack Oak (Quercus kelloggii) Found in hilly regions between 2,000 and 6,000 feet. Typically associated with conifers and pines, firs, or incense cedars.

bluoakBlue Oak (Quercus douglasii) Found in the hot, dry interior foothills. Generally found below 3,500′.

canyonliveoakCanyon Live Oak (Quercus chrysolepis) Evergreen oak found in foothills, steep canyons, and on slopes to 9,000′.

cloakCoast Live Oak (Quercus agrifolia) Evergreen oaks found in coastal fog zones and in grassland savannas.

mesa oakEngleman or Mesa Oak (Quercus engelmannii) Rare species found in scattered groups along the western edge of California deserts.

interiorInterior Live Oak (Quercus wislizenii) Evergreen species usually found on uploand slopes below 5,000 feet , in low foothills and in hot dry canyons.

island oakIsland Oak (Quercus tomentella) Found on the Channel Islands off the coast. Typically in foggy areas often on north or north west facing slopes.

oregonoakOregon Oak (Quercus garryana) Found away from coastal fog on slopes and open valleys, usually between 1,000 and 5,000 feet

valleyoakShreve’s Oak (Quercus parvula var. shrevei) Similar in appearance, often found growing adjacent to, and may produce hybrids with Coast Live Oak.

Valley Oak (Quercus lobata) A deciduous tree species that can grow very large. Found typically inland from the coastal fog region, usually below 2,000 feet.

Link to common shrub species.

 Illustrations by Michael Lee – Purchase his beautiful poster of the Native Oaks of California

SOD: Cleaning Tools & Equipment

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SOD: Cleaning Tools & Equipment

Topic:

Cleaning tools in SOD-infested areas.

Problem:

Can tools become infectious when used on trees infected by SOD? Yes, in particular when dealing with infected foliage, infested soil, and marginally with infected wood. The risk of spreading SOD is higher if soil or infected leaves are present on tires, shoes or tools. Wood is generally, not as infectious

Recommendation:

 Differently from other pathogens which produce invisible microscopic and often sticky spores that can remain viable even on apparently clean tools, the SOD pathogen produces spores that are normally lodged in plant debris, organic matter, and plant or soil residue.

In order to clean your tools, shoes, and vehicle tires you can follow this two-step procedure:tools1
  • Clean all visible organic residue, plant tissue or soil from the tool. A wire or plastic brush is generally effective. Removal of all visible debris will eliminate 99% of all infectious propagules.
  • Washing with a disinfectant that will kill any remaining spores. We have found that 70% ethanol, diluted bleach, and Lysol brand disinfectant are all effective. Lysol, which contains benzalkonium chloride, a powerful and effective antiseptic, is the product we use most often as ethanol tends to be the most expensive of the three, and bleach can cause corrosion of metal tools.

Note: it is important to ensure all parts that came in contact with infected plant material are visibly clean and that no organic matter remains. This is particularly crucial for tools like chainsaws or machinery like chippers. The physical cleaning step  (above) is critical, and can be effective even without washing although the application of a disinfectant may help). Washing first and then cleaning will be ineffective. Always wear safety glasses and protective equipment as recommended by the manufacturer and always follow the manufacturer’s directions, restrictions, and precautions on the product label.

Research behind the Recommendation:

To test the effectiveness of tool cleaning on the transmission of P. ramorum, a chainsaw and handsaws were “contaminated” with SOD-infected wood chips and saw dust. The saws were treated by different methods in an attempt to “decontaminate” them including, removing the visible organic matter with a wire brush or brushing followed by a disinfectant rinse. The disinfectants used were 10% ethanol, 5% bleach, and lysol (diluted as instructed on the label).

After cleaning, the saws were examined under magnification, any remaining bits of plant or organic matter plated on selective agar Petri dishes. The percentage of infectious propagules that grew on the agar plates is graphed below. Click to rebigulate the pdf.

Download (PDF, 17KB)

Links and References:

Disclaimer: Mention of any company, trade name, or commercial product does not constitute endorsement by the University of California or recommendation for use. Always follow the manufacturer’s directions, restrictions, and precautions on the product label.

What is Sudden Oak Death?

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What is Sudden Oak Death?
  • sod1SOD is an exotic disease caused by the microscopic pathogen Phytophthora ramorum, estimated to have been introduced into California 20-25 years ago from unknown region of the world.
  • P. ramorum was unwittingly introduced into California’s natural landscape when infected ornamental plants, such as Rhododendrons and Camellias (which carry the disease), were outplanted into the environment. On many ornamental plants, as well as many native forest plants (including California bay laurels and tanoak), the pathogen causes a disease called ramorum blight.  Ramorum blight is a foliar disease that often supports pathogen sporulation and spread, as the spores build up on the leaves and twigs of these hosts which can then be transferred to nearby oaks and tanoaks, causing sudden oak death.  Ramorum blight is rarely lethal.
  • In California, foliar infections on Bay Laurel leaves are mostly responsible for spreading the disease. Oaks and tanoaks are infected when in proximity of bay laurels. Oaks only get a stem infection; tanoaks can develop both stemsod5 and a foliar infection. Tanoaks are the only tree species that can spread the the disease and die from it as well. When P. ramorum infects oaks and tanoaks it destroys the cambium under the bark and effectively girdles the tree. Girdled trees are doomed, but can survive for 1 to 5+ years thanks to stored resources and their natural tolerance to drought.
  • Besides Tanoaks, California Coast Live Oak, Black Oak, Shreve’s Oak, and Canyon Live Oak are known to be infected by P. ramorum. Oaks are not infectious in nature and do not spread the disease further, but oak wood may be infectious if carried to an uninfested area.
  • P. ramorum can infect leaves and trunks of it’s hoasts without the need for wounds, but it does require rainfall and temperatures between 60 and 80F (25 to 27C). As a result most infections occur in the rainy season and in particular when spring temperatures rise (mid April to June).
  • Yearly infection levels will depend on the presence of rainfall in the Spring; while tanoaks and bay laurels are infected yearly, oaks are normally infected only in years with abundant Spring sod3percipitation.
  • P. ramorum is dispersed aerially usually at short distances 100 yrds (100m) or less, but occasionally up to 1-2 miles (1.7-3.5km).
  • Infectious airborne microscopic structures known as sporangia are produced during rain events on plant surfaces, primarily leaves, and can also accumulate in soil and stream water.
  • Besides the natural spread of the pathogen, movement of infected plants or plant parts, soil, and water may lead to new infestations. Soil and plant material on tools and equipment may vector the disease.
  • During dry periods, the pathogen survives within infected plant tissues, and by producing a thick walled resting structure called a chlamydospores. As a result SOD remains persistent in any site, but oak infection varies as it requires high infection levels of bay laurels.sod2sod6

Read More […]

After SOD Arrives in Your Area

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After SOD Arrives in Your Area

Topic:

Once SOD infections have arrived, what can be done?

Recommendation:

  •  If SOD is present in a portion of the property remember to:
    • Schedule all landscaping and construction operations to occur first in the SOD-free area.
    • Ensure that equipment is cleaned after work in the SOD infested area.
  • Minimize all activities and operations in the Spring. Fall is the best work to avoid spreading infection through disturbance. Pruning of large branches and stems in multi-stemmed oaks should occur possibly in late November, and never in February-June.
  • If the property is downwind and down slope from a dense mixed forest with significant infestation, ensuring that water runoff is properly channeled may be beneficial to avoid spread of the disease by water.
  • Oaks that have a buffer area of at least 10 yards (10m) around the main trunk, devoid of any bay laurels or rhododendrons, are less likely to become infected. Identify valuable oaks and clear a 10 yard area around them by removing all small and medium bays, or by pruning large bay laurel branches of large trees that may come into the “buffer”. Bay laurels need to be treated with systemic herbicides at least a couple of weeks before being cut down, to minimize re-sprouting. Avoid all overhead irrigation and summer irrigation near oaks. For oaks over 32″ in diameter (80cm), 50-65 ft (15-20m) may be appropriate.
  • If oaks are not infected they can be protected with a single yearly treatment of phosphonate in November and early December. If treating in the Spring for the first time, repeat in the Fall and switch to a single yearly Fall treatment. Treatments are not a cure but will increase resistance in about three quarters of treated plants.
  • Phosphonate treatments have no known damaging side-effects on the environment and their action enhances the natural defense mechanisms in the plant. If injecting the product, we recommend alternating with bark applications every other year, to minimize wounding.sod4
  • Ensure you or your tree-care specialist has attended a workshop on how to properly apply the product. Topical bark applications require:
    • Topical bark applications require removal of thick moss and tarping of shrubs or other small plants nearby.
    • Injections require knowledge of appropriate timing, number of injections per tree, and injection technique, injections should not be performed in the Spring.
  • No chemical treatment of bay trees is known to be effective, and other alleged treatments of oaks have been shown to be ineffective in controlled experiments.
  • Additional treatments such as carving or scribing the infection out of an oak, or amending the soil near oaks with a thin layer of compost are still untested and we cannot recommend them. In addition, wounds may be detrimental to the tree and facilitate further SOD infection. To minimize such risk do not attempt to remove lesions in the Spring when infection potential is high. Lesion removal from oaks is still completely untested and could have unknown side effects.

Research behind the Recommendation:

The arrival of SOD in an area may be very gradual over time or may occur in a single year. Knowledge of SOD biology and disease distribution may help you prepare for the arrival of SOD and may allow time to protect valuable trees. The SODmap Mobile app, available for iPhone and Android, is an invaluable tool for visualizing SOD infections on the ground, in real time.

Dead oaks represent a significant hazard to people and property and increased fire hazard, however oaks that are infected by SOD and are apparently green may be equally hazardous. In SOD-infested areas, monitor closely oaks and tanoaks that are near homes, access roads, paths, yards, and play areas, and remove trees that show significant SOD symptoms and may be hazardous due to their location. Dead standing oaks, whether still green or brown represent a serious hazard for homeowners and are documented to fuel hotter fires that can kill medium sized redwoods.

UC Berkeley researchers have surveyed a large number of Coast Live Oaks, Tanoaks, and California Bay Laurels and determined that significant natural resistance to SOD is absent or very rare. However all three species have individual plants that may be more tolerant than others. Currently resistant plants are not available on the market, but several oak species (Valley oaks Blue oaks, Oregon oaks) are not susceptible to SOD.

 

Links and References:

Before SOD Arrives in Your Area

Doug Schmidt All Posts, Features
Before SOD Arrives in Your Area

Topic:

What to do before SOD arrives in your area.

Recommendation:

To help prevent the spread of SOD in  ornamental plants you purchase, ensure that they come from a facility that is regularly tested for P. ramorum. Make sure that wood from trees in SOD-infected areas is not moved to uninfected locations. Ensure that shoes, vehicle tires, and tools are free of soil and organic debris that might harbor the disease.

Careful brushing tools or air-blowing all soil and organic matter from them is normally sufficient to eliminate the risk of contagion. If driving on dirt roads during the rainy season in highly infested areas, wash the vehicle before returning home or visiting SOD-free areas. Preventative application of phosphonate compounds may help slow the spread of the disease in uninfected trees.

Familiarize yourself with the distribution of SOD using our SOD Disease Distribution maps, SODmap Mobile app, or OakMapper, and learn to identify early symptoms on bay laurels and tanoaks. The California Oak Mortality Task Force is a comprehensive source for the latest information on SOD. Contact your county Department of Agriculture officer for testing and participate in the SOD Blitz Survey Project held each year from April to June.

Research behind the Recommendation:

P. ramorum is dispersed aerially usually at short distances 100 yards or less, but occasionally up to 1-2 miles. Movement of infected plants or plant parts, soil, and water may lead  to new infestations. The current distribution of SOD is updated on our maps each year. Only a lab test can confirm SOD and early detection of SOD is best to maximize chances of slowing its spread.

Links and References:

SOD: Bay Laurel Removal

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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: