“Concern has grown since severely stressed and even dead oak trees are becoming common observances,” said William Tietje, UCANR Cooperative Extension area natural resources specialist based inSan Luis Obispo County. “People are asking, ‘What can I do to help?'”
“Three options come to mind: deep water, mulch or do nothing,” said Tietje. Below, he and Steven Swain, UCANR Cooperative Extension environmental horticulture advisor for Marin and Sonoma counties, discuss the advantages and disadvantages of each option.
Q: Should oak trees be watered?
If the soil under your oak 12 to 18 inches down is dry and crumbly, the oak is out of water. A deep watering will invigorate the drought-stressed tree. Perhaps it seems ill-advised to water an oak tree during a drought. But think about it a minute. If you lose the oak, you lose substantial aesthetic and property values. Also lost are the many ecosystem services provided by this “keystone” structure, such as shade, soil nutrients, wildlife habitat and biodiversity. These take a very long time to replace.
Q: How do I deep water?
Deep watering of the drought-stressed tree is accomplished by moving a hose under the canopy of the tree during the day for one or two days at a low flow or trickle, such that the water percolates into the soil. Do this once or twice during the summer to early fall with at least a month between watering to allow the soil to dry, reducing the likelihood that fungi will attack the tree roots.
Q: Why mulch?
A prudent approach to the current drought and the maintenance of tree health is to conserve existing soil moisture as much as possible. Mulching under the tree helps to control moisture by keeping the soil cool and suppressing weed growth. Mulching also adds valuable soil microorganisms.
Q: Does mulch need to be composed exclusively of oak leaves and twigs?
A: No—Although the best mulch is the oak's natural leaf litter, other plant-based mulches can provide a similar benefit. In fact, there may be some advantages to having other plant components in the mulch.
Q: Can commercially available mulch provide the benefits of natural oak mulch?
A: Yes—Most any plant-based, commercially available mulch should be fine, especially mulches that are derived from multiple sources, such as municipal green-waste that has been composted. Composting allows some of the more reactive compounds that may be present in the mulch to be broken down by heat, chemical and biological means. There isn't necessarily a problem with using a single-source mulch (e.g., grape pressings, nitrolized sawdust [nitrolization allows nitrogen to remain available to the plant]), but because single-source mulches lack the variety of densities, chip sizes and nutrient compositions typically found in multiple-sourced mulches, there is a somewhat greater risk of problems using single-source mulches. Commercial compost may be listed as green-waste compost. The best recipe is 1 to 2 inches of compost overlain with 2 inches of coarsely chopped fir, redwood bark or wood, totaling 3 to 4 inches of mulch. Cover the area under the tree canopy at least up to the drip line. Don't mound the mulch against the tree trunk.
Q: Should mulch be mixed into the soil?
A: No—Although the oftentimes repeated advice that mixing mulch into the soil can cause a nitrogen shortage is mostly a myth, there are other reasons for not incorporating the mulch into the soil:
- The process can cause root damage to existing plants
- The process can degrade existing soil structure
- Organic matter breaks down over time, and in the process causes the soils in which it has been mixed to eventually shrink in volume. This is fine in some instances, but it can spell disaster if one has incorporated mulch into soils that were placed beneath sapling trees, for instance. In this case, the soil under the tree will shrink in volume, and the tree will sink into the soil. Water then pools around the base of the tree, and the tree eventually develops root rot.
Because mulches are incorporated naturally into soils by earthworms, springtails, and microorganisms, the benefits of digging mulches into the soil are pretty much balanced by the liabilities. In short, save yourself the work. Put the mulches on the ground and let the earthworms do the work for you.
Q: Is there any mulch that is categorically unacceptable?
A: Common sense should prevail—Coarsely chopped bark, wood and leaves is probably the best composition for a mulch, and that same material composted is about as ideal as one can get. The very best are oak leaf mulches, which exist, but are quite pricey and are typically derived by stripping wild oaks of their natural mulch base. Even less-than-ideal source material, such as eucalyptus leaves or black walnut roots, can be safely used if properly composted.
Q: If oak trimmings and dead leaf fall from native oak trees are used for mulch, is the presence of termites or the spreading of oak bark beetles a concern?
A: No—Termites do not live (for long) in wood chips. They require contiguous, solid wood to form colonies. The spread of termites in wood chips, or even in small branches, is therefore a non-issue. Every spring, summer and fall, oak bark beetles actively hunt stressed oak trees in central coast woodlands, and will find them from miles away. Worrying about spreading any of the various bark beetles that can attack oaks is a little like worrying about getting salt spray at the beach. They've been here for millions of years before us, and they'll likely be here millions of years after we're gone. That said, you may not want to encourage large colonies in your back yard by, for example, stockpiling large amounts of beetle infested firewood right beneath an oak tree.
If your tree appears healthy, with dense and green leaves over at least most of the canopy, it could be best to do nothing. In other words, “If it isn't broken, don't fix it.” Give proper considerations for the use of water during the current severe drought, and mandatory water-use restrictions, and deep water only if the tree shows symptoms of severe stress.
On the Central Coast, the predominant native oak trees are valley oak and blue oak, which drop their leaves in the winter, and the evergreen coastal live oak. Keep in mind that the two deciduous oaks, blue oak and valley oak, can respond to drought by undergoing leaf browning and leaf fall as early as July. This is a natural mechanism that the tree has evolved for water conservation. The tree is not dead and should be fine. All else considered, mulching that includes a green waste compost as described above could well be the most appropriate solution. Mulch conserves water as opposed to using water for deep watering, and mulching can improve important soil properties.
For more information, visit the UC Cooperative Extension San Luis Obispo County website http://cesanluisobispo.ucanr.edu and the UCANR oak woodland management website http://ucanr.edu/sites/oak_range.
This has California researchers worried and baffled. The bacterium Xyllela fastidiosa has been present in the state for more than 100 years and can sometimes be found in olive trees. Trials conducted by USDA and UC researchers from 2008 to 2011 showed that strains of the bacterium found in California did not cause disease in olives.
“It's complicated,” said Elizabeth Fichtner, an olive and nut crop expert with UC Agriculture and Natural Resources (UC ANR). “The Italians found X. fastidiosa in sick trees, but the causal agent of the disease still remains to be identified by scientists.”
There are different subspecies of X. fastidiosa. In California, X. fastidiosa subspecies fastidiosa causes Pierce's disease in grapevines and almond leaf scorch disease. Another subspecies, multiplex, causes disease in almonds but not grapevines. A possible reason why California olive trees are unaffected by the bacterium is that the genotypes found in Italy belong to a different subspecies named pauca, a subspecies group not known to occur in the United States. Strains of X. fastidiosa subspecies pauca cause a serious citrus disease in Brazil and Argentina. The X. fastidiosa subspecies known from California do not cause disease in citrus.
Fichtner is working closely with USDA researcher Rodrigo Krugner of the USDA San Joaquin Valley Agricultural Sciences Center in Parlier to better understand the potential threat to California olives and keep California growers informed.
Krugner traveled to Italy to view the olive trees with quick decline symptoms.
“They took me where it all started,” Krugner said. “Trees that were alive had scorched leaves, twig and branch dieback. I have been seeing olive trees in California with similar symptoms since 2008, but in Italy the disease is more severe. It's devastating.”
Surveys led by Krugner, dating back to 2008, showed that, among symptomatic ornamental trees in Southern California, 30 out of 78 were positive for X. fastidiosa.
“In the San Joaquin Valley, only 3 out of 121 were positive despite disease symptoms being the same between trees in the two regions,” Krugner said. “In greenhouse studies in which olive trees were inoculated with X. fastidiosa, the bacteria survived in the plants but produced no symptoms. In time, the bacteria disappeared.”
Krugner said he and Fichtner visited an olive orchard in the Fresno area where the trees were suffering from widespread leaf scorch symptoms and some dieback.
“The entire orchard had symptoms, every tree,” Krugner said. “But all samples were negative for presence X. fastidiosa by molecular tests and culturing.”
One possible explanation for the widespread and uniform distribution of the disease symptoms in the Fresno orchard was over watering and over working the soil, which may have damaged the roots facilitating the invasion of soil pathogens.
California olive growers and landscape managers with olive trees should be on the lookout for olive dieback symptoms. If new incidences of extensive dieback or scorch on olives are found, contact the local UC ANR Cooperative Extension farm advisor to facilitate early detection of potential new pathogens.
“Because of the regular movement of plants and plant materials across international borders, there is a constant flux of organisms,” Fichtner said. “We need to be aware of global trends in plant diseases and the establishment of insects that can spread disease.”
For more information about Xylella fastidiosa and olive quick decline syndrome in Italy, see an overview posted on the UC Berkeley plant disease page.
An initiative to manage endemic and invasive pests and diseases is part of UC Agriculture and Natural Resources Strategic Vision 2025.
The prestigious award, given each year by the Agricultural Sustainability Institute (ASI) at UC Davis, will be presented at a ceremony at UC Davis on April 23. UC Agriculture and Natural Resources, with its Sustainable Agriculture Research and Education Program, is a partner with ASI.
The keynote speaker at the awards presentation will be Navina Khanna, a UC Davis alumna and leader for food justice in California.
The Bradford–Rominger award honors individuals who exhibit the leadership, work ethic and integrity epitomized by the late G. Eric Bradford, a livestock genetics professor who gave 50 years of service to UC Davis, and the late Charlie Rominger, a fifth-generation Yolo County farmer and land preservationist.
Former students describe Fujimoto as a prophet and “energizer bunny of social change.”
“Isao began advocating for more socially just and environmentally sustainable forms of agriculture over 40 years ago,” said Mark Van Horn, director of the Student Farm at UC Davis. “At the time, it made him quite unpopular in some quarters, but he remained true to what he knew was right.”
In his early days at UC Davis, Fujimoto used the campus's signature red, double-decker buses to transport children of farm workers to school when public bus service was canceled. The incident sparked conversation about the need for the university to focus on California's rural communities, and led to creation of the Community and Regional Development Graduate Program at UC Davis in the mid-1970s.
Fujimoto was also instrumental in starting the Asian American Studies program on campus, and was mentor to many students who have become sustainable agriculture leaders in their own right. Throughout the 1970s, Fujimoto's home served as a local hub for community activism, with projects such as the Davis Food Co-op and the Davis Farmers Market starting out at his kitchen table.
“He has helped countless students understand the world around them and clarify their personal values and principles,” Van Horn said. “Most importantly, his actions have provided lessons and inspiration for those wanting to act upon their values and principles to bring about positive change in the world.”
Like Eric Bradford, Isao Fujimoto is a respected mentor and a consensus builder. Like Charlie Rominger, Fujimoto has consistently stood up for his beliefs, regardless of their unpopularity, and has done so with a kind heart and humble nature.
“The kind of commitment and sense of responsibility that Eric and Charlie had is a pretty remarkable trait,” Fujimoto said. “I find this award set up by the Bradford and Rominger families as a pretty significant marker of change in terms of broadening the scope of agriculture to include being conscious of the environment and using agriculture as a tool for building community.”
Past winners of the award include UC Agriculture and Natural Resource's advisor Rose Hayden Smith, specialist Ken Tate and advisor Mary Bianchi; and UC alumna Kelly Garbach.
Fujimoto will receive the award at the annual Bradford–Rominger Agricultural Sustainability Leadership Award Ceremony which begins at 5 p.m. in the Multipurpose Room at the Student Community Center at UC Davis. Khanna's keynote speech will address, “Claim Your Superpower: Meeting the Moment for a Winning Food Movement.” On April 24, Khanna will meet with UC Davis students to further discuss leadership in the food movement.
This event is free and open to the public. Students are encouraged to attend. Learn more about the award on the Agricultural Sustainability Institute's web site.
For more information, contact Aubrey White at 530-752-5299, firstname.lastname@example.org
The UC Agriculture and Natural Resources Statewide Integrated Pest Management (IPM) program teamed up with UC ANR farm advisors to develop a series of how-to videos that can help growers and pest control advisers monitor for pests and damage and determine if and when treatment is needed.
In one video, Sacramento Area IPM advisor Emily Symmes gives a brief overview of how to monitor for webspinning spider mites. Spider mites build up in stone fruit trees as the weather warms up. Late spring through summer is the ideal time to monitor for mites and their damage, which includes leaf stippling and webbing. If mites build up too much, leaves can drop, fruit may not fully develop, and branches and fruit can be exposed to sunburn.
Shoot strikes, or dead drooping leaf tips, are often seen on young peach and nectarine trees. In a second video, UC ANR farm advisor Janine Hasey explains how to monitor for shoot strikes and how to distinguish the culprits, Oriental fruit moth and peach twig borer. Although Oriental fruit moth and peach twig borer can bore into both foliage and fruit, they cause the most devastating damage by feeding on fruit. Early season monitoring and treatment can prevent future fruit loss.
In plum and prune orchards, leaf curl aphids and mealy plum aphids cause leaves to curl and become distorted. Aphids produce honeydew, which can lead to the development of sooty mold, causing fruit to crack and blacken. Aphids are often present when leaves start to grow. In his video, Rick Buchner, UC ANR farm advisor for Tehama County, discusses how to monitor for aphids and explains how to decide when treatment is warranted.
In a final video, UC ANR farm advisor Chuck Ingels teaches how to distinguish Phytophthora root and crown rot from bacterial canker. The two diseases are often confused because they both cause bark cankers. Phytophthora root and crown rot is confined to the lower trunk, but when a bacterial canker infection occurs in the tree trunk, the diseases can often be confused. Bacterial canker can be confirmed by cutting away the outer bark and looking for characteristic red flecks on the inner bark. Correct identification of these diseases will help in choosing a management strategy.
For homeowners, there are six key things to do to conserve landscape water, says Karrie Reid, UC ANR Cooperative Extension advisor, in San Joaquin County. Reid gives the following six tips:
- Tune up your irrigation system right away. When water is efficiently and accurately applied, less water is needed to keep plants healthy. Spray heads can get knocked out of alignment and end up spraying the sidewalk, street or driveway and running to the gutter. Check all spray heads to ensure they are hitting the target and twist those that aren't back into place. Some heads have adjustable angles of spray, which can be fixed with a tool available at a hardware store. Look for cocked heads, which spray water up into the air, and sprays blocked by grass or those that have sunk below grade. Make sure all spray heads are made by the same manufacturer and are from the same line so they deliver water at the same rate, otherwise they'll leave dry spots. Low-volume spray heads or rotators deliver water more efficiently.
- Avoid wasting water to runoff. If water runs off before the watering cycle finishes, split the cycle time. Set the timer to water in two, three or even four cycles at least an hour apart to allow the water to soak in. To ensure water isn't flowing below the root zone, check the watering depth after each cycle.
An irrigation scheduling worksheet created by Loren Oki, UC ANR Cooperative Extension specialist in the Department of Plant Sciences at UC Davis, and Darren Haver, UC ANR Cooperative Extension advisor in Orange County, helps fine tune irrigation timing. The worksheet is available for free online at the Center for Urban Horticulture website http://www.ccuh.ucdavis.edu.
- Switch to inline drip tubing for beds. Drip irrigation applies water where it is needed with less loss to the air. Be sure to lay tubing so water reaches plants' entire root zone.
- MULCH, MULCH, MULCH. Adding 3 or 4 inches of organic material such as wood chips, shredded bark or bark nuggets will improve soil health while retaining water and lowering stress on your plants. Place mulch away from the street curb to prevent heavy rains from washing it into the storm drains.