The Goldspotted Oak Borer (GSOB) (Agrilus auroguttatus) continues to kill native oaks in several areas of Southern California. Susceptible oaks include coast live oak (Quercus agrifolia), canyon live oak (Q. chrysolepis), and California black oak (Q. kelloggii). In many cases, GSOB has damaged or killed mature oaks valued for their beauty, wildlife habitat, and shade. Areas with large numbers of native oaks are particularly at risk. Unfortunately, oaks that are injured over several years from multiple generations of the GSOB often die.
Although GSOB was first identified in San Diego County in 2004 it wasn't until 2008 that oak deaths were linked directly to GSOB. By 2010, GSOB killed over 20,000 oak trees growing in forests, parks, and urban areas in San Diego County. Later infestations occurred in Idlyllwild (2012), Orange County (2014), and Los Angeles County (2015). The three most recent outbreaks have all occurred in San Bernardino County. The first occurred in Oak Glen in 2018 followed by infestations in California black oaks in the Sugarloaf area of Big Bear in August 2019 and in Wrightwood in early November 2019.
The GSOB is native to southeastern Arizona where it is not destructive to otherwise healthy native oaks. This may be due to natural enemies and/or resistant oak species that have co-evolved with GSOB. Damage. Damage occurs from larval feeding on the vascular (water and nutrient conducting tissues) system inside trunks and branches. Infested trees have black stained bark and may ooze sap underneath red bark blisters. Adult beetles leave a distinctive D-shaped exit hole.
Damage from GSOB adults feeding on leaves is not a major concern. Insect Identification. GSOB larvae are about 0.8 inches long, white and legless with two pincher-like spines on the end of their abdomen. Adult GSOB are smaller (about 0.4 inch long) and are mostly black with six gold spots on their forewings. Soft-bodied pupae resemble adults in size and shape and are found in the outer bark from late spring to early summer.
Prevention is important since there are no known control methods once trees become infested with GSOB. Keeping infected firewood onsite is the most effective way to stop its spread. Wood should never be moved offsite since this is the major method by which GSOB is spread. No known natural enemies have been identified and insecticides are not generally effective. Monitoring susceptible trees species and identifying and reporting new infestations early are both important.
If you believe there is an infested oak on your property please submit photos of the entire tree, a close up of a leaf (to confirm the species), and a close up of the surface of the bark on the main trunk. If possible, include a photo of an unsharpened #2 pencil tip next to any visible exit holes since are both around .15 inches wide. https://ucanr.edu/sites/gsobinfo/Help_Monitor/Report_Goldspotted_Oak_Borer_Symptoms/ A team of scientists from UC, California Department of Forestry and Fire Protection, CALFIRE and the U.S. Forest Service and other agencies are working collaboratively to reduce the devastation from this insect and identify effective biological control agents.
Did you know that many landscape problems are not caused by either diseases or insects? In the majority of cases the culprits are poorly drained soils, irrigation issues, trees in too tight of quarters or improperly pruned, and other non-living (abiotic) maladies. The good news is that most of these issues can be prevented. Follow these suggestions to steer clear of common abiotic problems in your landscape. Following these suggestions will also help your plants stay healthy and better able to fend off insects and diseases they would otherwise be vulnerable to.
Select plants that grow well in your climate zone. Choosing the right plant for the right place is a sure way to green up your thumb and keep your plants healthy and attractive. The Sunset climate zone map is a great resource for finding out what climate zone you are in. These zones are more accurate for most areas of California than USDA plant hardiness zones since they are smaller and take into account more relevant factors than do the USDA hardiness zones. An exception are mountain communities in which case either resource works equally well. Click here to find your Sunset climate zone https://www.sunsetwesterngardencollection.com/climate-zones and here to locate your USDA plant hardiness zone: https://planthardiness.ars.usda.gov/PHZMWeb/.
Select plants that will grow well under various ‘microclimates' around your home. Microclimates are smaller areas within your larger climate zone that further determine the suitability of a plant for a specific spot. Typical microclimates are sunny or shady areas, sections of your yard that get splashed by chlorinated water from a pool or spa, narrow strips in side yards that are too small for a large-growing tree, and poorly drained soil with not enough air to support plant growth. While all plants need adequate space, trees need the most. While a beautiful shade tree like a ornamental fig, magnolia, or rosewood might be tempting, they all need about 3,000 cubic feet of space underground and 50 feet or more above ground clearance at maturity. Otherwise roots can buckle sidewalks and driveways and branches can interfere with utility lines. There are many smaller-maturing trees such as acacia, mesquite, crape myrtle, and dozens of other species that won't cause these problems and are good choices for smaller areas.
Be sure to do adequate research on the expected size of a tree you're interested in before making a purchase. Care tags, UC Cooperative Extension Master Gardeners, and nursery and garden center personnel are all good sources of help to match a plant's requirements regarding space, sun, and shade and their tolerance to salt spray, problem soils and other restrictions with conditions in your own yard. Three excellent searchable plant databases for finding suitable plants for your climate zone and microclimate conditions are: Urban Forest Ecosystems Institutes (https://selectree.calpoly.edu/); California Native Plant Society (https://calflora.org/); and UC's California Center for Urban Horticulture Water Use Classification of Landscape Species (WUCOLS) (https://ccuh.ucdavis.edu/wucols-iv).
Apply the right amount of water at the right time. Many newly transplanted landscape trees, shrubs, and flowers die from the soil drying out too much between waterings. Their root systems are small and need to be kept moist for the first few weeks in their new home (or a full season for a tree). This holds true for drought-tolerant species as well as those that require more water. Once plants become well-rooted they should be watered less often but for longer periods of time. Water slightly below their current root system to encourage downward growth. This practice is especially important for trees since it promotes deeper roots better able to support and stabilize above-ground growth.
Since tree roots spread outward as well as downward, water should be applied beyond the dripline of the tree as well. Some soils never fully drain leading to low oxygen levels than hamper tree growth and can eventually kill the plant. To test for adequate drainage before planting a tree, dig a hole at least one foot deep and one foot wide and fill it with water. If it doesn't drain in one day or less a tree should not be planted at that site.
Hydrozone (place plants with similar water needs together). If your landscape is irrigated on an automated system, you can save water and reduce your water bill by grouping plants according to their water need. Divide your landscape into very low, low and medium water use hydrozones at the design stage and remember to replace any dead plants with new ones requiring a similar amount of water. This allows you to fine tune how often and how long to water each zone. Hydrozoning also helps you stay within a medium or even low water tier even if you have designated high water use areas for vegetables and fruits that generally require higher amounts of water than most landscape plants.
Prevent soil compaction and poor drainage. Avoid adding soil amendments to tree planting holes. Trees need adequate underground support as they grow upward and become heavier. Trees in either too small of planting hole or in holes where compost or other organic soil amendments have been added may grow in circles around the hole, never venturing outward beyond the constraints of the hole. Trees may eventually topple, causing injury to people and nearby structures. Instead, dig a hole at least twice the width of the container and at the same depth the tree you will be transplanting came in, remove the soil, gently plant the tree, tamping the original soil around the plant. Remember to water the newly planted tree in well and keep the rootzone moist the first season as discussed above.
Add soil amendments (including compost) to soil before planting small ornamental plants and edibles. They add valuable organic matter to the soil, improving aeration and drainage in heavy clay soils and water-holding capacity in sandier soils. Compost also adds valuable microbes that break down the organic matter improving overall soil health. Mix enough soil amendments into the soil so that the final volume contains about 40% soil amendments and 60% original soil. Make sure that the soil amendment is mixed into the soil evenly and completely. Do not add soil amendments to tree-planting holes to avoid circling roots.
Correct high pH (alkaline) soils. Southern California soils tend to be alkaline, with average pH ranges from 7.1 – 7.6. (Most plants prefer a neutral pH of 7.) Highly alkaline soils often tie up nutrients necessary for plant growth - such as iron, zinc, and manganese – making them unavailable to the plant. Once the pH is reduced, the problem is often resolved.
Avoid salt damage. Damage due to too much fertilizer or water high in certain salts (such as chlorinated swimming pool water) can cause brown leaf margins and leaves that look scorched. Over time, the damage gets worse and sensitive plants may die. One way to reduce the chance of too many salts building up in your soil is to not over-fertilize. For instance, woody trees and shrubs do not need to be fertilized at planting time and older trees may get all the nutrients they need from fertilizer applied to neighboring lawns and plants. .If symptoms of nitrogen deficiency (yellow newer leaves) do appear, apply no more than .25 lb/actual nitrogen per inch of trunk circumference. Leaching salts out of saline soils (applying water over what may be a few hours very slowly so it can drain salt build-up below the roots) may be necessary to avoid damage to your plants.
Take Care of Your Trees. Trees are likely the most highly valued plants in your landscape. They provide shade, reduce temperatures in hot urban heat islands, add beauty, and even help clean the environment by storing carbon dioxide. Prune trees correctly.
Don't top trees! Topping a tree is the process of giving a tree a virtual crewcut by making one or more horizontal cuts across the top of the tree (add photo) to shorten it. Why is topping trees harmful? Topping trees results in unstable, unsafe, and unattractive trees. It also reduces the ability for trees to reduce high temperatures and provide adequate shade in urban heat islands, sequester (store) carbon produced by fossil fuels, and provide wildlife habitat. In some cases, trees are topped because a tall tree that should not have been selected in the first place is growing into utility lines. In other cases, topping occurs due to a lack of knowledge about the dangers of topping and/or simply wanting to save money by going with the lowest bid. In all cases, topping should be avoided. The combination of improper balance and weak, poor-quality growth following topping creates a much higher likelihood of personal injury and property damage than occurs from properly pruned trees.
Trees should be properly thinned and pruned rather than topped. Proper pruning involves maintaining the natural integrity and balance of the tree. (For detailed information on proper pruning visit the International Society of Arboriculture's (ISA) consumer website: www.treesaregood.org). Contact a Certified Arborist (https://www.isa-arbor.com/Credentials.) who is trained in tree health and care if you are in doubt about caring for your landscape trees. S/he will determine the proper pruning and thinning procedures and otherwise assess the overall health of your tree.
Prevent Circled and Kinked Tree Roots. Circled, kinked tree roots can lead to stressed trees that drop limbs or topple, causing injury to people and structures. They occur when trees are left too long in containers, are planted in too small of a planting hole, or when the planting hole is filled with organic matter such as compost. In the latter case, roots will often prefer the higher quality compost to the surrounding native soil and never grow beyond its bounds. In some cases, one or more condition exists. To avoid these problems, make sure roots are not overgrown at the time of purchase and do not add soil amendments to the planting hole.
Avoid Mechanical Injury to Tree Trunks. Remove staking ties before they cut off the flow of water and nutrients in the vascular system of the trunk. If the tree will not support itself due to high winds, loosen the ties and move outward any stakes that may rub the trunk as the tree grows. Add mulch around trees (but avoid contact with trunks) to prevent damage from mowers and weed trimmers. (Mulch also reduces soil evaporation and weed seed germination).
Avoid Soil Grade Changes. Grade changes from construction activities should be kept several yards from trees. Even small changes in the depth of the soil can result in serious root system injury. As little as four to six inches of soil applied on top of the root zone of a mature tree can dramatically reduce the amount of oxygen available to roots and can kill a tree over time. Lowering the grade can cause lower trunk and root injury and reduce the nutrient and moisture supply to the roots.
The University of California Cooperative Extension San Bernardino County Master Gardener program is now accepting applications for the October 1, 2019 - February 18, 2020 Master Gardener class on Tuesday evenings in Redlands. (There are no classes on Dec. 25 and Dec. 31.)
The class provides 50 hours of training on sustainable landscaping and growing food in home, backyard, and community gardens and is taught by University of California experts and knowledgeable practitioners. Master Gardeners come from all walks of life and no college degree is required. What successful applicants have in common is a passion for sharing knowledge gained from the training class with residents of San Bernardino County. Accepted applicants agree to volunteer a minimum of 50 hours by June 30, 2021 via one or more outreach methods: answering email and phone helpline questions; making presentations at workshops and community events; staffing information booths at Farmers Markets and other non-profit events; writing blogs and promoting the program via social media; working with communities and schools to develop gardens; and working with Healthy Communities throughout San Bernardino County to encourage outdoor exercise and activities. Accepted applicants must pass two open-book exams, present a group class project, and pass a background check (approximately $30). The class fee (includes training materials) is $175. Visit our UCCE Master Gardener website for more information and to complete an online application: UCCE San Bernardino County Master Gardener Website. Applications must be received through the online system by or on August 31, 2019 to be considered.
There is a hybrid partially online option for residents of the high desert and mountains. Simply select this option on the application if this pertains to you. All costs and requirements remain the same as for in-class students.
Questions? Contact UCCE Master Gardener Coordinator Maggie O'Neill at firstname.lastname@example.org
We hope to hear from you!