Exotic Beetle Killing Oak Trees

Exotic Beetle Killing Oak Trees

An exotic beetle has invaded Southern California and targeted coast live oaks. The tiny western oak bark beetle has long been a threat to the oaks but this variety brings a fungus with them that is now wreaking havoc on the iconic oaks. These pests are about the size of a grain of rice. Beetle infestation is identified by the red sap and cream colored foam that pours from holes in the trunks and branches. This is the first symptom of infestation as the sap leaks from the insect entry holes. The beetles lay eggs which grow into larvae beneath the bark and feed on the widespread fungus rather than the tree itself. A single tree can be infested with many beetles at once which greatly increases the chances of death. If just the branches are infected, they can be pruned to stop the spread of the disease. The fungus called foamy bark canker disease blocks water and nutrients from circulating inside the oaks which causes the evergreen tree to turn brown. Once the sap and foam stop flowing, the only evidence of a problem is the small holes. The beetle is infecting trees in Los Angeles, Thousand Oaks, Orange, Santa Barbara, and Monterey counties. The disease is raising wildfire danger concerns as dead trees provide wildfire fuel. In addition to killing the prized trees, the wildlife that depend of the habitat of the oaks is also affected. Species include the Red-shouldered hawks, big, flycatcher’s mule deer, rodents, and quail. Additionally, these oaks are in danger of widespread wife fires due to the drought. Researches from the University of California have been unable to discover the origin of this insidious pest, but they are determined to find and eradicate the source. The insects can be further spread by moving infested firewood.

Orange County Register, Sunday May 11, 2014. Brooke Edwards Staggs, Staff Writer.

Care of California’s Native Oaks

Big tree

A big tree

Young native oak trees are much more tolerant and adaptable to environmental changes, while mature, established oaks are intolerant of change. This means that any major change in a mature oak’s environment may weaken the tree or kill it. To reduce the chance of this happening, leave the tree’s RPZ, or root protection zone, undisturbed. The RPZ is defined as one and a half times the area from trunk to drip line. A healthy root system for a mature oak begins with the formation of a tap root when the tree is a seedling. This allows the tree to have assessable moisture. As the tree grows, lateral roots extend horizontally (sometimes up to 90 ft.) past the dripline. This root system also forms fine roots typically within the top three feet of the soil, and these roots absorb moisture and nutrients. To support the natural dry weather conditions in the summer, the tree also grows deep vertical roots, usually within ten feet of the trunk, which helps support the mature tree. Therefore, mature oaks need protection from excessive water, inadequate drainage, over fertilizing, filling, paving, over pruning, and trenching within the root zone.

Excessive water and inadequate drainage can smother roots and encourages crown and root rot fungi. Conditions should be dry during summer months, therefore only drought-tolerant plants that require no summer watering should be planted around mature oaks. Even then, plants should be planted no closer than six feet from the base of the tree. Groundcovers such as cobbles, gravel, and wood chips can also be used because they don’t interfere with getting the right amount of moisture and oxygen. Usually, mature oaks don’t require any irrigation, except in cases where there is an impervious surface within the RPZ. If this is the case, occasional watering may be helpful – as an extension of the normal wet season. Otherwise, oaks should be watered only outside the RPZ. Moist, warm soil at the base of the tree encourages crown and root rot. If a tree needs to be watered, water only once or twice during the summer with a slow, all-day soaking. Shallow watering promotes shallow root growth and also encourages crown and root rot. Rot and root drowning may also be caused by placing a swimming pool, which acts as a dam, downhill from a mature tree.

Mature oak trees usually need no supplemental fertilization, because they get nutrients from the leaf litter that breaks down around the tree. If fertilizing is to be done, it should be done in late winter or early spring. Trees that have undergone recent severe pruning should not be fertilized. Sometimes when leaves yellow, it is not a sign of nutrient lack, but rather a sign of crown or root rot.

Trees that have fill or paving placed with the RPZ are more likely to have problems with soil compaction, which prevents water from soaking into the soil and inhibits gases from being exchanged between roots, soil, and atmosphere. Fill can also trap moisture in the soil and cause crown and root rot. Paving also usually requires excavation to create a stable base for the paving material. Both excavation and the compaction adversely affect the root system. If any paving is needed, decking on piers is the least intrusive to the oak’s root system. It’s also best to leave the natural soil grade within the RPZ and, if necessary, use retaining walls to keep soil out of the RPZ.

Another aspect of oak care is pruning. Excessive pruning affects an oak tree in a number of ways, from exposing interior branches to sun damage, encouraging new growth that is more easily attacked by mildew, and possibly causing decline in the health of the tree. Pruning should be done during winter dormant periods and only weak, diseased, or dangerous branches should be removed. Any wounds should not be sealed, as recent research shows sealants do more harm than good. It is recommended that pruning should only be performed by a certified arborist.

Oak trees also need protection from trenching within the RPZ. Trenching can sever important tree roots when underground utilities are installed. Especially detrimental is when multiple trenches are cut through the roots. If utilities are installed within the RPZ, they should be dug by hand or they should be bored at least three feet below the surface.

Oak tree care also includes being aware of diseases and insects, which can impact the health of the tree. The tree can usually be saved if the disease is caught early. Crown rot is caused by a microscopic fungus and is promoted by saturated soils and poor soil aeration. Symptoms of crown rot are decline in tree vigor, twig die-back and wilting, lesions that ooze a dark-colored fluid, and abnormally yellow leaves. Oak root fungus, or Armillaria root rot, is almost always found in California oak trees, but doesn’t usually affect the tree unless the tree is over watered or weakened in some way. Trees affected by root rot can have branch die-back or yellowing and thinning of foliage. To avoid these symptoms, avoid summer irrigation. Mistletoe is a parasitic plant that can harm oak trees, usually be causing structural weaknesses and branch breakage. Major infestations are difficult to control and it’s best to consult an arborist for advice on mistletoe or any unexplained decline in an oak tree. Insects can also harm oak trees; especially pit scales, oak moths, and other leaf eaters. These insects weaken the tree and make it susceptible to disease. Consult an arborist when insect infestation causes a change in leaf color, twig die-back, substantial leaf loss, or sticky or sooty foliage and branches.

If all of these issues are addressed in an oak tree’s care, the oak should be healthy and last a considerable amount of time.

Oak Trees

Donna Giustizia in Ontario, Canada, is asking that the oak trees near her daughter’s school be cut down due to the risk of a nut allergy reaction from the acorns off the trees. Because she felt strongly about the possible hazards, she appeared before the Vaughan, Ontario, City Council to ask the council to remove the trees. Giustizia heads the school’s allergy committee, and although the school says it is nut-free, she feels that the school isn’t doing enough to protect its students.

The Vaughan City Council has said that they will prepare a report about the nut problem that will be read at their next meeting. Giustizia has said that she is not requesting that the entire town become nut free, but wants the school to be free of all nuts, including acorns.

When asked about the possibility that acorns are a hazard to children with nut allergies, Dr. Clifford Bassett, medical director of allergy and asthma care at New York University, said he has never encountered a case of an allergic reaction by playing with acorns found on the ground. “There’s no relationship between acorns and peanuts,” said Bassett.


Oak Trees


Apple gall

According to Michele Warmund from the Missouri University College of Agriculture, Food and Natural Resources, galls on oak trees, frequently seen in autumn months, are harmless. Galls appear as round, brown growths, especially prevalent on oaks. Warmund has had inquiries about the growths from property owners and thinks that the unseasonably warm spring may have contributed to the increase in the production of galls this year. Property owners want to know if they should spray insecticides or trim trees to try to get rid of the galls. She says, “Do nothing. Just sit back, relax, and enjoy them. Mother Nature will probably take care of them as leaves drop and insects seek shelter underground during the winter.”

Oak galls are caused by a gall midge, which looks like a long-legged fly. Whey they first form in the spring, they are magenta or red. In the fall they turn brown and crusty, and at that time the gall bursts, releasing a larvae that buries into the soil to overwinter while pupating. In the spring, adults emerge to lay eggs and start the cycle over again.

Galls can also be found on other plant species like hickory, walnut, chestnut, and elderberry.


Oak Trees Damaged in Fire

Old tree

Old tree

Two oak trees on the Auburn University campus were burned in the early morning hours in November. A video showed the trees rolled with toilet paper and two men passing the trees just before the fire started. A week after the fire, most of the trees’ foliage had browned and begun to fall off. The trees were examined by members of Auburn University’s Trees Task Force and were found to have severe injury and death of most of the rootstalk and rhizomic shoots, damage to the bark of the trees at ground level, leaf scorching, and damage to surrounding trees and shrubs. Because of the timing of the fire, new leaves will probably not form on the tree until next spring.

The trees were already in severe decline due to a case of poisoning last year, and the task force said that the fire would likely further weaken the trees.  “Everyone takes a lot of pride in the oak trees and no one wants to see something like that happen” said Police Capt. Tom Stofer.


Mapping to Proactively Reduce Sudden Oak Disease and Death

Designed to help protect historic heritage trees, a comprehensive map, called SODMAP, details the spread of sudden oak death – a tree-killing microbe. The disease is spreading from the forested hills into residential areas all over the Bay Area, and it has already killed hundreds of thousands of oak trees from southern Oregon to Big Sur. The map is to be updated annually to track the movement of the disease, which will help cities and counties plan projects to keep the disease from spreading and to preserve healthy trees.

The disease, most aggressive during the rainy season, was discovered in Mill Valley in 1995. It kills both big oak trees and the smaller tan oaks. Other plants, like the California bay laurel, camellia, and rhododendron, are host plants of the disease. Any oak less than 0.6 miles from the disease is at a high risk of becoming infected. The disease is also spread through the water. Arborists and ecologists predict that in 25 years, 90% of California’s live oaks could die from the disease.

This map may counteract this worst-case scenario. “People need to know that if they can take action before their trees are infected, then they can really slow down the rate of infection and minimize the number of trees that are infected,” says MatteoGarbelotto, a UC Berkeley forest pathologist who helped create the map. “The biggest hindrance to protecting oaks is that people don’t think about it until the trees are infected, so the more we let people know there are these tools, the more they will know they can do things to make things better.”

There are three recommendations to help mitigate the spread of the disease:
1. Remove California bay laurels that are near oaks – this increases the survival rate of oaks tenfold.
2. Use phosphonate spray – it’s effective against the disease.
3. Avoid big projects like soil removal, grading, or tree pruning during the rainy season in infected areas.