Foamy bark canker disease poses threat for Marin’s live oaks
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Martha Proctor
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There is a new cause for concern regarding coast live oaks (Quercus agrifolia) in Marin County and elsewhere in drought-stricken California — a foamy bark canker disease caused by a fungus, Geosmithia pallida.
This disease was first identified in Europe as early as 2005, but was not found in Southern California until 2012 and showed up in Napa, Sonoma and Marin counties last year. The spores of the fungus hitch a ride into the tree’s vascular system on the Western oak bark beetle.
This offending small brown beetle is native to California. It burrows through bark in drought-stressed or recently wounded coast live oak trees, excavating shallow tunnels under the bark across the grain of the wood. Female beetles lay their eggs in the tunnels; the developing larvae mine galleries that branch out from the egg-laying gallery within the phloem (inner bark) close to the surface.
Symptoms of foamy bark canker disease include wet discoloration on the trunk and main branches of the infected coast live oak tree. This discoloration surrounds the entry holes that the beetle makes to burrow into the tree. Multiple perfectly round holes, approximately 30 to 40, can be seen in an infected tree. If the outer bark of an infected tree is peeled back, bark necrosis surrounding the entry hole is visible. As the disease advances, a reddish sap may be seen oozing from the entry hole, followed by a prolific foamy liquid. This foamy liquid, the cause of which remains unknown, attracts butterflies and ants and may run as far as 2 feet down the trunk. Oaks with infected branches can sometimes be saved, while oaks with infected trunks die.
No known methods are in place to control the fungus or the beetle. However, there are two preventive measures that can protect at-risk trees. First, because the beetle is only attracted to drought-stressed, mostly urban trees, it is critical to irrigate trees in years affected by drought. The amount of water depends on the size of the tree and the composition and drainage capacity of the soil.
Native California oaks have evolved in a Mediterranean-type climate where there is little rainfall between late spring and early autumn. As such, they generally do not require irrigation during this dry period and, in fact, may be adversely affected by supplemental watering since warm-moist conditions can favor harmful diseases.
Oaks should be watered only within the root protection zone, defined as the area just inside the drip line (the outermost edge of the tree’s foliage). It is critical that the trunks remain dry. If the winter season is unusually dry, then supplemental irrigation in the early spring can complement natural rainfall. Water deeply, to 1 to 2 feet, in the outer two-thirds of the root zone.
Although oaks are prone to crown and root rot with overwatering, it is now thought advisable to water drought-stressed oaks once a month in July and August with deep, slow waterings to wet the soil to 8 inches down, letting the soil dry between waterings. Frequent shallow watering not only encourages crown and root rot, but it also promotes the growth of ineffective shallow roots near the surface.
A second method involves spraying high-value trees with an insecticide such as Astro or Onyx.
Homeowners should assess their coast live oaks for health and indicators of drought stress. Even applying a minimal amount of water can help reduce a tree’s attractiveness to beetles and increase its natural resistance. Be aware that pruning drought-stressed coast live oaks when the beetles are active could attract them.
Freshly cut live oak firewood should be tightly covered with clear poly sheeting for about three months to seal in and kill bark beetles. Do not move the firewood to another location as the bark beetles could infect healthy trees.
Although the beetles are not known to burrow into trees of other species or other varieties of oak, it may be too early to be sure.