- Author: Steven Swain
Sudden oak death (SOD) is a disease syndrome that has killed millions of native oak trees (Figure 1) along the west coast of the United States, from Big Sur in California up to Southern Oregon. The disease may involve several organisms, but its main driver is the fungus-like organism (known as water mold), Phytophthora ramorum. This plant pathogen is spread in the springtime by windy rainstorms. It infects the bark of oak trees, frequently creating bleeding trunk cankers that interfere with water uptake and sugar transport.
Death of SOD-infected trees can be accelerated by attacks from bark and ambrosia beetles. In the absence of beetle attacks, infected oaks may take years to die.
Many common disorders (other than P. ramorum infections) can cause damage that resembles SOD, so laboratory testing is needed to confirm the diagnosis.
Sudden Oak Death Host Range
Sudden oak death isn't always sudden, nor does it infect just oaks. The potential host list of P. ramorum includes hundreds of plant species, many of which are natives of California's woodlands and forests. In most of these host species, the pathogen simply causes small necrotic spots on the leaves (Figure 2). In a few instances, notably in certain susceptible Rhododendron cultivars, the disease may progress from the leaves down into the stem and kill the plant.
Phytophthora ramorum has evolved as a foliar pathogen, primarily spreading from the leaves of infected hosts such as bay laurel (Umbellularia californica), tanoak (Notholithocarpus densiflorus), and rhododendrons. Therefore, it does not spread easily to other plants from oak trunk cankers. Thus, when oaks become infected, they are referred to as terminal hosts. Tanoak acts as both a foliar host, spreading spores from its leaves in wet and windy weather conditions, and a terminal host, developing cankers that are almost always lethal to the infected plant (Figure 3). Perhaps because infected tanoak leaves rain spores onto their own trunks, their SOD survival rates are among the lowest of all trees that may become afflicted by the disease in North America.
Despite its fearsome reputation, SOD doesn't always kill infected oaks. Valley oak (Quercus lobata), blue oak (Q. douglasii), and Oregon white oak (Q. garryana) are not known to develop cankers in nature, while coast live oak (Q. agrifolia), black oak (Q. kelloggii), Shreve oak (Q. parvula var. shrevei), and interior live oak (Q. wislizeni) are considered susceptible. Recent studies have demonstrated that there may be considerable variance in some coast live oaks, with measured resistance varying from about 16-40%. Many of these resistant or tolerant trees do become infected but are able to defeat the pathogen before trunk cankers enlarge to life-threatening sizes. Casual observations suggest that resistance levels seen in coast live oaks are likely similar to those in Shreve oak and black oak populations. Because interior live oak trees grow in comparatively hotter, dryer, environments than the other susceptible oaks, they almost never become infected, so resistance levels have not been studied. Tree age seems to play a role too. Oak trees under four inches in trunk diameter at chest height are not typically susceptible to infection by P. ramorum.
Treatment Approaches
Many different treatment approaches have been trialed, a few of which have shown promise.
Prevention
Potassium phosphite compounds (AgriFos, Reliant, Garden Phos, etc.) work best as preventive treatments. Most of the efficacy achieved by these compounds appears to be the result of stimulating the trees' natural defensive systems, although individual oaks vary widely in their immune responses. Thus, the application of potassium phosphite compounds is a bit of a gamble as to whether it will actually help an individual tree. Trees that are already showing symptoms of infection when treated have a significantly lower survival probability than trees that appear healthy during treatment.
There are two different recommended application methods for potassium phosphites—surface sprays made directly to the bark (using a surfactant such as Pentrabark) and trunk injections. Some applicators have claimed increased efficacy from the trunk injections, albeit at the cost of potential damage to the tree from wounding. Others have claimed equivalent efficacy with repeated bark sprays, when carefully timed. Application of calcium to the root zones of oaks treated with potassium phosphite has been shown to further improve resistance rates.
Potassium phosphite treatments should be made in the spring and fall, regardless of the application method used. This is because the uptake of potassium phosphite by the tree is dependent on high transpiration rates. High transpiration rates in turn depend on both adequate available soil moisture, and warm and sunny weather, preferably with a light breeze.
Another preventive treatment option that has proven to be effective is removal of foliar hosts that are near highly valued oak trees (Figure 4). Removing bay laurels that have foliage within about 30 feet of an oak trunk decreases the chances of that oak becoming infected. Removal is especially effective for small, understory foliar hosts, such as young bay laurels, poison oak, and rhododendron. This is not a recommendation for the wholesale removal of bay laurel trees, which are important parts of the California forest ecosystem and should be retained where appropriate. This management approach should only be adopted after careful consideration of the ecological function of the forest or woodland as a whole.
Management of Active Infections
Bark scribing, or cutting away the outer portion of infected bark to let the infection site dry out, was previously touted as an effective treatment for infected oaks. However, rigorous testing has since shown that it does not significantly increase the odds of an oak surviving. Similarly, the application of whitewash to tree trunks has not been shown to make measurable improvements to survival of treated oaks as compared to no treatment at all.
Mefanoxam (Subdue Maxx, Stergo MX, etc.) is a fungicide with a proven record of suppressing Phytophthora activity. It has been used to successfully treat infected plants, keeping them alive as long as treatment continues, typically over the span of a year or two. However, in most cases, once treatment stops, Phytophthora begins growing again within several months, and the infected plants will eventually succumb to the disease. While mefanoxam fungicides may be effectively used in certain limited situations, they should not be considered effective curative treatments. Resistance to this active ingredient has developed repeatedly, rendering it unacceptable for long-term use.
In summary, no silver bullet exists for preventing SOD, and there are no effective tools that will reliably save a tree that is already showing symptoms of infection. This does not mean that any oak that exhibits SOD trunk cankers or bleeding symptoms will die. Bleeding from the bark is a normal response to substantial damage, whether from a pest, pathogen, or, sometimes, even mechanical damage. Even if bleeding is the result of infection by P. ramorum, symptomatic trees have recovered with no intervention, usually in association with a few dry years. The most effective tools for managing SOD are preventive, in nature. For more information and best management practices, please see the California Oak Mortality Task Force website at: https://www.suddenoakdeath.org.
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- Author: Belinda J. Messenger-Sikes
Because mistletoes can damage trees, you may decide to do something about mistletoes infesting your trees. The first step is to find out whether you're dealing with broadleaf or dwarf mistletoe. Mistletoes differ in their life cycles, the damage they cause and management methods. UC Cooperative Extension Advisors Igor Lacan (San Mateo and San Francisco Counties), Steven Swain (Marin County) and Ed Perry (Stanislaus County, retired) have updated and expanded Pest Notes: Mistletoes. It includes more about identification of broadleaf and dwarf mistletoe, the latest information about where specific mistletoes grow in California, as well as details about how dwarf mistletoe infest trees and under what circumstances. The publication has management options for both types of mistletoes, information on community-based mistletoe management, a new section about the ecological benefits of broadleaf mistletoes, and new references.
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Symptoms of anthracnose vary by plant host and weather conditions. High humidity and dense canopies can exacerbate this common disease. Management relies on planting resistant cultivars of landscape plants along with careful maintenance of susceptible cultivars, such as pruning and removal of fallen leaves and twigs.
Authors Jim Downer (UCCE Ventura County), Steven Swain (UCCE Marin County), and Amanda Crump (UC Davis Plant Sciences) recently revised Pest Notes: Anthracnose which includes a list of anthracnose-resistant trees as well as more information about the life cycle of Anthracnose and how it spreads, and an updated section on chemical control.
- Author: Steven Swain
[From the August 2015 issue of the UC IPM Green Bulletin]
California has a number of native bark beetle species that individually do only minor damage as they bore through the outer bark to reach the inner bark (phloem) and wood surface. Bark beetles are relatively small insects; most species are smaller than a grain of rice. When their population densities are low, these beetles typically attack only the most stressed pines. However, when their population densities are high, they attack and kill healthier trees (Figure 2). If drought conditions continue and beetle populations grow, the beetles can attack in numbers large enough to overwhelm tree defenses.
Deep-red fresh sap around open tunnels typically indicates that a more serious invasion is in progress. Severely stressed trees cannot fight back, and may produce little or no sap at all. Instead, all you may find are piles of frass in the bark fissures or on the ground. Regardless of whether the tree is defending itself or not, little can be done to stop an infestation once the beetles are in the tree.
Since little can be done to treat a bark beetle infestation, preventive measures are the best practice. The care a pine receives can have a significant impact on its survival when bark beetle populations are on the rise. If the tree is of high aesthetic value, consider irrigating it in late spring or early summer. Place a soaker hose just inside the dripline, cover it with mulch if possible, and run the hose until the soil is moist to a depth of 12 inches. Wait until the upper 12 inches of soil is dry before irrigating again.
Learn more about various beetles that attack trees at UC IPM's Pest Notes: Bark Beetles or the tree-boring beetles page.
- Steven Swain, Environmental Horticulture Advisor, Marin & Sonoma counties, svswain@ucanr.edu
/span>- Author: Steven Swain
- Author: Karey Windbiel-Rojas
[From the July 2015 issue of the UC IPM Retail Nursery and Garden Center IPM News]
What are Pantry Pests?
Beetles and moths are the two most common groups of pantry pest insects encountered in California. The most common pantry moth is the Indian meal moth. Adults are small (about 1/3 inch), with characteristically broad-banded
Several species of beetles feed on stored food products, including the warehouse beetle, sawtoothed grain beetle (Figure 2), and the drugstore beetle, just to name a few.
While adult pantry beetles and moths are easy to tell apart, it is often the larvae that are found in the grain or flour; and these can be challenging to distinguish. Beetle larvae (Figure 3) are either grublike and legless or have three distinct pairs of legs, all located near the head. Moth larvae (Figure 4)
Damage
Pantry pests consume food but also can contaminate food with their bodies and by-products. Indian meal moth larvae produce frass (excrement) and webbing, and some beetle larvae produce secretions that give food a disagreeable odor and taste, or microbes that can produce carcinogenic compounds. Warehouse beetles have setae (hairs) that can irritate the mouth, throat, and stomach of people who eat infested products.
The keys to controlling these pests in the home are prevention and sanitation. Some important considerations include:
- Inspect newly-purchased grain products (including bird seed and dried pet food), nuts, dried fruits, potato chips, spices, and other foods of plant origin for signs of infestation.
- Seal any infested products in a plastic bag and dispose of them in an outside trash receptacle.
- Transfer pest-free products to hard-walled containers (glass, metal, hard plastic) that can be tightly sealed. Plastic bags aren't adequate, as they develop tiny holes over time and because they can't always be reliably sealed.
- Vacuum up any spilled products within food storage areas, including any crumbs or debris in shelving cracks and crevices or beneath shelf paper. Follow this up with a good wipe-down with soapy water.
Some pantry pests can survive on other items in the house; if the infestation persists despite cleaning up the kitchen areas, consider inspecting the rest of the house. Art made of grain, nuts, or seeds; woolen articles; stored furs; animal skins; rodent droppings; and even uncontained aspirin or other pharmaceuticals can be used as a food source by some pests. Others can survive on dead insects, so wall voids and attic spaces may have to be checked as well.
Use traps to monitor
Many home improvement stores and retail garden centers sell pheromone traps which can be used to monitor for pantry pests and to help in managing pest outbreaks. Using traps alone, however, will not successfully control pantry pests if the measures above have not been taken to clean up and prevent pest invasion and reintroduction. Also, pheromone traps are pest-specific, so identify pests first, read labels carefully, and be certain you are steering customers to the right trap. Place the traps within or near the previously-infested area and to check them weekly. Most traps remain effective for about three months. Newly trapped insects are a sign that stored foods may be infested and should be inspected or that hidden pupating larvae from a previous infestation have emerged as adults.
Find information on identifying, preventing, and managing pantry pests on the UC IPM website at www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7452.html.
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