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| Leaf affected by Leaf Blight |
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Species affected by leaf blight are foliar hosts. Many California natives have become foliar hosts. Hosts range from the common Bay Laurel (Umbellularia californica) to the Coast Redwood (Sequoia sempervirens). Darkened, necrotic leaves are commonly found on foliar hosts. Necropsy may spread down to the stem depending on the species. Dark spots surrounded by a light halo may also be seen on host species such as the Bay Laurel. Needle bearing species like the California Nutmeg (Torreya californica) and Coast Redwood may reveal the pathogen both through dead needles and needles appearing smashed together, this is leaf blight. Recent discoveries keep adding to a growing list of possible hosts including some species in the fir family (Abies).
The and other researchers have been attempting to determine the . Because SOD is a disease that has emerged recently, many details of the disease cycle are not yet understood. Here is one .
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How Can Sudden Oak Death be Stopped?
Currently there is no cure for Sudden Oak Death, although the research continues. One fungicide is approved both as a preventative measure and to slow onset of the disease of recently affected organisms (trees). Another preventative measure involves spraying a specific chemical on oak trunks and lower limbs. The hope is the substance will prevent the pathogen from entering treated trees. Another method of control is injecting a particular chemical into the base of oak trunks. It is hoped this method will prolong lifespans of treated trees. However, many of these treatments can have adverse impacts on the person applying it, or on pets, birds, insects, and other animals, if not applied carefully. These products need to be applied properly to prevent impacts, and should only be applied by a qualified professional. Check out the for more information for homeowners.
Other ongoing research includes studies centering on California’s fire regime and whether recent changes in fire frequency has aided Phytophthora ramorum’s success. Initial studies found infested areas occur in locations where fire has been absent for an extended period of time. The pathogen is most common and abundant in urban areas correlating with an infrequent fire regime. But additional research on historic fire regimes and ground truthing is needed to obtain a more accurate picture and reliable conclusions on this matter.
Other studies, some of which have occurred at Sonoma State University, attempted to find connections between historic land uses and the spread of the pathogen. Still other studies have attempted to link smoke with inhibition of the pathogen and its reproduction levels. Research on the pathogen when exposed to smoke has been investigated by U.C. Berkeley. However, none of these studies have been conclusive. Working theories include these two beliefs: (1) exposure to smoke only prevents growth in the pathogen while actively sporulating, and (2) inhibition is more likely with increased exposure to smoke. Problems arise with these theories including the lack of wildfires during the wet season when the pathogen is reproducing. Another problem with these theories is the inconsistency of smoke patterns in wildland fires. The inconsistency of smoke exposure during a wildland fire would make possible inhibition of the species even less likely.
The Marin County Fire Department has also investigated the fire regime issue. The department has found infested areas with Phytophthora ramorum where prescribed burns have occurred. This disproves the theory that fire itself could be the key to stopping the spread of Phytophthora ramorum.
Besides losing a piece of our cultural heritage and an important food source for wildlife, the die off of numerous trees raises other concerns. Increased tree mortality could increase fire risks to communities. Dead and down trees suffering from Phytophthora ramorum add significant amounts to fuel loads which previously did not exist. Just as fire suppression has played a part in altering our fire regime, so too could this newly increasing fuel load. However, instead of the decrease we see as a result of fire suppression, an increasing fuel load could increase fires.
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What Can You Do?
As a visitor to places with Sudden Oak Death be aware of your surroundings and what you can do to stop its spread. Now that you know the signs of SOD, be aware of the vegetation around you.
When you notice signs that may be SOD take precautions such as washing and disinfecting your shoes, tires, and anything that can transport vegetative particles and dirt. Disinfection can be accomplished with a diluted bleach solution.
Phytophthora ramorum reproduces through spores and wet winters encourage reproduction. Periods of high dispersal rates of Phytopthora ramorum are during windstorms and rain events. Be especially vigilant about disinfecting shoes, etc., after storms.
Wood brought from outside of the park for building a beach fire must be pine or almond. Do not bring oak, fir, redwood, madrone, or tanoak unless they are certified to be free of Phytophthora ramorum. Because Phytophthora ramorum may be found on a large number of trees and other plants and in soil, there are often restrictions on the transportation of these items from areas affected by Sudden Oak Death in order to limit the spread of this pathogen. When in doubt, do not remove infected material from where it is found. Further details on regulated plants and plant parts can be found at the United States Department of Agriculture's Animal and Plant Health Inspection Service's . Your County Agricultural Commissioner is one of the best sources for answers to specific regulatory questions pertaining to your area.
Spread the word about SOD, especially to those who engage in hiking, biking, and driving in and out of infected areas.
Remember the importance of oaks. In addition to being a part of our cultural heritage they are a keystone species in our ecological communities. Whole ecosystems of plants, animals, and fungi are dependant on the survival of this one species.
Visit the California Oak Mortality Task Force web site for more information on Sudden Oak Death.
Learn more by reading Bay Nature Magazine's October-December 2006 Watch Your Step: Tracking the Spread of Sudden Oak Death. If you garden, you may be interested in Bay Nature's What Gardeners Should Know About Sudden Oak Death. Additional information and links on Sudden Oak Death may be found at Bay Nature's Resources for Learning About Sudden Oak Death page.
Reports
Spatial distribution and impacts of Phytophthora ramorum and Sudden Oak Death in Point Reyes National Seashore, by Max Moritz, Tadashi Moody, Benjamin Ramage, Alison Forrestel, February 7, 2008 (1,565 KB PDF)
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