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Sudden Oak Death is one of the most recent tree diseases to emerge in North America. First described in 1995, the fungal infection is now spread along the coast from Monterey north to California?EUR??,,????'???s Oregon border. Caused by an organism called Phytophthora ramorum, the infection may be one of the most frustrating diseases tree professionals are forced to deal with.
Spores are spread by rainwater across California?EUR??,,????'???s north coastal woodlands, where they are seriously impacting college campuses, parks and other managed landscapes. Oak trees can be infected for months before symptoms become obvious. Traditional fungicides and pesticides have limited effectiveness. Recent research suggests that a multi-pronged approach works best. This means long-term monitoring of large areas followed by trimming, debris removal and some chemical application.
One ongoing research effort is led by Vacaville, Calif.-based Phytosphere Research. The firm is developing techniques to limit the spread and impact of P. ramorum in its Northern California stronghold. The following excerpts are reprinted with permission. To learn more, visit the company?EUR??,,????'???s web site at phytosphere.com. ?EUR??,,????'??+ Erik Skindrud
Landowners, superintendents and managers are seeking ways to protect coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and tanoak (Lithocarpus densiflorus) from Phytophthora ramorum, the pathogen that causes sudden oak death.
Managing the pathogen in tanoak stands is difficult because P. ramorum can complete its entire disease cycle on this host alone. Management in coast live oak and black oak is accomplished differently because the disease needs a host to complete the cycle. With these two trees, selective pruning and/or removal of California bay laurel is one technique that produces good results.
Potassium phosphite (also known by the trade name Agri-fos) is a selective, systemic fungicide with a high level of environmental safety and very low non-target toxicity. It has shown good levels of efficacy against P. ramorum canker in various trials.
Although phosphite can be injected directly into tanoak trunks, in this study we are applying phosphite as a spray to the trunk. For this application, the phosphite solution is combined with a relatively high amount (2.5% by volume) of an organosilicate surfactant (trade name Pentrabark). The trunk spray application allows one to treat a larger number of trees in a shorter time compared to trunk injection, so the spray method is more suited to treating multiple trees. We have used both an ATV-mounted sprayer and an electric backpack sprayer mounted on a custom cargo bicycle to apply calibrated amounts of spray solution to the treated trees.
For both types of sprayers, we used custom-made telescoping spray booms that allowed us to apply the spray high on the trunk – up to a height of about 20 ft.
TeeJet air induction nozzles are used to provide relatively large spray droplets and minimize drift. Nonetheless, some overspray and drift is inevitable.
We are currently using a spray program that includes two applications the first year (winter and spring), followed by annual reapplication. The reapplication interval may be extended later in the study if the one-year reapplication interval is shown to be effective. If funding is continued, we expect to observe these plots for at least 3 years.
To treat all 256 stems in the five sprayed plots described above required 39 gallons of spray solution. This volume of spray solution contains about 19 gallons of Agrifos and 1 gallon of Pentrabark.
Francisco Uviña, University of New Mexico
Hardscape Oasis in Litchfield Park
Ash Nochian, Ph.D. Landscape Architect
November 12th, 2025
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