The Sierra snowpack is deep, the reservoirs are full and Gov. Jerry Brown has declared the drought officially over. This is a huge relief for California drought-stricken conifer forests and oak woodlands. But, pardon a pessimistic twist on an old saying, “Every silver lining has a dark cloud.” The heavy and lingering rainfall creates an increased risk of Sudden Oak Death (SOD) disease spreading from infected California bay laurel trees into susceptible oaks.

The disease spreads in water, primarily rainwater dripping from infected bay laurel leaves onto coast live oaks and a few other susceptible tree species. Research has shown that the majority of infections occur when conditions are right: mild to warm and rainy.

Last year, Matteo Garbelotto, director of the UC Berkeley Forest Pathology and Mycology Laboratory said the drought had curtailed the spread of the disease somewhat but there were some new outbreaks. Increased rainfall gives the disease more opportunities to spread.

SOD is present in over 500 miles of coastal forests, hence the help of volunteers to survey such a large region is absolutely necessary. The annual SOD Blitz gives volunteers the opportunity to help.

Volunteers at 23 locations from Mendocino to San Luis Obispo are working again this spring to locate and map the disease. This is the 11th year of the ongoing effort and the seventh year in which Napa will participate. The Napa Blitz is on May 6.

The disease apparently arrived in California from Europe in infected nursery stock and quickly escaped into oak woodlands. There is no way to eradicate it. The best we can do is to manage it.

Here are several ways to do that:

— Assess the risk of infection in a given location.

— Select high-priority oaks for protection.

— Employ disease prevention and management practices that are known to be effective.

A primary function of the SOD Blitz is to assess disease risk at a given location. It can be a pleasant half-day outing for Blitz volunteers, especially working in pairs.

After a one-hour training session, volunteers receive a packet of small envelopes with forms for recording tree locations and other data for each California bay laurel sampled. Then they head off to locations of their own choosing to collect samples of bay leaves showing the typical infection symptoms.

The free smartphone app, SOD Map Mobile, is a big help in disease risk assessment and in filling out the sample collection forms. When opened at the location where the user is standing, the app provides the disease risk based on previous surveys and the GPS coordinates that can be written on the collection form.

Volunteers return the samples to a local drop-off location within 24 hours, where they are picked up and taken to the UC Berkeley Lab for analysis. Results are released in October, just before the treatment season begins. Halloween to Christmas is the best time frame to apply the systemic fungicide to oak trees selected for protection.

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Once property owners determine the degree of risk, they can make informed decisions for prevention and management.

There is only one preventative chemical treatment registered in California, produced by two different companies: Agri-Fos (Monterey; Agrichem Manufacturing Industries) and Reliant (Quest Products Corp.) Both contain phosphonate, the active ingredient.

The material can be injected directly into tree trunks or applied by the “basal bark” spray method when combined with Pentrabark (Quest Products. Corp.), a surfactant that helps the phosphonate and water solution soak into the vascular tissue.

Updated recommendations now include soil incorporation of granular gypsum at a particular rate and radius around the base of the tree. It has been shown to enhance effectiveness of the phosphonate.

Dr. Garbelotto says, “While a few hundred people participate in the SOD Blitzes, the data they generate is used by hundreds of thousands, making it one of the most useful and consequential citizen-science programs in the country.”

The Napa SOD Blitz is on May 6 at 9:30 a.m. at the Napa Resource Conservation District conference room 1303 Jefferson Street, Suite 500B. There is no pre-registration or fee. Just show up and take the opportunity to learn and make a positive contribution.

A wealth of information on SOD is available at this link: https://nature.berkeley.edu/garbelottowp/?page_id=148

Bill Pramuk is a registered consulting arborist. Visit his website, www.billpramuk.com, email questions to info@billpramuk.com or call him at 707-226-2884.

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