Plant pathologists find growing number of plants affected by sudden oak death

The newly discovered disease, Sudden Oak Death (SOD), is quickly gaining a reputation, and it’s not a good one. SOD is tenacious and lethal, using as many as 26 different plants as hosts and spreading in ways scientists don’t completely understand. Now, recent research suggests that SOD is capable of using an even greater number of host plants than previously thought. While this is not necessarily good news, it does help shed light on why SOD has been so quick to spread.

“SOD is deadly for oaks and it’s impacting many other species as well,” states Matteo Garbelotto, an extension forest pathologist and adjunct professor at the University of California, Berkeley, and a leading researcher on SOD. Not long after the isolation of the microbe causing SOD by U. C. Davis Professor Dave Rizzo in 2000, plant pathologists began to suspect that while oaks were the direct victims of the disease, other plants were involved in spreading it. Plants from the rhododendron family were among the first host plants identified. “What we hypothesized and what we’re now confirming,” says Garbelotto, “is that SOD is not spreading via the oaks, but is instead using a huge range of native plants for reproduction.”

In fact, research by Garbelotto and Rizzo indicates that nearly all of the main tree species in California’s forests, as well as forest shrubbery and undergrowth, may act as hosts for SOD. SOD appears to use the leaves, branches and stems of these plants to reproduce, resulting in lesions and leaf discoloration. It doesn’t kill the host plant outright, but scientists say repeated SOD infections are likely to weaken the plant over time, negatively impacting its growth and making it susceptible to other diseases and insects.

And the more host plants SOD is able to use, the greater its potential impact on California’s forests and ecosystems. Says Garbelotto, “SOD’s reproductive strategy may make it able to persist indefinitely in infested forests and may affect the success of future regeneration and restoration efforts.” While these new developments are worrisome, they are not without hope. “The more we know about how SOD is spreading, the greater the chances for finding a way to control it,” says Garbelotto.

And there is great news for those interested in learning more. Those working on the front lines of SOD will be sharing what they know, what they don’t know, and what they hope to learn when they convene for a special online meeting and discussion forum, “Sudden Oak Death – How Concerned Should You Be?” April 21 – May 4, 2003 at http://sod.apsnet.org. It’s free and anyone with an interest in SOD is invited to participate.

There is also a full report on the recent research by Garbelotto and his colleagues on APS’s website at www.apsnet.org. The American Phytopathological Society (APS) is a professional scientific organization dedicated to the study and control of plant diseases with 5,000 members worldwide.