Preventative Phosphonate Treatment for Sudden Oak Death Effective for Up to Two Years

       OAKLAND, Calif., Jan. 22 (AScribe Newswire) -- Treatment with the fungicide phosphonate was effective in stemming the spread of sudden oak death - a tree disease that has killed thousands of oaks and tanoaks in 14 California coastal counties - for up to 2 years, according to a series of studies by UC Berkeley researchers published in the January-March 2009 California Agriculture journal.

       Conversely, in the studies an alternative treatment comprised of azomite soil amendments and a lime bark wash was ineffective in stopping the spread of sudden oak death.

       The alternative treatment "is like treating pneumonia with orange juice," says Matteo Garbelotto, UC Berkeley adjunct professor of soil pathology, whose lab conducted the studies.

       The January-March 2009 issue of the University of California's California Agriculture journal is available online: http://californiaagriculture.ucop.edu/0901JFM/toc.html; for a free copy, e-mail janet.byron@ucop.edu.

       First discovered in Marin County in 1995, sudden oak death is caused by the exotic water mold Phytophthora ramorum, which is carried by more than 100 plants species; it kills tanoak trees and four types of oaks (black canyon, coast live and Shreve's). California bay laurel trees are a major transmission source of P. ramorum in forests that they share with oaks and tanoaks.

       The UC Berkeley researchers conducted three studies: a direct comparison of the two treatments (phosphonate and azomite soil amendment/lime bark wash) on potted oak trees in greenhouses; a field study that measured the efficacy of treatments 6, 12 and 18 months afterward; and a treatment efficacy study using potted plants that had been "wounded" and artificially infected with P. ramorum. In the field study, the authors developed a new method in which branches of infected trees were brought back to the laboratory, so that living trees would not be placed at risk.

       "Our [studies] indicated that phosphonate treatments had a significant effect at 6,12 and 18 months post-application," wrote Garbelotto and co-author Doug Schmidt of UC Berkeley. "Because most P. ramorum infections occur in late winter and spring, 18 months of coverage will adequately protect trees for 2 years."

       Phosphonate is the only legally registered treatment for sudden oak death in California. It is recommended to prevent the disease in areas where P. ramorum has been found, but it does not cure trees that have already been infected. The azomite soil amendment/lime bark wash treatment is not registered, but has been marketed as a natural way to bolster tree defenses against sudden oak death.

       "Azomite appeals emotionally to a lot of people," says Janice Alexander of the California Oak Mortality Task Force. "Now we'll be able to tell them that it doesn't work."

       The phosphonate treatment is considered environmentally friendly. It is applied either by drilling holes in trees and injecting the fungicide into the tree cambium, or topically to tree bark in mixture with a surfactant called Pentrabark.

       Garbelotto and his colleagues have been offering sudden oak death treatment workshops for professional tree-care specialists and property owners.

       "We advise treating oaks with no sign of infection when symptoms show up on neighboring bay laurels," Garbelotto says. "We also advise that even if some oaks have died in a grove, it may be possible to protect those that have not been infected."

Etiology and Evidence of Systemic Acidification In SOD Affected Forests In California

Data on pH from 34,700 soil samples taken from a wide range of agricultural and forest soils in California indicate that between 14 and 21% of the soils are acidic (pH < 6.0) and 3 to 4% are strongly acidic (pH < 5.0). However, a subset of samples taken from SOD-affected sites indicates that 72% of these soils are acidic and 4% are strongly acidic (median pH = 5.7; n = 132). The soils from these sites were also found to be consistently low in Ca and very high in soluble Al and Fe. Spatial analysis reveals a strong coastal gradient in soil pH with the lowest pH values found near the coast. Strong coastal gradients are also
apparent in soil Ca, which is lowest near the coast, and in soil Al, which is highest near the coast. Precipitation chemistry data from this region also reveal a coastal pH gradient much like that found in the soils. Similar coastal gradients in precipitation pH have been reported from the Olympic peninsula, from southeast Alaska, and from Scandinavia.

    These results lend further support to the theory that systemic acidification is adversely affecting the
health of the trees and soils in SOD-affected forests. The situation described here in California is not unlike that in other regions of the world where aging forests are experiencing decline. From this and other work (e.g., studies at Hubbard Brook), we strongly believe that the cause (and the definition) of SOD is still an open question, and that the scope of SOD research should be expanded to include studies of acidification
by cryptogams in the context of forest and soil ecology.