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 lit 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.”
Also in the January-March 2009 issue of California Agriculture journal:
• New pistachio varieties evaluated for California orchards
• Sanitation standards revised for navel orangeworm
• Dairy cattle should be tested for health conditions
• Allelopathy in sudex cover crops may damage subsequent cover crops
• Biomass crops can disinfest and remediate soils
California Agriculture is the University of California’s peer-reviewed journal of research in agricultural, human and natural resources. For a free subscription, go to: http://californiaagriculture.ucop.edu, write to calag@ucop.edu or call (510) 642-2431 x33.