Arboriculture & Urban Forestry 33(5): September 2007 315 observed to fail when they were applied directly into or below such imperfections. Injections need to enter the outer xylem of the tree (last three rings), because these are the rings ac- tively involved in acropetal translocation of fluids into the leaves. Because translocation in coast live oaks occurs at high pressure, positive injection systems, in which pressure is con- stantly applied until the product is absorbed, are likely to be the most effective. It is best to inject phosphites during clear warm days when plants are known to be physiologically more translocative. Rainy or cold days should be avoided, because physiologically inactive plants will absorb the products more slowly. The data clearly indicated that the best options to control SOD were provided either by preventive treatments or by treatment of trees only recently infected. Unfortunately, because infection in adult trees can remain latent for several weeks or months, estimating the time since infection may be difficult or impossible. Hence, we strongly recom- mend preventive treatment of trees at risk of becoming in- fected. A combined understanding of the epidemiology of SOD and of the temporal dynamics of treatments’ efficacy is es- sential for best planning of chemical treatments (Rizzo and Garbelotto 2003). We have shown that phosphite treat- ments can be effective as long as 8 months. It is likely phos- phite treatments will be effective for 2 to 6 years as re- ported for other tree species (Hardy et al. 2001), but until the length of efficacy of treatments is clearly studied, we recommend at least one yearly treatment. Three to 6 weeks from application may be necessary to obtain maximum con- trol in adult trees. Oak infections appear to have a peak of infection in May through June and potentially another peak in February through March, especially in the milder coastal ar- eas. In such areas, two treatments may be recommended each year: one in November and one in March each year. In areas characterized by colder winters, a fall treatment in November followed by a spring treatment in March may be recom- mended for the first year followed by a single treatment in March for subsequent years. If a particularly wet year is predicted, two treatments should always be prescribed every- where, because conditions for infection may be extremely favorable. Besides chemical treatments, it is necessary to embrace other approaches to slow the spread of this exotic disease in California and elsewhere. Ensure all plant material is planted is certified as “free of pathogen” and do not move any plant material within the infested area or between infested and uninfested areas. Infected plant material should be locally disposed of by burning or composting. Debris exclusively made up of wood can be chipped and locally broadcasted in the dry season (rather than piled and covered by a tarp) to ensure rapid drying. It has been shown, in fact, that the patho- gen does not survive in the woody debris of oaks and tanoaks if dry (Davidson et al. 2005; Swain et al. 2006). Finally, all infectious hosts such as bay laurels, tanoaks, rhododen- dron spp., camellias, and the lower branches of redwood trees should be removed at least from an area 10 m (33 ft) in radius around the most valuable oak trees. Inoculum reduc- tion appears to be one of the best options to curtail infec- tion on coast live oak, a plant species that does not carry the pathogen in an infectious form, i.e., oak-to-oak contagion has not been reported or observed. The presence of tolerance or partial resistance to SOD is currently being investigated and may be another tool to control this devastating forest disease. In this study, we developed and tested a relatively novel application method, i.e., bark application of a fungicide in combination with an organosilicate surfactant. This applica- tion is user- and environment-friendly while remaining rela- tively inexpensive. We believe this application may allow for treatments of large number of trees without the need of drill- ing into the plant trunk. This application may enhance our outlook on chemical treatments of forest trees and widen it from a purely landscape tree perspective. Preventive treat- ments of selected trees by application of phosphite + Pentra- bark™ in a forest setting may represent a possible way of protecting natural populations of trees endangered by this introduced pathogen. Acknowledgments. This study was performed thanks to a com- petitive grant funded by the USDA Forest Service Pacific South- western Station and by a grant from the Gordon and Betty Moore Foundation, both to MG. We are grateful to Valley Crest Tree Company for donating 50% of the trees used in these experi- ments and to the Sunnyside Nursery and to Lucasfilm Ltd. for al- lowing placement of study plots within their properties in Marin County, CA. LITERATURE CITED Davidson, J.M., A.C. Wickland, H. Patterson, K. Falk, and D.M. Rizzo. 2005. Transmission of Phytophthora ramo- rum in mixed-evergreen forests of California. Phytopa- thology 95:587–596. Dodd, R.S., D. Hüberli, V. Douhovnikoff, T.Y. Harnik, Z. Afzal-Rafii, and M. Garbelotto. 2005. Is variation in susceptibility to Phytophthora ramorum correlated with population genetic structure in coast live oak (Quercus agrifolia)? The New Phytologist 165:203– 214. Erwin, D.C., and O.K. Ribeiro. 1996. Phytophthora Diseases Worldwide. APS Press, American Phytopathological So- ciety, St. Paul, MN. Fernandez-Escobar, F.J.B.R., M. Gallego, and M. Benlloch. ©2007 International Society of Arboriculture
September 2007
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