Arboriculture & Urban Forestry 33(5): September 2007 Arboriculture & Urban Forestry 2007. 33(5):309–317. 309 Phosphite Injections and Bark Application of Phosphite + Pentrabark™ Control Sudden Oak Death in Coast Live Oak M. Garbelotto, D.J. Schmidt, and T.Y. Harnik Abstract. In each of seven fully controlled experiments, potted California, U.S. coast live oak trees were artificially inoculated with Phytophthora ramorum, the agent of a tree disease commonly referred to as sudden oak death. Phosphites were applied to trees using a range of application approaches either as preventive or therapeutic treatments. Soil drenches and bark application of phosphites were ineffective; foliar application of phosphites amended with surfactants were effective only at times and always caused phytotoxicity. On the other hand, injections of phosphites and bark applications of phosphites + the organosilicate surfactant Pentrabark™ (Agrichem, Medina, OH, U.S.) were consistently effective in suppressing bark colonization by this pathogen without causing phytotoxicity. This is the first study describing the use of a chemical treatment amended with an organosilicate surfactant for topical bark applications. Key Words. Chemical treatments; disease control; forest disease; forest management; Phytophthora ramorum; Quercus agrifolia; SOD. The use of phosphite fungicides has become common prac- tice in some agricultural or orchard crops severely affected by Phytophthora diseases. In particular, citrus and avocado or- chards have been treated with such compounds for decades in Africa, Asia, Australia, and even the United States and Eu- rope (Guest et al. 1995; Erwin and Ribeiro 1996; Hardy et al. 2001). The active ingredient in such fungicides is phosphonic acid, and although its mode of action is complex and still not completely understood, most researchers agree the effect of phosphonic acid is the induction of defense mechanisms in the plant rather than direct antimicrobial action. The effects of phosphite treatments include enhanced lignification, in- creased cell wall thickness, and enhanced production of sec- ondary plant metabolites, some of which are known to pos- sess antimicrobial properties (Guest and Grant 1991). Phos- phites are applied as extremely soluble neutralized salts of phosphonic acid. The compounds first move acropetally as a salt in the outer xylem into the leaves and then basipetally as phosphonic acid through the cambium in the trunk and roots. Efficacy of the compounds is thus maximized on breaking down of the salt into cations and phosphonic acid occurring in the leaves. Some researchers place a group of phosphite- like compounds characterized by a further direct effect on microbial growth into a different category called phospho- nates (Hardy et al. 2001). In this study, we compared the effect of both types of compounds and we refer to both as phosphites. Because of their mode of action, aimed at enhancing the defense mechanism of treated hosts, rather than at directly arresting or killing the microbial disease agent, phosphites are excellent candidates not only for agricultural treatments, but also for control of soilborne pathogens in natural ecosystems. Secondary effects, in fact, are mostly on the plant itself (phy- totoxicity, changes in flowering or fertility, potential changes in mychorrhization rate) without any consequence to the ex- isting forest microbial community. Finally, phosphites are excellent candidates for treatment in natural ecosystems be- cause they have extremely low toxicity to invertebrates, aquatic organisms, or animals, including humans. Phosphite applications have been extensive in Australian wildlands in- vaded by the aggressive exotic soilborne pathogen Phytoph- thora cinnamomi (Hardy et al. 2001), and some successful treatment attempts are reported for oaks infected by the same pathogen in southern Europe (Fernandez-Escobar et al. 1991). Phosphites are normally applied as injections on trees and as foliar sprays on herbaceous shrubs. The most common side effect, phytotoxicity, varies depending on the plant spe- cies. Sudden oak death (SOD) is a devastating forest disease caused by the newly described pathogen Phytophthora ramo- rum (Garbelotto et al. 2001; Rizzo et al. 2002). Although its origins are still unknown, the pathogen is present in three distinct areas: in coastal forests of California and southern Oregon, in European plant nurseries, and in nurseries of the ©2007 International Society of Arboriculture
September 2007
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