310 Garbelotto et al.: Controlling Sudden Oak Death west Coast of the United States and Canada. It is a generalist pathogen that causes foliar blotches and extensive branch dieback on tens of plant species both in the wild and in nurseries. In California forests, this pathogen produces abun- dant infectious airborne propagules on the leaves of bay lau- rel (Umbellularia californica, Lauraceae) and on the petioles and leaves of tanoak (Lithocarpus densiflorus, Fagaceae), whereas it sporulates less abundantly on the leaves and twigs of several species. The disease on such hosts can vary from an aggressive dieback, resulting in death of the infected plant, to a modest foliar blotch or blight. During periods character- ized by wet conditions and mild temperatures, the stems of several oak species and of the related tanoak may be infected. Stem infections almost invariably end with a lethal girdling of the entire plant as the pathogen thrives in the sugar-rich cam- bial layer and sometimes can reach the outer xylem of in- fected plants, which are incapable of walling off cambial infection. Such lethal trunk cankers do not allow for sporu- lation of the pathogen and are thus not involved in its dis- persal. For spread of the disease to occur at the landscape and regional scale, infectious foliar hosts need to be present. Long-distance movement of the pathogen is linked to the trade of infected nursery plants as clearly shown by Ivors et al. (2006). Although related to the soilborne P. cinnamomi, P. ramorum, whose life cycle is dominated by an aboveground (e.g., aerial) phase, has a unique biology and epidemiology among forest Phytophthora species. Girdled oaks and tanoaks can survive up to 2 years after girdling. It takes from 2 months to 1 year for the pathogen to infect and girdle an adult tree. Although variation in suscep- tibility to SOD has been reported for the two most severely affected hosts, coast live oak (Dodd et al. 2005) and tanoak (Hayden and Garbelotto 2005), it is unclear whether the less susceptible trees are capable of surviving the infection or whether they may experience a relatively more prolonged, but equally lethal, disease. From an ecological perspective, tanoaks are the most severely affected trees with an average of 30% mortality in the areas infested by the pathogen and with the loss of entire adult populations in certain areas (Maloney et al. 2005). Average mortality is lower for coast live oaks, and trees away from infectious hosts such as bay laurels or tanoaks, until now, appear unaffected (Swiecki and Bernhardt 2002; Davidson et al. 2005). Despite the intermediate susceptibility to infection of oaks, urban development in California has favored mixed ever- green forests characterized by a significant component of both bay laurel (infectious host) and coast live oak (terminal host). For this reason, SOD has seriously impacted many coastal communities of California, infecting and killing tens of thousands of oak trees at the urban–wild interface. Many of these trees are integral components of residential proper- ties and are commonly located near roads, homes, camping ©2007 International Society of Arboriculture sites, and recreational areas. Infected trees are prone to be windthrown even before they are dead attributable to the enhanced activity of subsequent wood decay agents once the tree is girdled. The development of a chemical treatment appears as a necessary tool to slow the potential extinction of the very susceptible tanoak and to protect valuable oak trees from infection and death. This article reports on a series of studies in which we tested the efficacy of phosphites to control SOD on coast live oaks. The aims of such studies specifically were: 1) to determine the efficacy of phosphite treatments in coast live oak against P. ramorum, including potential unwanted phytotoxicity; 2) to compare the efficacy of different application methods, including a thoroughly novel approach; 3) to compare the efficacy of preventive versus early and late therapeutic treatments; and 4) to compare the efficacy of different phos- phites. MATERIALS AND METHODS Experimental Setup Coast live oak trees in 15 gal (3.9 L) pots were tested at three experimental sites, two in Marin county and one in Alameda county. Trees were drip-irrigated with well, unchlorinated water and kept under a shade cloth intercepting 50% of solar radiation to recreate conditions favorable to infection by P. ramorum. Trees were 2 to 4 m (6.6 to 13.2 ft) tall with calipers ranging between 2 and 8 cm (0.8 and 3.2 in). Trees were in part purchased by the University of California and in part donated by Valley Crest Tree Company (Calabasas, CA) and were placed at the study sites at least 1 month be- fore the beginning of each trial. At the very end of each experiment, to ensure safe disposal of inoculated wood, por- tions of the stem artificially inoculated with P. ramorum were cut off and autoclaved. Similarly, before being discarded, soil from each pot was tested for the presence of the pathogen by standard baiting techniques using D’Anjou or Bartlett pears (Erwin and Ribeiro 1996). Soil was never found to be in- fested by the pathogen and thus was discarded without auto- claving. Inoculation Techniques Three isolates of P. ramorum, namely 0–4, 0–7, and Pr217, were used in trials 1 through 4. Because no significant dif- ferences of the tested treatments were observed among iso- lates (see “Results”), only Pr217 was used in all other trials and all results are presented ignoring pathogen isolate as a variable. The identity of the isolates was confirmed by mi- croscopic observation of the characteristic large terminal and lateral chlamydospores produced by P. ramorum and by DNA sequencing of the ITS region of the nuclear ribosomal operon as described in Rizzo et al. (2002). All three isolates
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