Arboriculture & Urban Forestry 42(2): March 2016 to have a mutualistic relationship with this insect. It is suggested that these fungi aid the beetle in over- whelming host defenses (Raffa and Berryman 1983), providing nutrition (Six and Paine 1998; Adams and Six 2007), and perhaps a more favorable environment for beetle development (Lee et al. 2006a). For exam- ple, axenic MPBs are capable of entering and mining host trees, but offspring are apparently insufficiently nourished and attacks do not result in successful reproduction (Six and Paine 1998). In addition, fun- gal symbionts of MPBs have been shown to cause tree mortality by themselves (Mathre 1964; Basham 1970; Shrimpton 1973; Strobel and Sugawara 1986; Yamaoka et al. 1995). It appears that the combined action of the beetle and the fungi result in a more rapid host death (Amman et al. 1989; Yamaoka et al. 1995; Solheim and Krokene 1998; Lee et al. 2006a). In a study using western white pine (WWP), P. monticola, trees in situ in northern California, U.S., TREE-äge® (4% [wt/wt] emamectin benzoate (EB), Syngenta Crop Protection, Greensboro, North Car- olina, U.S.) injected trees had significantly reduced adult MPB gallery length compared to untreated trees (10.1 ± 2.1 and 120.6 cm ± 13.4 SE, respec- tively), and did not appear to be girdled by adult beetle activity (Strom et al. unpublished). However, significant tree mortality was observed in the study. To investigate potential causes of tree death, study trees were felled, wood samples collected, and fungal isolates cultured. The isolates were submitted for identification to Drs. Michael Wingfield and Wil- helm de Beer of the Forestry and Agricultural Bio- technology Institute (FABI), Pretoria, South Africa, who confirmed isolates were two species: G. clavigera and L. longiclavatum, both of which are common associates of MPB (Lee et al. 2005; Lee et al. 2006b). In this study, the authors consider two factors that may impact tree survival: 1) injected formulation and 2) MPB fungal associates. Applied correctly, tree injections have been found to be both useful and effective against insects and pathogens. Doccola et al. (2011b) demonstrated that green ash (Fraxinus pennsylvanica Marsh.) successfully compartmental- ized tree injection sites following treatment with TREE-äge for emerald ash borer (Agrilus planipennis Fairmaire). Furthermore, hemlock woolly adelgid (Adelges tsugae Annand)-infested eastern hemlock (Tsuga canadensis L.) recovered with new growth following imidacloprid tree injection (Doccola et al. 85 2012). Other researchers have not documented tree mortality from injections, and have generally found little or no phytotoxicity with currently available products when used according to label (Grosman et al. 2002; Smitley et al. 2010; Doccola et al. 2011a; Doccola et al. 2011b; McCullough et al. 2011). Although Ophiostomatoid species vary in their pathogenicity to trees, a few do kill trees and this is especially true of species exotic to the host. Among these are Ophiostoma novo-ulmi (causal agent of Dutch elm disease), Ceratocystis fagacearum (Bretz) Hunt (causal agent of oak wilt), and Raffaelea lau- ricola (causal agent of laurel wilt). In addition, the native black stain root disease (Leptographium wageneri) can be a lethal vascular wilt pathogen of conifers in California, U.S. (Wagener and Mielke 1961). All of these species are vectored by insects that bore into trees. Furthermore, MPB-associated fungi were reported to cause injury and mortality following inoculations in lodgepole pine, jack pine, and lodgepole × jack hybrids (Strobel and Suga- wara 1986; Yamaoka et al. 1990; Yamaokoa et al. 1995; Lee et al. 2006b; Rice et al. 2007a; Rice et al. 2007b; Rice and Langor 2009). It is suggested that once fungal associates colonize the sapwood, they impede water and mineral transport in these tissues (Amman 1978). However, the authors are not aware of previous research regarding the virulence of G. clavigera and L. longiclavatum in P. monticola. In this study, researchers report on the effects systemic of injections of TREE-äge, Alamo, and Arbotect to WWP seedlings and the subse- quent challenge of these treatments via inocula- tions with G. clavigera or L. longiclavatum. The effects of the three systemic pesticides on the in vitro growth of G. clavigera were also investigated. METHODS Fungal Isolation Trees cut from a field study in the Modoc National Forest, Alturas, California, U.S., were the source of wood samples used in the fungal isolations. Sample chips were removed from wooden block samples of TREE-äge-treated and untreated trees as de- scribed in Shigo (1986) using a No. 11/4 concave wood gouge. Chips were placed into Difco™ Malt Extract Agar (MEA) and incubated in the dark at 25°C for one week. Samples were then examined ©2016 International Society of Arboriculture
March 2016
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