Arboriculture & Urban Forestry 42(2): March 2016 DISCUSSION & CONCLUSIONS The two Ophiostomatoid fungi isolated from western white pine, Grosmannia clavigera and Leptographium longiclavatum, caused mortality of P. monticola seedlings following inoculation. Seedlings faded as early as 40 days following in- oculation with G. clavigera, whereas seedlings generally faded more slowly following inoculation with L. longiclavatum. These results may differ in mature trees, as trials were only performed on seedlings. However, Krokene and Solheim (1998) demonstrated that the results of inoculations of two- and four-year-old Norway spruce seedlings, with four bark beetle-associated blue-stain fungi, largely agreed with previous results of inoculated 40-year-old Norway spruce trees with the same fungal strains. These, as well as the current re- sults, suggest that inoculation of seedlings can be a reliable bioassay to predict the pathogenicity of blue-stain fungi associated with bark beetles. As L. longiclavatum is a recently described species, information on its virulence is not extensive. Lee et al. (2006b) proved its pathogenicity on 98- to 130-year-old lodgepole pine (P. contorta), sug- gesting that it may contribute to the mortality of MPB-infested pines. Rice et al. (2007b) compared the virulence of the three main associated blue- stain fungi—Ophiostoma montium, G. clavigera, and L. longiclavatum—in lodgepole pine, jack pine, and lodgepole × jack hybrids, concluding that these species are about equally competitive and virulent, finding no significant differences in lesion lengths between G. clavigera and L. longi- clavatum. The results of the current study differ from this, as a significant difference was observed in mortality and extent of circumferential lesions over time between the two species, with G. clavig- era being more virulent. This difference could be due to the variability of virulence in geographi- cally distant strains of G. clavigera and L. longicla- vatum, attributed to host-specific effects (western white pine versus jack and lodgepole pines and their hybrids), or could also be attributed to the amount of inoculum used for infection. Further research including the addition of a second inocu- lation point on the same seedling with L. longicla- vatum, may be helpful in elucidating this question. This injection experiment used a newly devel- oped technique, Micro I.V.’s, to inject the pesticide 91 treatments, and from researchers’ observations it performed effectively. Of the 5 ml of TREE-äge, Alamo, and Arbotect placed into the Micro I.V.’s precise uptake into each seedling was not calcu- lated. However, results indicate that the volume of solution administered and infused into the xylem tissues was sufficient to show effects in the xylem (e.g., lesion length). Seedling autopsies also revealed discoloration from dye, a compo- nent in the TREE-äge and Alamo formulations, visually confirming that the vascular tissues were exposed to the injected product. Seedling mortality was not observed following injection with TREE-äge, therefore, the method of injec- tion and application of TREE-äge per se is not likely to cause injury to western white pines. Since the isolates were obtained from TREE-äge- injected trees attacked by MPB, researchers strongly suspect the involvement of G. clavigera and L. longiclavatum and other MPB associates in tree mortality. Furthermore, it was demon- strated that G. clavigera and L. longiclavatum are pathogenic with differences in virulence on P. monticola seedlings. Field studies to evaluate tree protection using TREE-äge and Alamo in P. monticola trees attacked by MPB are currently underway, and will help determine the generality of these findings, and, more specifically, pro- vide an indication of their relative importance in determining mortality of larger trees in situ. In vitro studies provide useful information in the development of effective injection treatments for tree protection. These results demonstrate that TREE-äge alone did not provide any repression of G. clavigera, and are consistent with the recovery of isolates of G. clavigera and L. longiclavatum from treated trees. Repression of the fungi was evident in both Alamo and Arbotect injections, particularly at the higher concentrations (Table 2), suggesting that similar chemistries could be used for systemic treatment against blue-stain fungi. Research uti- lizing injection of both insecticide and fungicide chemistries is currently being conducted to evalu- ate protection against MPB/blue-stain associates in situ with P. monticola, the results of which will help to determine efficacy in tree protection. In the interim, it is advised that tree stewards and arbor- ists consider both vector and microbial associ- ates when treating trees with systemic pesticides. ©2016 International Society of Arboriculture
March 2016
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