Forest Health and Biotechnology Possibilities and Considerations (2019) / Chapter Skim
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3 Mitigating Threats to Forest Health
3 Mitigating Threats to Forest Health
Pages 53-92
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From page 53... ...
. When outbreaks of insect pests and diseases affect only one or a few tree species, the larger impact of such pests is directly related to the dominance of the host species.
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management practices to prevent or decrease the infestation. The enhancement and use of genetic resistance can proceed through the development and deployment of selective-resistance breeding within either the native species or from closely related nonnative species or the development and deployment of resistance using biotechnology.
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, many attempts have been unsuccessful. For example, eradication of white pine blister rust (Cronartium ribicola)
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. As of 2018, it is generally acknowledged that white pine blister rust will have a permanent presence in North America.
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. Management practices that favor native predators of the insect pests (conservation biological control)
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Ash Containment and management strategies related to the emerald ash borer (EAB) have focused on pre vention of further dispersal and diminishing insect pressure through insecticides and biological control
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Pruning focuses on protecting existing trees and their genetic diversity but does not increase the genetic resistance of future progeny. Maintaining diverse forests and planting mixed stands where the site naturally supported multiple species is another management tool to promote ecosystem resistance to insect pests and pathogens.
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From page 60... ...
Many forest tree species have at least some genetic resistance, even if at low frequencies, to most pathogens or insects (Lattanzio et al., 2006; Sniezko and Koch, 2017; Woodcock et al., 2017; see Box 3-2)
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. Whitebark Pine A selective breeding program for whitebark pine with resistance to white pine blister rust has been ongoing since the 1990s in the Oregon and Washington portions of the species' range.
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62 FOREST HEALTH AND BIOTECHNOLOGY BOX 3-2 Continued FIGURE 3-2 Overview of the theoretical expectations upon which The American Chestnut Foundation backcross breeding program was initially based. NOTE: The illustrated "recovery" of the native American chestnut with each round of backcrossing is based on theoretical expectations that are rarely achieved in practice unless DNA marker-assisted selec tion is used in every generation.
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FIGURE 3-3 Testing whitebark pine for resistance to white pine blister rust at the U.S. Forest Service's Dorena Genetic Resource Center.
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. As with white pine blister rust, genetic resistance to the mountain pine beetle has been found (Six et al., 2018)
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Ash All of the ash species native to North America have some susceptibility to the EAB, with the widely distributed green ash and white ash being very susceptible. Monitoring forests for individual trees with intact canopies after most of the ash in the monitored plot have died from EAB was initiated a few years after EAB was detected in Michigan in 2002 (Knight et al., 2012)
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Field tests will be needed to determine the efficacy and durability of the resistance from the selective-breeding programs. The time line for restoration with genetically resistant ash will depend on the search for additional selections and the results in the field trials.
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For example, in Asia, what resistance is present in the native white pine species, where presumably the pines and the white pine blister rust fungus have coevolved? It would be helpful to know whether similar resistance exists (even at low frequency)
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However, only two tree species modified using biotechnology had been grown outside of field trials in the United States in that time.1 The first tree species to reach this stage was papaya (Carica papaya)
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Mutagenesis Although the frequency of naturally occurring gene sequence polymorphisms is high in most forest trees, methods have been developed for increasing variation by inducing mutations in the DNA sequence (direct mutagenesis)
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In the future, novel synthetic genes could be of value for forest health, particularly for generating highly specific resistance to attacks by insect pests and pathogens if and when natural resistance genes are overcome by newly evolved variants of pests or pathogens. The committee is aware of the possibility of the eventual creation of such novel synthetic genes; however, the relevance to this report is purely hypothetical.
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Many tree species have structural and developmental differences that either required adaptation of methodology or precluded application altogether. Furthermore, many characteristics common to trees (e.g., dormancy, wood formation, and obligate outcrossing)
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. Whitebark Pine No effort to date has focused on utilizing biotechnology to impart genetic resistance in whitebark pine.
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FIGURE 3-6 Wild-type American chestnut seedlings (left) , Darling transgenic American chestnut seedlings (middle)
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utility of biotechnology for developing resistant white bark pine would be if new types of genetic resistance (of a durable nature) not found in whitebark pine were identified and transferred into whitebark pine.
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. This recapitulates the success with this gene against the chestnut blight, Cryphonectria parasitica.
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Seed collection and testing of seedlings from hundreds or thousands of parent trees may be necessary. Those steps must then be followed with the development of breeding or orchard populations that have useful types and degrees of genetic resistance and sufficient genetic diversity to use for restoration (e.g., Sniezko et al., 2012; Dudley et al., 2017)
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. The case of whitebark pine with resistance to white pine blister rust demonstrates the variation in the time it takes from resistant parent identification to deployment of resistant seedlings based on geography and parent genetics (Sniezko et al., 2007, 2011)
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, which shows broad efficacy against fungal pathogens such as the Cryphonectria blight in American chestnut (Zhang et al., 2013) and Septoria leaf spots in Populus (Liang et al., 2001)
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In many tree species it can take 5–10 years or more until flowers are produced and the gene can be passed on to the next generation. The third step is field testing.
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Biological control agents can suppress insect pest populations or mitigate the effects of a fungal pathogen. However, experience with the American chestnut, whitebark pine, ash, and poplar indicates that these practices will be insufficient to curtail the loss of affected tree species.
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For whitebark pine, there are still no programs for drought tolerance or resistance to the mountain pine beetle. Recommendation: Entities concerned about forest health should devote resources to identifying resistant trees within a population that have survived a pest outbreak.
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2009. Sexually mature transgenic American chestnut trees via em bryogenic suspension-based transformation.
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2017. Emerald ash borer biocontrol in ash saplings: The potential for early stage recovery of North American ash trees.
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2006. The backcross breeding program of the American Chestnut Foundation.
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2017. Classical biological control of insect pests of trees: Facts and figures.
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1997. White pine blister rust control in North America: A case history.
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2007. The influence of white pine blister rust on seed dispersal in whitebark pine.
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2015. Verbenone decreases whitebark pine mortality throughout a mountain pine beetle outbreak.
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2007. Variation in resistance to white pine blister rust among whitebark pine families from Oregon and Washington -- early results and implications for conservation.
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2007. Availability of an Environmental Assessment for the Proposed Release of Three Parasitoids for the Biological Control of the Emerald Ash Borer Agrilus planipennis in the Continental United States.
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1995. Transfer of T-DNA from Agrobacterium to the plant cell.
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