Resistance breeding against nonnative pathogens in forest trees — current successes in North America

Abstract

Nonnative invasive pathogens have decimated North American forests for over 100 years, and additional pathogens continue to arrive. These pathogens are destined to be permanent fixtures in the ecosystem. In many cases, management activities have been unsuccessful in slowing the spread of these pathogens or in restoring forests. Genetic resistance potentially provides an invaluable management tool for restoring these species or using them in plantations. Although native tree species are highly susceptible to some nonnative pathogens, a low frequency of resistance is present in even those North American host species most affected. Classical breeding methods can produce genetically diverse and resistant populations for reforestation or restoration of natural forests. However, any operational program for developing resistant populations of forest trees must contend with relatively long generation times, as well as the long-lived nature of trees and the potential of the pathogen to evolve. Western white pine (Pinus monticola), PortOrford-cedar (Chamaecyparis lawsoniana), American chestnut (Castanea dentata), and American elm (Ulmus americana) are examples of species for which there are successful breeding programs for disease resistance. Examples from these species, with a particular focus on western white pine and white pine blister rust [Cronartium ribicola] resistance, will be used to illustrate some of the successes in operational programs, as well as to discuss some of the research needs and current unknowns in developing durable resistance. Some of these programs have been ongoing for 50 years, while the Port-Orford-cedar program began only in the last decade. Resistant seedlings from several programs are now being used in reforestation and restoration. 279Résumé : Les organismes pathogènes envahissants exotiques ont décimé les forêts nord-américaines depuis plus de 100 ans et de nouveaux agents pathogènes continuent d'arriver. Ces agents pathogènes sont destinés à devenir des résidents permanents de l'écosystème. Dans plusieurs cas, des activités de lutte contre ces agents pathogènes n'ont pas réussi à ralentir leur dispersion ou à restaurer les forêts. La résistance génétique a le potentiel pour être un outil d'aménagement inestimable pour la restauration de ces espèces ou pour leur utilisation en plantation. Même si les essences indigènes sont très sensibles à certains agents pathogènes exotiques, un faible taux de résistance est présent, même dans les essences nord-américaines les plus affectées. Les méthodes classiques d'amélioration génétique peuvent produire des populations génétiquement diverses et résistantes pour la reforestation ou la restauration de forêts naturelles. Cependant, tout programme opérationnel de développement de populations résistantes d'arbres forestiers doit composer avec des durées de génération relativement longues, ainsi qu'avec la longévité des arbres et le potentiel d'évolution des agents pathogènes. Le pin argenté (Pinus monticola), le faux cyprès de Lawson (Chamaecyparis lawson...

Insect outbreak shifts the direction of selection from fast to slow growth rates in the long-lived conifer Pinus ponderosa

“…Tree improvement programs supply seed resources for managed tree plantations, and for restoration purposes after natural and human-caused disturbances (e.g., fire, severe drought, and reclamation). These programs have traditionally focused on selection on growth-related traits (52), although efforts to breed for tree resistance against insects and pathogens are currently in place (53). Our results indicate that the traditional focus on fastgrowth by tree breeding programs may reduce survival under intense, unpredictable stress (54).…”

Genetic Resistance to Fusiform Rust in Southern Pines and White Pine Blister Rust in White Pines—A Contrasting Tale of Two Rust Pathosystems—Current Status and Future Prospects

“…Incorporation of multiple R genes and different types of partial resistances in an elite seed orchard appears to be the most effective approach for long-term WPBR durability [13,98]. However, currently we do not know how many genes or quantitative trait loci (QTL) underlie white pine partial resistance as well as the genetic effects contributed by each gene or QTL through their interaction with environmental factors.…”

“…Fortunately, genetic resistance to these rusts has been discovered in all pines species studied [6][7][8][9][10][11][12][13][14][15]. Resistance breeding programs, begun over 50 years ago, continue to produce trees with resistance in the U.S. and Canada [9,16,17].…”

Effects of Host Variability on the Spread of Invasive Forest Diseases

“…Usually, in the initial phase of an invasive pathogen epidemic, either resistance in the host population is completely lacking or resistant host genotypes are at too low of a frequency to reduce the effects of the pathogen [97,98]. Successively, the continued exposure to the pathogen will select for less susceptible host genotypes, which will increase in frequency.…”