No abstract

Restoration of American chestnut (Castanea dentata) depends on combining resistance to both the chestnut blight fungus (Cryphonectria parasitica) and Phytophthora cinnamomi, which causes Phytophthora root rot, in a diverse population of C. dentata. Over a 14-year period (2004 to 2017), survival and root health of American chestnut backcross seedlings after inoculation with P. cinnamomi were compared among 28 BC3, 66 BC4, and 389 BC3F3 families that descended from two BC1 trees (Clapper and Graves) with different Chinese chestnut grandparents. The 5% most resistant Graves BC3F3 families survived P. cinnamomi infection at rates of 75 to 100% but had mean root health scores that were intermediate between resistant Chinese chestnut and susceptible American chestnut families. Within Graves BC3F3 families, seedling survival was greater than survival of Graves BC3 and BC4 families and was not genetically correlated with chestnut blight canker severity. Only low to intermediate resistance to P. cinnamomi was detected among backcross descendants from the Clapper tree. Results suggest that major-effect resistance alleles were inherited by descendants from the Graves tree, that intercrossing backcross trees enhances progeny resistance to P. cinnamomi, and that alleles for resistance to P. cinnamomi and C. parasitica are not linked. To combine resistance to both C. parasitica and P. cinnamomi, a diverse Graves backcross population will be screened for resistance to P. cinnamomi, survivors bred with trees selected for resistance to C. parasitica, and progeny selected for resistance to both pathogens will be intercrossed.

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With future global temperatures predicted to increase, the relationship between a host, pathogen, and environment, becomes less predictable and epidemics may pose a greater risk to forests worldwide. Resistance breeding is an important disease management tool, but because tree species require long breeding times, it is necessary to develop techniques for testing current pathogen isolates against their hosts. Pitch canker disease of pines, caused by the pathogen Fusarium circinatum, is no exception and represents a threat to pine forests and commercial plantations worldwide, as it thrives at warm temperatures and high humidity. We tested growth of 15 F. circinatum isolates in culture at three temperatures: 25, 27, and 31 • C. We also evaluated the sporulation and pathogenicity of eight of the isolates on two susceptible Pinus elliotti (slash pine) open-pollinated families and one tolerant open-pollinated Pinus taeda (loblolly pine) family. Our results showed significant differences among isolates in the temperature and pathogenicity tests. All isolates showed a significant decrease in growth at 31 • C, although some showed similar growth at 25 and 27 • C. Several of the new isolates tested were more pathogenic than the isolates that the USDA Forest Service Resistance Screening Center (RSC) had been using. The new isolates have now been incorporated into their operational screening program.Also, both pathogen and some of hosts have whole-genome drafts available [16][17][18] that would facilitate future projects designed to understand host-pathogen interactions at the genomic and molecular level.Due to the wide-scale planting of southern pines and their economic importance, there is an urgent need to understand, not only the genetics and host-pathogen interactions, but also interactions with the environment in order to develop practical strategies for the management of pitch canker and other diseases. Resistance to pitch canker is quantitative and heritable [19,20]; therefore, commonly used management strategies involve breeding and deployment of resistant host material. With future global temperatures predicted to increase [21][22][23], new climate variations and extreme weather events are predicted to increase [24][25][26][27][28], which may lead to increased disease incidence [29] and compromise the host's defense response [30][31][32][33]. These changes in environment may also alter the relationships between host, pathogen and environment [34]. This disruption may also change the geographical distribution of pathogens, bringing them into areas where they are currently absent. In the future, a larger number of outbreaks and possible pathogenic variants may be expected as new environmental conditions resulting from climate change may favor survival and fitness of some pathogens. In contrast, plant disease management strategies under new climate conditions are lagging [35].The USDA Forest Service Resistance Screening Center (RSC) in Asheville, North Carolina has been screening seedlings of pine and other forest tre...