Abstract
Hybridization is a potential tool for incorporating stress tolerance in plants, particularly to pests and diseases, in support of restoration and conservation efforts. Butternut (Juglans cinerea) is a species for which hybridization has only recently begun being explored. This North American hardwood tree is threatened due to Ophiognomonia clavigignenti-juglandacearum (Ocj), the causal fungus of butternut canker disease (BCD), first observed in 1967. Observational evidence in some wild J. cinerea populations indicates that naturalized hybrids of J. cinerea with Japanese walnut (Juglans ailantifolia) may be more tolerant to BCD than non-admixed J. cinerea, but this has not been formally tested in a controlled trial. We aimed to examine potential BCD tolerance within and between J. cinerea and J. cinerea × J. ailantifolia hybrids and to determine if there is a difference in canker growth between BCD fungal isolates. Five-year-old J. cinerea and hybrid trees were inoculated with two Ocj fungal isolates collected from natural infections found in two different sites in Indiana, United States, and a blank control (agar only). Measurements of both artificially induced and naturally occurring cankers were taken at 8-, 12-, 20-, 24-, and 32-month post-inoculation. Differences in canker presence/absence and size were observed by fungal isolate, which could help explain some of the differences in BCD severity seen between J. cinerea populations. Smaller and fewer cankers and greater genetic gains were seen in hybrid families, demonstrating that hybrids warrant further evaluation as a possible breeding tool for developing BCD-resistant J. cinerea trees.
Highlights
Native and non-native diseases and pests are increasingly threatening ecosystems, especially forests, across the globe (Ennos, 2015; Early et al, 2016)
The objectives of this study were to examine potential butternut canker disease (BCD) tolerance within and between non-admixed J. cinerea (“J. cinerea”) and J. cinerea × J. ailantifolia hybrids (“hybrids,” unless otherwise noted) and to determine if there is a difference in canker growth between isolates of Ophiognomonia clavigignenti-juglandacearum (Ocj)
The percent of individuals with canker growth present at the inoculation site strongly increased over time (χ2 = 186.87, p < 0.0001; Figure 1A)
Summary
Native and non-native diseases and pests are increasingly threatening ecosystems, especially forests, across the globe (Ennos, 2015; Early et al, 2016). There are concerns that hybrids could be detrimental to both the target species and its ecosystem through potential invasion (Muhlfeld et al, 2014), outbreeding depression (genetic incompatibilities or reduced fitness; Allendorf et al, 2013), and genetic swamping (loss of local adaptations by genetic dominance from another species; Allendorf et al, 2013). Desirable traits, such as disease and pest resistance conferred through hybridization, may be one of few remaining tools to save some species (Sniezko and Koch, 2017). The American Chestnut Foundation (TACF), one of the leading organizations in this effort, has been breeding and backcrossing C. dentata hybrids for three generations over 30 years and is currently trialing hybrids with increased resistance in several restoration sites in the eastern United States (TACF, 2020)
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