Susceptibility to butternut canker disease in field trials is influenced by disease level and hybridization

Butternut (Juglans cinerea) has been decimated due to butternut canker disease (BCD). To date, much of the observed disease resistance in butternut occurs in hybrids with Japanese walnut (J. ailantifolia), a non-native species which was introduced to the US and naturally hybridizes with butternut. We screened a US range-wide collection of butternut and hybrid butternut, including over 300 open-pollinated families, for susceptibility to BCD. Trees were planted in Indiana, US, in two areas (with high or low disease levels) and allowed to become naturally infected. After 10 years, severity of disease symptoms was assessed using a five-point rating scale. We used logistic regression to evaluate factors, like ancestry (butternut or hybrid butternut), on the probability of a tree being rated as resistant. Across all sites, ancestry was a significant predictor of resistance. Our results also demonstrate heritability of disease susceptibility on sites where there was sufficient disease pressure for trees to display disease symptoms. While the most resistant families are hybrid, some butternut families do show modest levels of resistance based on estimated breeding values for each family. Variation in disease level across experimental plantings highlights the importance of considering site influence on disease development. Identifying sites less conducive to disease development may aid in butternut conservation and restoration.