Defence responses in the xylem of Ulmus americana cultivars after inoculation with Ophiostoma novo-ulmi

Earlier, it has been shown that cultivars of American elm (Ulmus americana) can differ in their susceptibility to Dutch elm disease (DED) and in their ability to compartmentalize infection. To gain a better understanding of how certain factors of compartmentalization influence disease susceptibility, histological and histochemical studies were performed on five cultivars of American elm and two wild-type seedling populations. There were a variety of differences in barrier zone formation and barrier zone characteristics among the cultivars which may help explain variability in resistance to DED. Timing of barrier zone production may be one factor that helps determine whether a tree survives infection. At 20 days postinoculation (DPI) in 2015, “New Harmony,” which had one of the highest mean disease severity ratings (DSR), was the only cultivar to have no barrier zones present in the samples examined. Barrier zones were present in all trees examined in 2016 for the two cultivars with the highest mean DSR, with many of the trees at 100% permanent wilt at 90 DPI, providing evidence that the formation of barrier zones does not ensure the tree will survive infection. When examining stem sections of these cultivars from 2016 for autofluorescence under blue light, which is indicative of phenolic compounds, they displayed significantly less autofluorescence than “Valley Forge,” which had the lowest DSR. Another important finding from this work is that despite having weak or discontinuous barrier zones, cultivars can still have relatively low DSR. “Prairie Expedition” and “Princeton” had multiple samples which had barrier zones which were breached or circumvented. When a barrier zone was breached, these cultivars often formed a subsequent barrier zone. Findings from these examinations help illustrate the complex nature of compartmentalization in American elm and how a variety of factors are affecting disease resistance.