Five cultivars and two populations of wild-type seedlings of American elm (Ulmus americana), 3 and 4 years old, were examined for differences in their abilities to compartmentalize and resist infection by artificially inoculating with Ophiostoma novo-ulmi.Morphological characteristics of tree defence, often referred to as the compartmentalization of decay in trees model, were used as a conceptual framework, with particular emphasis on the limiting of tangential spread of infection within the xylem and barriers that limit spread outwards to cells formed after infection. To investigate the change in functional xylem over time, 3-year-old trees were assessed at multiple time points following inoculation for hydraulic conductivity. Three and four-year-old cut trees were placed in 0.1% w/v safranin O for 18 to 24 hr to indicate functional xylem. Transverse sections of the stained stems were used to calculate per cent of sap-conducting xylem area and the per cent of circumference conducting of first formed cells and later formed cells. At each collection time, trees were assessed for disease severity on a 1-12 scale, based on the percentage of permanent wilt in the crown. There was considerable variation between cultivars in disease severity and their capacity to localize and resist infection. "Prairie Expedition," which had the lowest disease severity rating in 2015 and the second lowest in 2016, consistently limited the spread of infection into newly formed xylem and had functional xylem around the entire circumference of the stem at 90 days post-inoculation. "Valley Forge" in 2016 had the lowest overall disease severity ratingand was the only cultivar to consistently limit the tangential spread of infection within extant xylem. This research identifies key characteristics that some cultivars have to resist and limit infection and provides new information that can be used in disease screening programmes to evaluate other cultivars and older plant material.

Cryptodiaporthe corni is the causal agent of a destructive disease called golden canker, which affects Cornus alternifolia, known as the pagoda or alternate-leaved dogwood. Due to the association between Cr. corni and pagoda dogwood, we sought to determine whether this fungus was capable of living as an endophyte in pagoda dogwood and causing this disease. Forty asymptomatic stems of plants growing in nature were sampled from five sites across Minnesota. Cryptodiaporthe corni was present in more than half (62.5%) of the stems. Asymptomatic nursery material also was sampled, and the fungus was isolated from a small percentage (20%) of them. Inoculations carried out in the field and greenhouse suggested the endophytic isolates of Cr. corni were capable of causing disease. Asymptomatic stems of trees in the field inoculated with non-colonized (control) grain seed developed golden canker as frequently as those inoculated with grain seed colonized by Cr. corni, suggesting that the fungus was already present in these plants. In greenhouse pathogenicity trials an isolate of Cr. corni obtained from an asymptomatic stem was capable of causing golden canker disease, thus demonstrating causality, fulfilling Koch's postulates. The taxonomic placement of Cr. corni within Cryphonectriaceae was determined. Phylogenetic analysis of the ITS rDNA and β-tubulin gene regions, along with morphological characteristics, suggested Cr. corni is distinct from other genera within this family. Therefore, we propose a new genus, Aurantioporthe, as well as the new combination, A. corni, to accommodate this species within Cryphonectriaceae.

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.

Avocado (Persea americana), an important fruit crop, is under threat from an invasive disease, laurel wilt. The pathogen, Raffaelea lauricola, spreads rapidly in the xylem of infected trees and causes a lethal vascular wilt. A previous study showed that variation in susceptibility to the disease exists among different races of avocado, with the West Indian race being most susceptible. To help elucidate potential explanations for differences in susceptibility, xylem characteristics were examined for fourteen avocado cultivars from the Guatemalan, Mexican, and West Indian botanical races. Samples of each cultivar were assessed for vessel size, vessel density, vessel aggregation, and xylem-specific potential hydraulic conductivity. The West Indian race had significantly greater mean vessel diameters, mean maximum vessel diameters, and xylem-specific potential hydraulic conductivities than the Guatemalan and Mexican races (p < 0.05), which in turn did not differ for any of these variables (p > 0.05). There were no significant differences among the races for vessel aggregation or vessel density. Cultivars of the Mexican and Guatemalan races generally had smaller mean vessel diameters, mean maximum vessel diameters, and mean xylem-specific potential hydraulic conductivities than the West Indian race; however, there was considerable variation among cultivars of the Mexican race. Statistically significant differences in vessel grouping indices and vessel solitary fractions were evident among some cultivars but to lesser extents than were found for vessel size. This study indicates that larger vessel diameters and greater potential hydraulic conductivities exist in the West Indian, compared with the Guatemalan and Mexican races. We suggest that these attributes may be contributing factors in the greater susceptibility to laurel wilt that is evident in the West Indian race.

Traditional screening of American elm (Ulmus americana) for resistance to Dutch elm disease (DED) often requires many years between initial propagation of trees and inoculation of older trees in the field. Previously published studies have found an association between smaller vessel diameters and increased resistance to DED, but further validation was needed to determine whether it could provide a rapid screening method to identify candidate trees for further testing. This investigation examined xylem characteristics in main stems of three‐year‐old trees for five cultivars and two wild‐type seedling populations of U. americana with varying levels of resistance to DED. Cultivars with low disease severity ratings tended to have smaller vessel diameters and higher vessel densities than cultivars with high disease severity ratings. Xylem characteristics were also assessed in branches and main stems of larger plant material. Data suggest that the use of main stems is preferential to branches when evaluating large trees, as main stems provided more resolution in differentiating between the genotypes. Results from this study indicate that there is potential for the use of xylem characteristics, such as vessel diameter and vessel density, for selecting trees with putative resistance. However, caution should be used due to the potential effects of the environment, such as the effect of water availability and its impacts on xylem development.