American elm cultivars: Variation in compartmentalization of infection by Ophiostoma novo‐ulmi and its effects on hydraulic conductivity

Abstract

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.

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

“…Findings on DSR at all time points were previously reported in Beier et al. () Table . A summary of the DSR at 90 DPI is presented in Table .…”

“…The process of staining the trees with safranin O is described in Beier et al. (). In brief, cut trees were placed in 10 mM KCl with 0.1% w/v safranin O for 18–24 hr.…”

“…or O. novo‐ulmi Braiser. Despite the seemingly endless losses, there are American elm genotypes that show resistance to DED (Beier, Held, Giblin, Cavender‐Bares & Blanchette, ; Townsend, Bentz & Douglass, ; Townsend, Bentz & Johnson, ). Scientists have long searched for the mechanisms responsible for this increased resistance.…”

“…While studies conducted by Beier et al. () provided a macroscopic view of differences in compartmentalization, histological assessments were warranted to further discern differences among cultivars.…”

Breeding and scientific advances in the fight against Dutch elm disease: Will they allow the use of elms in forest restoration?

“…Vascular wilt syndrome DED is mostly associated with xylem hydraulic failure and possibly dysfunction of cambium and parenchyma in chronic phases of the disease (Ouellette and Rioux 1992). Within days or weeks of inoculation with DED fungi, there is a decrease in sap flow, leaf water potential, transpiration, photosynthetic capacity and/or nonstructural carbohydrate concentrations, associated with the embolization of outermost, current-year xylem vessels transporting most of the sap (Newbanks et al 1983;Oliveira et al 2012;Li et al 2016;Beier et al 2017). Progressive fungal invasion of sapwood causes complete failure of the hydraulic system, which kills a large part of the crown and eventually results in stem dieback.…”