Different Responses in Vascular Traits between Dutch Elm Hybrids with a Contrasting Tolerance to Dutch Elm Disease

Abstract

The ascomycetous fungus Ophiostoma novo-ulmi is the causative agent of the current Dutch elm disease (DED) pandemic, which has ravaged many tens of millions of European and North American elm trees. Host responses in vascular traits were studied in two Dutch elm hybrids, ‘Groeneveld’ and ‘Dodoens’, which show different vascular architecture in the secondary xylem and possess contrasting tolerances to DED. ‘Groeneveld’ trees, sensitive to DED, possessed a high number of small earlywood vessels. However, these trees showed a poor response to DED infection for the earlywood vascular characteristics. Following infection, the proportion of least vessels with a vessel lumen area less than 2500 µm2 decreased from 65.4% down to 53.2%. A delayed response in the increasing density of vessels showing a reduced size in the latewood prevented neither the rapid fungal spread nor the massive colonisation of the secondary xylem tissues resulting in the death of the infected trees. ‘Dodoens’ trees, tolerant to DED, possessed a low number of large earlywood vessels and showed a prominent and fast response to DED infection. Vessel lumen areas of newly formed earlywood vessels were severely reduced together with the vessel size : number ratio. Following infection, the proportion of least vessels with a vessel lumen area less than 2500 µm2 increased from 75.6% up to 92.9%. A trend in the increasing density of vessels showing a reduced size was maintained not only in the latewood that was formed in the year of infection but also in the earlywood that was formed in the consecutive year. The occurrence of fungal hyphae in the earlywood vessels that were formed a year following the infection was severely restricted, as revealed by X-ray micro-computed tomography imaging. Possible reasons responsible for a contrasting survival of ‘Groeneveld’ and ‘Dodoens’ trees are discussed.

Xylem Embolism and Pathogens: Can the Vessel Anatomy of Woody Plants Contribute to X. fastidiosa Resistance?

“… Schematization of the pathogen spread (represented in brown color) in the xylem tissue of susceptible or resistant plants: ( A ) xylem of the Phaeomoniella chlamydospora –susceptible grapevine cultivar Thompson Seedless, characterized by wide and long xylem vessels (photo from [ 100 ], bar = 1 mm); ( B ) xylem of the Ophiostoma ulmi –tolerant elm hybrid Dodoens, capable of a fast reaction to infection that includes the formation of much smaller vessels and an increase of their density (photo from [ 101 ], bar = 500 μm); ( C ) comparison of the xylem between the Raffaelea lauricola –susceptible avocado cultivar Simmonds (I and II) and the resistant one Duke (III and IV), in which besides differences in vessel length and diameter (C I and C III), any significant difference in the vessel’s density and organization is reported (photo from [ 102 ], bar = 1500 μm); ( D ) vessel ending (red) at the petiole level, which acts like filter for emboli and pathogens, especially at the junction between leaf and stem; adjacent long and open vessels (blue) can facilitate the circulation of emboli or pathogens. …”

Selection of Elms Tolerant to Dutch Elm Disease in South-West Romania

“…), elm morphological wood characteristics, host chemical reaction, the bark and wood microbiome, the momentary attractiveness for bark beetles, individual phenotypic plasticity, etc. [37][38][39][40]. The effect of these factors can vary with time, therefore long-term tests are needed to understand the tolerance/resistance to DED of the European elm genotypes.…”