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.
Highlights
Since the outbreak of Dutch elm disease (DED) in the 1920s, many bred cultivars with varying degrees of resistance were planted in an attempt to mitigate the negative impacts of the pandemics and to restore the previous abundance of elm trees across both the European and North American continents [2,3]
Two Dutch elm hybrid cultivars with contrasting responses to DED were used, i.e., sensitive trees of ‘Groeneveld’ [(Ulmus × hollandica 49) × U. minor ssp. minor 1] [24]
After two weeks of fungal growth, a portion of the mycelium was subjected to DNA extraction and to fungal subspecies determination using a PCR-RFLP procedure [26] together with the mating type test determination proceeded by the cultivation method [27]
Summary
Wild trees of the genus Ulmus, showing a low demand for soil and climatic conditions, resistant to mechanical damage, and frequently cultivated in urban areas, were heavily affected during the two waves of Dutch elm disease (DED). This wilt disease is caused by ascomycetous fungi of the genus Ophiostoma. Numerous attempts to control the disease focused on the reduction in elm bark beetle (vector) populations and the application of fungicides and on the development of resistance to DED through breeding programs [1,2,3].
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