Identifying Fraxinus excelsior tolerant to ash dieback: Visual field monitoring versus a molecular marker

Abstract

Ash dieback (ADB) caused by the pathogen Hymenoscyphus fraxineus is the cause of massive mortality of Fraxinus spp. in Europe. The aim of this work was to check for the presence of the molecular marker for ADB tolerance in mapped healthy‐looking F. excelsior trees, and to compare its occurrence in trees exhibiting severe ADB symptoms. Monitoring of 135 healthy‐looking F. excelsior on the island of Gotland, Sweden, showed that after 3–4 years 99.3% of these trees had 0%–10% crown damage, thus remaining in a similar health condition as when first mapped. After 5–6 years, 94.7% of these trees had 0%–10% crown damage. Molecular analysis of leaf tissues from 40 of those showed the presence of the molecular marker in 34 (85.0%) trees, while it was absent in 6 (15.0%) trees. Analysis of leaf tissues from 40 severely ADB‐diseased trees showed the presence of the molecular marker in 17 (42.5%) trees, but its absence in 23 (57.5%) trees (p < .0001). The results demonstrated that monitoring of healthy‐looking F. excelsior is a simple and straightforward approach for the selection of presumably ADB‐tolerant ash for future breeding. The cDNA‐based molecular marker revealed moderate capacity on its own to discriminate between presumably ADB‐tolerant and susceptible F. excelsior genotypes.

Diversity of secoiridoid glycosides in leaves of UK and Danish ash provide new insight for ash dieback management

“…development of robust genetic markers for ADB 25 . Recently, infrared spectroscopy has been shown to distinguish bark extracts of ash trees with low, intermediate and high susceptibility to ADB 26 .…”

Marker-Trait Associations for Tolerance to Ash Dieback in Common Ash (Fraxinus excelsior L.)

“…The selection for and deployment of disease-tolerant tree genotypes are keys to the restoration of sites destroyed by forest disease epidemics and help prevent the functional extirpation of threatened tree species [51] and associated biodiversity. It seems possible to achieve this with the ongoing ash dieback epidemic by identifying ash with improved tolerance using both phenotypic and genotypic selection methods [17,25]. However, phenotypic selection methods, although being simple and straightforward, are relatively slow as they generally involve scoring crown damage in a large number of trees or following trees over a long time to identify putatively resistant individuals [17,23,25].…”

“…Secondly, 70 plus trees (trees with superior properties), originating from different regions of southern Sweden, still remaining in a seed orchard established before the ash dieback epidemic arrived were included [17]. Finally, we used 113 ash genotypes pre-selected phenotypically for their tolerance to ash dieback from southern Sweden and Gotland [25], of which 72 were tolerant with low disease severity scores (disease severity <10%) (Supplementary Table S1).…”

“…For seed orchard trees, the disease severity assessment was based on a six-grade scale [17], ranging from 0 (none) to 6 (very serious damage) based on a visual scoring of crown damage to get a measure of health status. For the 113 ash trees from southern Sweden and Gotland described by Menkis et al [25], the scoring system was based on a visual monitoring of health status where trees were classified as either tolerant (0%-10% crown damage) or susceptible (more than 10% crown damage). The remaining trees were phenotyped according to Kirisits and Freinschlag [35].…”

“…However, only three markers [15] were able to significantly predict tolerance to the disease. These molecular markers were used to identify trees with a reduced susceptibility to ash dieback in Danish, British and Swedish ash populations [22,25] and had a moderate predictive capacity. These results are highly encouraging, but for efficient marker-assisted selection several more markers are needed in addition to the available markers.…”