April 22, 2016

expert reaction to Nornex report on ash dieback

A group of researchers have reported the identification of ash trees which are tolerant to ash dieback, a disease which is spreading through the UK.

Prof. Steve Woodward, Department of Plant and Soil Science, University of Aberdeen, said:

“I welcome the report: the work has provided some exciting and very valuable results. Moreover, I am very pleased to see that the headline results are mainly focused on the tree itself, rather than the pathogen. What we want from all of this work is healthy trees, in resilient woodlands that can withstand pathogen attacks. It’s certainly fascinating and necessary to understand the pathogen biology, but the tree must be the main focus.

“It’s pleasing that the authors of the report have used the phrase ‘strong tolerance’, rather than resistance for the ash trees. Although a little vague in this context, tolerance is not implying that the trees do not get infected by Hymenoscyphus fraxineus. In my experience, it seems all common ash (Fraxinus excelsior) trees are susceptible to infection, but their responses differ. At the first sites where ash dieback was reported, in Lithuania, many healthy looking ash were left standing about 20 years ago, when the badly diseased mature trees were felled. Sadly, almost all of the trees that were left in those first fellings are now either dead or showing severe dieback.

“There is clearly great variation in the extent to which trees are able to ‘tolerate’ the presence of the pathogen. A rate of 3% of trees show tolerance doesn’t sound high but ash does seed prolifically, of course, so perhaps survivors will eventually fill some of the gaps left by more susceptible individual trees.

“The three genetic markers for tolerance present in a small proportion of ash give a super tool for marker-led selection and breeding. The greater presence of these markers in the British ash population relative to as from other European countries could be related to the refugia from which the UK ash population spread following the last ice age. UK ash are derived from a western refugium; ash further east in Europe repopulated from more eastern refugia. This idea could be tested by testing trees from the Iberian peninsula for the presence of the ‘tolerance’ markers. It is useful to see that these markers may also be present in other species of ash – does that include F. mandschurica and other Asian species?

“The statement that ‘Strains of H. fraxineus present in Europe are closely related to each other but distantly related to isolates from Japan’ is rather misleading. My reading of this point is that the isolates which invaded Europe were not from Japan; H. fraxineus is also present in the far east of Russia, in China and Korea. The use of ‘distantly related’ without a suitable qualifier is not accurate.

“The report makes no mention of the Emerald ash borer, which is present on the continent of Europe (spreading westwards from Moscow in Russia; it may have already reached Belarus. It is NOT present in Sweden, as suggested by a recent paper). Emerald ash borer is native to the same areas of far east Asia as H. fraxineus, but spread to North America, probably in the late 1990s/early 2000, where is devastating all native species of ash. It is unfortunately highly likely that the Emerald ash borer will gradually spread across Europe, to the west. The ash trees remaining after the effects of H. fraxineus will certainly provide suitable food sources for the borer. It is essential, therefore, that the rewarding efforts made to understand ash dieback we see reported here, are continued with emphasis on the emerald ash borer.”