Occurrence of Hymenoscyphus pseudoalbidus on infected ash logs

Abstract

Ash decline induced by Hymenoscyphus pseudoalbidus is an emerging disease that severely affects Fraxinus excelsior stands in Europe. There has been an invasive spread of the disease from east to west in Europe over the last decade. Wood discoloration on infected trunks has been reported, but few data are available on the involvement of H. pseudoalbidus in such symptoms. Transport and trade of ash logs could introduce the pathogen into disease-free areas and therefore accelerate its dissemination. The aim of this study was to assess the prevalence and severity of H. pseudoalbidus in ash logs in infested areas located in the northeast of France and to clarify the role of secondary pathogens in ash decline. The results showed that prevalence of H. pseudoalbidus on collar lesions was high in the study area. The pathogen was able to produce conidia from infected wood. Thus, export of ash logs could represent a potential risk for spreading the disease. Involvement of Armillaria spp. in the decline process was confirmed, while no Phytophthora-induced collar lesions were found. Studying both disease prevalence and the age of callus tissues surrounding collar lesions in 60 ash stands enabled the origin of the disease in the study area to be determined.

Ash Dieback on Sample Points of the National Forest Inventory in South-Western Germany

“…Collar necroses occurred more frequently in plots with a higher proportion of ash on the total basal area of all tree species, which is related to increased amounts of inoculum. Moreover, the probability for root collar infection was significantly greater for heavily defoliated trees, which is in agreement with other studies that demonstrated a relationship between collar necroses prevalence and crown symptoms [15,16].…”

“…On average, planar terrain is more humid than steep ground, as water takes much more time to drain off. Thus, the higher collar necroses prevalence at more planar sites is in agreement with other studies that indicated increased risks of collar necroses on wet or humid sites or sites that are periodically flooded [15,16,18,19]. Moreover, the prevalence was highest in the Upper Rhine Valley and Lake Konstanz region, where ash usually grows on humid or wet sites.…”

“…Necroses can also occur on ash root collars, which are often followed by extensive butt rot and which decisively drive mortality. H. fraxineus was frequently detected in such collar necroses and is probably the causal agent of this symptom [15][16][17][18][19][20][21]. However, there are other pathogenic fungi that frequently colonize ash collar necroses, especially species of honey fungus (Armillaria spp.).…”

“…However, there are other pathogenic fungi that frequently colonize ash collar necroses, especially species of honey fungus (Armillaria spp.). It is assumed that honey fungi act mainly as secondary pathogens in ash collar necroses but play an important aggravating role in this pathosystem [15][16][17][18][19][20][21]. Infections by these fungi are often followed by wood decay and a reduction of the stability of the trees, which can be a crucial risk for forest visitors, forestry staff, and adjacent infrastructure [5,21,22].…”

The susceptibility of Asian, European and North American Fraxinus species to the ash dieback pathogen Hymenoscyphus fraxineus reflects their phylogenetic history

“…While most of the native Fraxinus species are susceptible to the borer, F. quadrangulata is considered resistant (Tanis and McCullough 2012), but this species seemed to be highly susceptible to ash dieback in our study. Regarding potential pathways of introduction of H. fraxineus to North America arrival via airborne ascospores seems less plausible than introduction via import of infected but asymptomatic plants, seeds (Cleary et al 2013a) or wood (Husson et al 2012). Given the higher genetic variation (Cleary et al 2016;Gross et al 2014b) and presumably also higher virulence of East Asian strains (Gross and Sieber 2016), introductions from East Asia would be especially critical (Landolt et al 2016).…”