Influence of site and stand factors on Hymenoscyphus fraxineus‐induced basal lesions

Abstract

Hymenoscyphus fraxineus is an invasive fungus in Europe and causes a severe decline affecting ash, which began in the late 1990s. One of the symptoms associated with the disease is lesions in the outer bark of the collar area. However, the aetiology of these basal lesions, and in particular the relative roles of H. fraxineus and Armillaria species, is still controversial; moreover, little is known about the influence of environmental factors on the disease epidemiology. This study therefore surveyed 42 plots located in northeastern France, in an area affected by ash decline since 2008, in order to determine which environmental factors condition the severity of lesions associated with H. fraxineus on ash collar. The spatial pattern that is a consequence of the invasive spread of the disease was taken into account in the analysis, using a spatial hierarchical Bayesian model fitted by integrated nested laplace approximation (INLA). Results show that while basal lesions are tightly associated with H. fraxineus, their severity is influenced by the Armillaria species present in the plot. Sites with vegetation indicating moist conditions, or more humid topographical positions, were associated with more developed basal lesions.

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

“…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].…”

Fungal colonization patterns in necrotic rootstocks and stem bases of dieback‐affected Fraxinus excelsior L.

“…It has also been proposed that H. fraxineus is able to enter ash stem bases through bark, which represents an additional way of infection, possibly causing basal lesions and collar rots from which H. fraxineus has frequently been isolated (Chandelier, Gerarts, San Martin, Herman, & Delahaye, ; Husson, Caël, Grandjean, Nageleisen, & Marçais, ; Matsiakh, Solheim, Hietala, Nagy, & Kramarets, ). In Europe, ash collar rots are being observed with increasing frequency, and these rots are likely to accelerate tree death in all age classes (Enderle, Peters, Nakou, & Metzler, ; Enderle, Sander, & Metzler, ; Husson et al, ; Langer, ; Langer, Harriehausen, & Bressem, ; Marçais et al, ; Marçais, Husson, Godart, & Caël, ; Metzler, ; Metzler & Herbstritt, ; Muñoz, Marçais, Dufour, & Dowkiw, ; Skovsgaard, Thomsen, Skovgaard, & Martinussen, ). Infections of stems and branches seem to represent an epidemiological dead end for the fungus, since the formation of fruiting bodies is usually limited to leaf rachises, petioles, leaflet veins and rarely also to young shoots (Gross, Zaffarano, Duo, & Grünig, ; Kirisits & Cech, ).…”

Do higher summer temperatures restrict the dissemination of Hymenoscyphus fraxineus in France?

“…The disease is influenced by environmental conditions. High site humidity also enhances the severity of collar canker on the ash trees (Marçais, Husson, Godart, & Caël, ). Moreover, Kowalski and Bartnik () have shown that the optimal in vitro growth temperature of the fungus is around 20°C.…”