Development of Ash Dieback in South-Eastern Germany and the Increasing Occurrence of Secondary Pathogens

Abstract

Abstract:Since its first identification in Poland in 2006, the ascomycete Hymenoscyphus fraxineus has caused massive dieback of Fraxinus excelsior in the countries of eastern, northern and central Europe. This work shows the development, expansion, and severity of the disease in south-eastern Germany for a period of four years, starting in 2010. Differences between habitats, as well as age classes have been captured. The presence and the amount of potentially resistant trees were proven over the years, to determine how high the resistance level might be. Typical disease symptoms are the wilting of leaves, necrotic lesions in the bark and reddish discolorations of branches and stems. In addition, stem necroses also appear by infection with species of Armillaria. Therefore, special attention has been given to Armillaria species in affected ash stands but also to other secondary pathogens, like ash bark beetles. It is shown that breeding galleries of Hylesinus fraxini are only found in trees that have recently died and thus Hylesinus fraxini is still acting as a secondary opportunistic pathogen. In contrast, Armillaria spp. can be considered as serious pathogens of weakened ash trees. In different ash stands, typical symptoms of infection can be found. A relationship between stem base necrotic lesions and vitality was examined. It is shown that necrotic lesions severely contribute to accelerating the mortality of ash trees. In addition to the high infection pressure by H. fraxineus, the high inoculum of Armillaria in the soil facilitates further infections and, thus, likewise endangers the survival of potentially resistant trees. In the following years, forest conversion and seed harvest in affected ash stands will have to be urgently considered to avoid tree gaps on a large scale. Furthermore, infection assays of potentially resistant trees with ensuing breeding programmes should be initially started for the conservation of this ecologically and economically important tree species.

Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic

“…However, clear predictions of percentage mortality have proved elusive as the disease progresses slowly in mature trees (Lenz, Bartha, Straßer, & Lemme, 2016;McKinney et al, 2011), and in the early stages of infection, it is hard to distinguish resistant trees from escapees. Many new data on this topic were published in 2017.…”

Negative correlation between ash dieback susceptibility and reproductive success: good news for European ash forests

“…The symptoms are clearly recognisable, and thus, it has been successfully used to evaluate, monitor, and quantify the disease level. Significant variation among clones in crown damage has previously been documented in the present trial with an estimated broad-sense heritability of around 0.4 (McKinney et al 2011), and the same approach to assess disease intensity has been applied in many other studies (Enderle et al 2014;Kjaer et al 2012;Lenz et al 2016;Lobo et al 2014;Marçais et al 2017;Muñoz et al 2016). We therefore consider average clonal level of crown damage as a good estimate for the ability of the clones to withstand infections of H. fraxineus.…”

“…Genome-wide population diversity in H. fraxineus indicates an eastern Russian origin of European ash dieback (Sønstebø et al 2017), where it is asymptomatically associated with Asian Fraxinus species (Cleary et al 2016;Nielsen et al 2017). High mortality due to the pathogen in the European ash stands of Fraxinus excelsior regardless of age (Bakys et al 2009;Lenz et al 2016;Skovsgaard et al 2010) raises concern about the future existence of the species.…”

Statistics of ash dieback in Latvia

“…The overall mortality of affected ash in Latvia (Table 1) was lower than in Lithuania and Germany (6.9 vs. 8.7 and 10.1% year -1 , respectively; cf. Lenz et al 2016;Vasaitis and Enderle 2017), suggesting a higher resistance of the Latvian ash stands against the pathogens. Still, it was considerably higher than observed for "veteran" (i.e., long lived; ca.…”