Seasonal Pattern of Lesion Development in Diseased Fraxinus excelsior Infected by Hymenoscyphus pseudoalbidus

Abstract

Ash dieback is a recent widespread disease on ash (Fraxinus sp.) that is causing important economic and ecological losses throughout Europe. The disease is initiated by the ascomycetous fungus Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea). The main aim of this study was to investigate seasonal pattern of lesion development associated with ash dieback. We present data on the spread of 324 natural lesions in ash shoots, branches and stems surveyed over a 32 month period. Most lesions were active and showed the greatest rate of growth during the summer; however, lesions were active throughout the year. Tree mortality was high, with more than a third of the surveyed trees dying during the study. Although many lesions permanently ceased to develop, the rate at which new lesions emerged was greater than the rate at which lesions entered a resting phase. The most common cause for a lesion going into a permanent state of rest was that it had encountered a branch-base. Genotype analysis showed that multiple infections can occur in a single tree given that different genotypes were identified in different lesions as well as in single lesions. A weak positive correlation was noted between tree health and tree size and a weak negative correlation was noted between tree overall health and lesion activity. The lower limit for H. pseudoalbidus growth in culture was between 4.0°C and 0.5°C.

Fungi on stems and twigs in initial and advanced stages of dieback of European ash (Fraxinus excelsior) in Poland

“…Failures in isolation of H. fraxineus after natural infection from advanced necroses have also been reported elsewhere. In Sweden, H. fraxineus was detected only occasionally in both initial and advanced necroses after natural infection (Bakys et al 2009a), and it was recorded in only 17-64 % of lesions depending on the habitat (Bengtsson et al 2014). In Ukraine, H. fraxineus was detected only in 5.6 % of symptomatic shoots (Davydenko et al 2013).…”

“…Such analysis was expected to: (1) determine the frequency of H. fraxineus as the potential primary causal agent of F. excelsior dieback; (2) identify fungal species that benefit from earlier, primary infection by H. fraxineus or initially colonize the tissue independently and cause lesions similar to those of H. fraxineus; and (3) create the basis for studies on interactions between H. fraxineus and other fungal colonizers that contribute to progress of necrosis and disease (Bengtsson et al 2014). Relationships between host and fungi are particularly important in relation to global warming, with consequences for host and pathogen physiology, reproduction, survival, spatial and temporal distribution, resource availability and competition (Clark 2013).…”

Seasonal development of lesions caused by Hymenoscyphus fraxineus on young Fraxinus excelsior trees in Latvia

“…The number of countries affected by ash dieback continues to increase and already exceeds 20 (Gross et al 2014). This dieback affects stands of different ages and compositions (Pliura & Heuertz 2003, Schumacher et al 2010), yet young stands are the most susceptible to infection, and hence are critical for development of the pathogen (Skovsgaard et al 2010, Bengtsson et al 2014.…”

“…Primary symptoms of the disease are macroscopic cankers on leaves and leafstalks (Skovsgaard et al 2010), brown spots on buds (Bengtsson et al 2014), and wilting of leaves and/or top shoots (Schumacher et al 2010). These symptoms are followed by the formation of necrotic lesions spreading along rachises onto shoots, branches, and stems, resulting in dieback of the affected parts of a tree (Bakys et al 2009, Skovsgaard et al 2010, Bengtsson et al 2014.…”

“…These symptoms are followed by the formation of necrotic lesions spreading along rachises onto shoots, branches, and stems, resulting in dieback of the affected parts of a tree (Bakys et al 2009, Skovsgaard et al 2010, Bengtsson et al 2014. Skovsgaard et al (2010) noted, however, that cankers may also appear before wilting and dieback of shoots occurs.…”