Ash dieback is an emerging disease of Fraxinus excelsior in Germany. To date, economical damage is significant in nurseries, which also contribute towards spread of the disease, but damage to forests is increasing. The study presents the results of mycological and histological investigations on three hundred 3-year-old nursery ash saplings. The infection rate by the causative pathogen was determined for bark, outer and inner xylem, the pith and also separately for the above-ground portion and root system of the plants. The invasion and colonization strategy of the fungus in the woody stem was examined. In addition, the presence of soil-borne Oomycetes as possible primary or accompanying causal organisms was investigated. The results verify the dominant role of Chalara fraxinea as a causal agent of ash dieback and rule out the role of Oomycetes in the disease process. We conclude that C. fraxinea is not primarily endophytic in nature and spreads very effectively in the central stem tissues, which enables colonization of the woody stem in all three dimensions. Infections arising in the upper part of plants can thus spread extensively to lower parts.

Zusammenfassung In den Jahren 2005 und 2006 wurden in einem großen Dresdener Stadtpark mehrere auffällig vitalitätsgeschwächte bzw. rasant abgestorbene Rot-Buchen (Fagus sylvatica) untersucht. Als Schadverursacher konnte ein in der verfügbaren Literatur bislang ausschließlich als Saprophyt charakterisierter Schlauchpilz, die Schwarze Buchenkohlenbeere (Hypoxylon cohaerens), identifiziert werden. Die Symptomatik des Wirtes, die morphologischen und physiologischen Eigenschaften des Pilzes in situ und in vitro sowie der mögliche Einfluss prädisponierender Faktoren werden vorgestellt. Da H. cohaerens als Parasit pathologische Bedeutung erlangt und diagnostisch mit anderen Schlauchpilzen verwechselt werden kann, werden abgrenzende Merkmale zu verwandten Arten herausgearbeitet. Die veränderte Bedeutung des Pilzes für die forstliche und baumpflegerische Praxis wird diskutiert.Schlüsselwörter Hypoxylon cohaerens · Fagus sylvatica · Vitalitätsschwächung · Differenzialdiagnose · Holzzersetzung Noteworthy decline and wood decay on beech by the largely unknown ascomycete Hypoxylon cohaerensAbstract In the years 2005 and 2006 several declining and recently died beech trees were investigated in a large city park in Dresden. The ascomycete Hypoxylon cohaerens, who is exclusively characterized as a saprophytic fungus by literature, was identified as a conspicuous cause of the disease. The symptoms of the infection, the morphologically and physiologically characteristics of the fungus in situ and in vitro as well as the factors of predisposition are presented in the article. Since H. cohaerens attains pathological importance and can be mistaken for some other ascomycetes, the distinguishing marks to related species are described. The changed importance of the fungus is evaluated for practice.

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