Studies on the pathogenicity of Chalara fraxinea
Ash dieback is an emerging disease which was first observed in Poland in the 1990s but which is now spreading westwards to other European countries. A newly described fungus, Chalara fraxinea (EPPO Alert List), was frequently found associated with the disease but its pathogenicity remained unclear, as other potentially pathogenic fungi (Cytospora, Diplodia, Fusarium, Phomopsis) were also detected in declining ash trees. Studies were recently carried out in Poland to demonstrate the pathogenicity of C. fraxinea to common ash (Fraxinus excelsior). Artificial inoculations were performed in 3 field experiments on a total of 48 ash trees. Ten different Polish isolates of C. fraxinea were inoculated by using wood sticks colonized by mycelia. Inoculated ash trees were cut after 2, 3 or 12 months and studied in the laboratory. On all inoculated trees, necrotic lesions of very variable sizes developed and wilting occurred occasionally. Symptoms observed after artificial inoculation were identical to those seen after natural infections. C. fraxinea was reisolated from 39 of the 48 inoculated trees. It was also frequently found together with Alternaria alternata, Cytospora sp., Coniothyrium olivaceum, Diplodia mutila, Fusarium spp. and Phomopsis. It is also noted that because cankers are perennial, C. fraxinea may be outcompeted by other microorganisms, which either cause secondary cankers or act as biological control agents (e.g. in Petri dishes a pronounced inhibition of C. fraxinea was observed in the presence of Diplodia mutila). It is concluded that an artificial inoculation with C. fraxinea results in extended necrotic lesions under field conditions, with pronounced symptoms very similar those of natural infections. Thus, the fungus is considered to play a key role in ash dieback.
Kowalski T, Holdenrieder O (2008) Pathogenicity of Chalara fraxinea. Forest Pathology 38(6), in press.