Diaporthe oncostoma causing stem canker of black locust in Hungary

In Hungary, black locust (Robinia pseudoacacia) is an introduced, fast growing tree covering ca. 18 % of total forested lands. It is considered a tree species that resists the vast majority of plant pathogenic microorganisms (Keresztesi, 1988). Nevertheless, a canker disease was first observed in 1998, in a test plot of new varieties and at a forest nursery in Helvécia, a village in the Great Hungarian Plain, and then observed in 2000 and 2001 in 2-year old plantations in the northeastern and central part of the country, in Mezõladány, Inárcs and Csévharaszt. The frequency of disease occurrence ranged between 8 - 25%, but was as high as 40% in Helvécia. As a consequence of bark necrosis and cankers exposing the xylem of the trunks of 2 to 3-year old trees, trunks broke and eventually many trees died. Stromatic pycnidia, 150 - 300 µm in diameter, were found in the necrotic bark. The cause of cankers was identified as Phomopsis oncostoma (Thuem.) Traverso, teleomorph: Diaporthe oncostoma Fuckel. Its alpha conidia measured 10.3 x 2.6 µm (7.5 - 12.5 x 2.5 -3.0 µm), beta conidia, however were not found. The fungus was pathogenic to 2-year-old stems of black locust following inoculation of stems with mycelial agar plugs from a monoconidial culture. After 12 days, sunken and greyish necrotic lesions developed on the infected stems. In cankered bark, the formation of stromatic pycnidia was observed. The phloem and cambium were brownish and necrotised. No necrosis developed around control wounds. Phomopsis oncostoma was successfully reisolated from the lesions.

Some predisposing factors might play a role in the high frequency of severe disease observed at these sites. Practices such as winter pruning of side stems of young trees or spring planting using root cuttings or seedlings followed by drought might have resulted in high disease incidence. Thus, predisposition may be of great importance in disease.

Until now, D. oncostoma has been considered a saprotrophic or weak parasitic fungus which plays some role in natural pruning and self-thinning of black locust forests in Hungary. However, this fungus has been reported as a causal agent of canker and severe dieback disease of black locust in Russia (1953) (Scerbin-Parfenenko, 1953) and in Greece (1999) (Michalopoulos-Skarmoutsos & Skarmoutsos, 1999).