Diaporthe oncostoma causing stem canker of black locust in Hungary

Abstract

In Hungary, black locust (Robinia pseudoacacia) is an introduced, fast-growing tree covering ≈ 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 north-eastern and central part of the country, in Mezõladány, Inárcs and Csévharaszt. The incidence of disease occurrence ranged between 8 and 25%, but was as high as 40% in Helvécia. As a consequence of bark necrosis and cankers exposing the xylem of the trunks of 2–3-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 (teleomorph: Diaporthe oncostoma). Its α conidia measured 10·3 × 2·6 µm (7·5–12·5 × 2·5–3·0 µm); β 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 necrotized. 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 (Scerbin-Parfenenko, 1953) and in Greece (Michalopoulos-Skarmoutsos & Skarmoutsos, 1999).