Taxonomy and Pathogenicity of Phoma exigua var. populi var. nov. Causing Necrotic Bark Lesions on Poplars

A Phoma disease causing necrotic lesions on poplars has been observed in The Netherlands in the period of 1993- 96. The disease symptoms as well as the morphology of this Phoma correspond with previous records from Germany and Italy. The disease has been ascribed to Phoma urens Ell. & Ev. but the American holotype of/*. itrens contained a Sderophoma species. In vivo and in vitro the European fungus most resembles the plurivorous Phoma exigua Desm., therefore it is classified as a variety of the latter. The differentiating characteristics are presented. In pathogenicity tests on Populus (x) euramericana "Robusta", Phoma exigua var. populi proved to be an opportunistic pathogen, causing distinct necrotic lesions after inoculation on injured cuttings. In a pathogenicity test on nine clones of four poplar species using two isolates of P. exigua var. populi differences of susceptibility were observed. Among five clones of P. (x) euramericana tested, disease development was most severe on the clones ʹRobustaʹ, ʹFlorence Biondiʹ and ʹAgathe Fʹ. The clones ʹEllertʹ and especially ʹSpijk" proved to be less susceptible. The two P. exigua var. populi isolates showed differences in pathogenicity when tested on two Populus nigra clones. Inoculation with the most pathogenic isolate showed a moderate to high susceptibility of the P. nigra clones ʹAnkumʹ and ʹVereeckenʹ, respectively. Populus (x) interamericana ʹDonkʹ and Populus trichocarpa ʹBlomʹ proved to be moderately susceptible to both P. exigua var. populi isolates