Phomopsis alnea, the cause of dieback of black alder in Italy.

Black alder (Alnus glutinosa) occurs naturally on moist soils and along streams and rivers throughout most of its range. Groves of this species in north-central Italy were recently found to be attacked by the mitosporic coelomycete Phomopsis alnea, which causes perennial stem cankers and dieback. The fungi Melanconium apiocarpium and a Hymenopsis sp. were also frequently found on necrotic alder tissue, occasionally invading the living bark. All these fungi were tested for pathogenicity in two seasonal inoculation trials on seedlings of black alder, Italian alder (Alnus cordata) and green alder (Alnus viridis) that were either normally watered or water-stressed. Phomopsis alnea actively colonized the seedlings and reproduced the symptoms observed in the field. The other fungi behaved as weak parasites, only occasionally spreadings to apparently healthy bark tissue. It appears that these fungi are saprobes, commonly colonizing bark and twigs already killed by P. alnea. Symptoms caused by P. alnea in the field were exacerbated on dry sites and by seasonal drought stress. On artificially inoculated and water-stressed seedlings, both the incidence and severity of P. alnea also increased, causing extensive mortality. The data provide evidence for the belief that P. alnea becomes a factor in the dieback of natural black alder woodlands when trees are first impaired by wounding agents and then subjected to unusual extended drought.