Epidemics of Peronospora sp. on Alpine wormwood (Artemisia umbelliformis Lam.) in Italy

A new disease was observed at end July/beginning August 2004 and 2005 on Alpine wormwood (Artemisia umbelliformis, Lam.) grown in Maira Valley (Cuneo, NW Italy). It was characterized by scattered leaf chlorosis followed by brown discoloration and necrosis; these symptoms quickly extended to the whole foliage killing the plant. Sporangiophores were visible on lesions; they were 408-566 um long, erect, with a basal compartment 1.15-1.7 times larger than main axis and dicotomic ramifications (branching angle 28 to 73°, 54.6° in average). Distal branches were mucros-shaped and ended in one sterigma bearing one sporangium. These were egg-shaped, non papillated, 17-24x29.3-46 um. Mass fructifications were ash-grey coloured. The pathogen could not grow in vitro. Water-flooded sporangia did not release zoospores even after cold shock. These features suggest that the pathogen be ascribed to the Peronospora genus. When artificially inoculated on A. absynthium and A. umbelliformis, disease symptoms were observed after 7-8 days incubation at 18/24°C on A. umbelliformis only, on which the parasite completed its life cycle. Some biological aspects such as biological specialization, inoculum surviving, sources, dispersal and environmental conditions conducive to the disease are briefly discussed. The disease is not described on Artemisia sp.