Aralia japonica (synonym Fatsia japonica), belonging to the Araliaceae family, is a foliage plant highly valued in Italy for landscape and interior decoration. In the fall of 2002, a leaf blight disease was observed on plants grown in pots that were maintained under shade at a density of 15 to 20 pots per m2 at a nursery located in central Italy (Teramo Province). Typical symptoms were tan-to-dark brown leaf spots and rapid blighting of foliage under moist conditions. Chlorotic zones around necrotic lesions were common, and considerable leaf drop was associated with the disease. Affected plants were rarely killed, but the presence of lesions on mature plants reduced aesthetic quality and market value. The disease occurred on 70% of the plants. A fungus identified morphologically as Alternaria panax (2) was consistently isolated from infected leaves on potato dextrose agar (PDA). The fungus grows slowly and sparsely on PDA and produces a light brown mycelium, a characteristic red diffusible pigment in the agar medium, and rare conidia under 12-hr photoperiods. Measurements were carried out on conidia formed from single-spore isolates grown on autoclavated host tissue on water agar (LWA) at 24°C for 10 days. In LWA culture, conidia were borne singly or in chains of two to four conidia. Conidia produced in culture were smaller than those formed on the host and were highly variable in shape. They appeared obclavate, ellipsoidal, and obpyriform and pale to dark brown with relatively short or false beaks. Conidial bodies were 14.4 to 48.0 μm long (average 30.5 μm) and 7.2 to 12.0 μm wide (average 9.9 μm) with 3 to 10 transverse and a few longitudinal septa. Length of appendages was 9.6 to 26.0 μm (average 16.0 μm). Pathogenicity tests were performed by inoculating leaves of healthy Aralia japonica and Schefflera actinophylla plants by placing mycelial disks (5 mm in diameter) directly on wounded leaf tissues. Uninoculated, wounded plants served as controls. Four plants of each species were used. Plants were covered for 72 h with plastic bags and maintained in a growth chamber at 20°C (12 hours per day of fluorescent light). Control plants were maintained similarly. The first lesions developed on leaves of inoculated plants of both species after 7 days. A. panax was consistently reisolated from the lesions. The pathogenicity test was carried out twice. The presence of A. panax on Aralia japonica has been reported in Japan, Korea (2), and the United States (1) but to our knowledge, this is the first report of A. panax on Aralia japonica in Europe.
References: (1) S. Alfieri et al. Index of plant diseases in Florida. Bull. 11:52, Florida Department of Agriculture and Consumer Services, 1984 (2) S. H. Yu et al. Ann. Phytopathol. Soc. Jpn. 50:313, 1984.