First Report of Leaf Spot Caused by Alternaria alternata on Hydrangea macrophylla in Italy.

Abstract

Hydrangea macrophylla is cultivated as an ornamental and also used in the landscape. During the fall of 2005, leaves and young stems on 12-month-old plants (cvs. Hanabi, Nigra, and Zaffiro) grown in pots in several gardens and commercial nurseries in the Piedmont (northern Italy) had extensive necrosis. In many cases, 4-mm-diameter spots on the upper side of the leaves were surrounded by a chlorotic halo, which turned progressively black. Lesions often coalesced into 3- to 8-cm-diameter necrotic areas. Initial necrosis developed mainly on the leaf margins and near the petioles. Severely affected plants were defoliated. Infected plants rarely died, but the presence of lesions reduced the aesthetic quality and subsequently the commercial value. The disease occurred on 30 to 50% of the plants. Leaf spots contained dark brown, multicellular, pear-shaped conidia. Conidia were 19.2 to 36.5 μm (average 26.3 μm) long and 7.7 to 11.5 μm (average 8.9 μm) wide, with 3 to 4 longitudinal cross walls and an average of 4.4 single cells. A fungus identified on the basis of its morphological characteristics as an Alternaria sp. was consistently isolated from symptomatic leaves onto potato dextrose agar. DNA was extracted from mycelium (Nucleospin Plant Kit, Macherey Nagel, Brockville, ON, Canada) and PCR was completed using Alt-for/Alt-rev primers (3), which amplified a part of the gene that encodes for the protein Alt a 1, the major allergen produced by the genus Alternaria. A 305-bp fragment was amplified, sequenced, and the sequence was subjected to BLASTn analysis (1), which confirmed that the isolate belonged to the genus Alternaria and to the alternata group (3). The nucleotide sequence has been deposited in GenBank (Accession No. EF446670). Pathogenicity tests were performed by spraying leaves of healthy potted H. macrophylla plants, cvs. Zaffiro (6-month-old) and Hanabi (12-month-old) with a spore suspension (105 conidia/ml). Plants sprayed with water only served as a control. Ten plants per cultivar were used for each treatment. Plants were covered with plastic bags for 5 days after inoculation and maintained at 20°C for an additional 7 days. Plants were transferred outdoors and kept at temperatures ranging from 19 to 25°C. The first foliar lesions developed on leaves 15 days after inoculation, whereas control plants remained healthy. Alternaria sp. was consistently reisolated from these lesions. The pathogenicity test was completed twice. The presence of Alternaria sp. on Hydrangea spp. was reported in the United States (2), whereas A. hortensiae was observed in Spain (4). To our knowledge, this is the first report of Alternaria sp. belonging to the alternata group infecting H. macrophylla in Italy. The disease is currently spreading in other Italian areas.