First Report of Leaf Blotch Caused by Marssonina coronaria on Apple in Italy

During the early summers of 2001 and 2002, in Forno Canavese in northwest Italy, a leaf disease was observed on the old apple cv. Furnas in a domestic orchard. Lesions on the upper side of the leaf were brownish, irregular in size and shape with somewhat dendritic margins, became black, and often coalesced with time. On the underside, lesions were smaller with more definite margins. Beginning in July, scattered acervuli (95 to 170 μm) were observed erupting through the epidermis on the upper side of leaves. Conidia were ampule shaped, 1-septate, constricted at the septum, hyaline, guttulate, and 6.1 to 8.4 × 14.6 to 22.0 μm. Severely diseased leaves abscised prematurely. The fungus was identified as Marssonina coronaria (Ellis & J.J. Davis) J.J. Davis, teleomorph Diplocarpon mali (1) although the conidia were slightly shorter than those originally described for this fungus. Monoconidial isolates were obtained by spreading mini-suspensions of conidia taken from acervuli on malt agar (MA) and transferring single-germinated conidia to MA, potato dextrose agar, V8 agar, or apple leaf agar (ALA). The fungus grew slowly, producing small colonies on V8 and ALA only. On ALA medium, after 3 months incubation at 20 to 22°C, the colonies were 5 to 7 mm in diameter with light brown, irregular margins and dark brown centers bearing acervuli. Conidia from pure cultures were collected, suspended in sterile, distilled water (250,000 ml-1), and sprayed on the leaves of three ‘Golden Delicious’ apple shoots maintained in a mist chamber at 20 to 25°C for 2 weeks. In two independent experiments, the fungus reproduced symptoms like those observed on ‘Furnas’ and was reisolated from acervuli. No symptoms were observed on water-treated controls. To our knowledge, apple leaf blotch has not previously been reported in Italy. Although now it is a minor disease, it could become more important in sustainable crops because of its relatively low sensitivity to copper fungicides (2), the only products that can be used under that program.