FIRST REPORT OF LEAF SPOT CAUSED BY COLLETOTRICHUM KAHAWAE ON AMERICAN SWEETGUM LIQUIDAMBAR STYRACIFLUA IN ITALY
A. Garibaldi, G. Gilardi, S. Franco-Ortega, M.L. Gullino
Liquidambar styraciflua (American sweetgum, Altingiaceae) is a deciduous tree native to warm temperate areas of eastern North America and tropical montane regions of Mexico and Central America. In Italy it is a popular ornamental tree. During summer–fall 2014, a new leaf spot was observed on 30-year-old plants of L. styraciflua, at temperatures ranging between 15 and 26°C and high relative humidity. All of the five plants in a private garden close to Biella (northern Italy) were affected. Seventy to eighty percent of leaves showed necrotic spots, 10 to 60 mm in diameter, with a purple margin, often interesting the margins of the leaves. Colletotrichum sp. was consistently recovered with a frequency of 70% from several isolations from infected leaf tissues carried out on potato dextrose agar (PDA) amended with 25 mg/l of streptomycin sulphate (Bailey and Jeger, 1992). Hyaline, cylindrical, aseptate and thin walled conidia (13.0 to 18.7 x 3.8 to 5.9, average 16.2 x 4.7 μm size) were abundantly produced in acervuli in a gray mycelium on PDA medium. DNA was extracted with E.Z.N.A. Plant DNA Kit (Omega Bio-Tek) and PCR reactions were performed using primers ITS1/ITS4, and primers T1 (O’Donnell and Cigelink, 1997) and βt2b (Glass and Donaldson, 1995). The 499 bp and 725 bp products were sequenced (GenBank accession Nos. KT375326 and KT375325, respectively) and confirmed to correspond to Colletotrichum kahawae. Pathogenicity tests were performed under growth chamber at 24 to 26°C and 12 h photoperiod by spraying L. styraciflua leaves with 1x105 conidia/ml or sterile water covered with plastic bags for 5 days. About 7 to 10 days after inoculation, lesions developed only on inoculated leaves and Colletotrichum sp. was consistently reisolated. This is the first report of Colletotrichum kahawae on American sweetgum in Italy as well as worldwide. Due to the increased number of report of C. kahawae in Italy on species such as olive, mango and cultivated rocket, the adoption of preventative strategies to contain the spread of this pathogen is suggested.