First Report of Diaporthe eres Causing Stem Canker on Peach (Prunus persica) in Italy

Italy represents the second worldwide producer of peaches with around 71,012 ha of harvested area with a large number of varieties and 1.3 million tons/year production. During June 2016, symptoms of stem canker, with brown to black lesions and associated chlorosis of leaves, were observed on 9-year-old peach trees (Prunus persica [L.] Batsch) cv. Amiga. Samples were randomly collected from two symptomatic orchards in Savigliano (Piedmont, Italy), where 35% of the trees were affected. Isolation was performed from the margins of necrotic stems lesions on potato dextrose agar (PDA) amended with 0.025% streptomycin sulfate. Petri dishes were incubated at 25°C for 5 days. Monoconidial isolates were subsequently transferred onto PDA at 25°C in the dark for 7 days. Colony morphology showed white aerial mycelium and dark pigmentation in the center. Conidia were collected from pycnidia and they showed typical morphology of a Diaporthe sp. (Gomes et al. 2013), with unicellular, aseptate, hyaline, biguttulate, and elongated alpha conidia, and beta conidia aseptate, filiform, and hyaline. DNA was extracted from monoconidial culture and ITS region (accession no. KX676493), beta-tubulin gene (KX676492), and translation elongation factor 1-alpha (TEF1-α; KX676494) genes were amplified and sequenced following current revision of the genus (Udayanga et al. 2014). The sequences were blasted in GenBank obtaining 100% homology with strains of Diaporthe eres Nitschke for ITS region (KC343095), 99% for beta-tubulin (KC344041), and 100% for the TEF1-α gene (KC343804). To further confirm the identity of the species, DNA sequences were aligned with CLUSTAL W with related species of Diaporthe (e.g., D. amygdali, D. helianthi, D. phaseolorum) and other species found on P. persica, and a phylogenetic analysis with the neighbor joining method based on maximum composite likelihood model (bootstrap 1,000) was performed using MEGA6. The phylogenetic tree confirmed the identity of the isolates to the species. Pathogenicity tests were performed on two strains in greenhouse on 1-year-old plants of P. persica cv. Amiga. A plug 4 mm in diameter covered by mycelium was taken from a 7-day-old PDA culture and was placed on wounded stems of 1-year-old plants. They were immediately covered with a sterile gauze pad saturated with sterile distilled water, and then parafilmed. Control plants were prepared similarly using uncolonized agar plugs. The experimental trial was performed on five inoculated and five control plants. The pots were placed in the greenhouse at 25°C and canker lesions similar to the symptoms in orchards were observed 15 days after inoculation. Control plants were asymptomatic. D. eres was reisolated from the 10 inoculated stems and the pathogen identification was confirmed by molecular analysis as described above. D. eres was previously reported on peach in Greece (Thomidis and Michailides 2009) and in Italy on grapevine (Vitis vinifera) (Cinelli et al. 2016). To the best of our knowledge, this is the first report of D. eres causing stem canker on P. persica in Italy. By considering the primary role of peach production in Italy, further studies should be conducted to better understand and control the disease. © 2017, American Phytopathological Society. All rights reserved.