First Report of Leaf Spot and Stem Rot Caused by Diaporthe eres on Hydrangea macrophylla in Italy

Abstract

HomePlant DiseaseAhead of PrintFirst Report of Leaf Spot and Stem Rot Caused by Diaporthe eres on Hydrangea macrophylla in Italy PreviousNext DISEASE NOTE OPENOpen Access licenseFirst Report of Leaf Spot and Stem Rot Caused by Diaporthe eres on Hydrangea macrophylla in ItalyA. Garibaldi, G. Tabone, I. Luongo, and M. L. GullinoA. GaribaldiCentre of Competence AGROINNOVA, University of Torino, 10095 Grugliasco, ItalySearch for more papers by this author, G. TaboneCentre of Competence AGROINNOVA, University of Torino, 10095 Grugliasco, ItalySearch for more papers by this author, I. LuongoCentre of Competence AGROINNOVA, University of Torino, 10095 Grugliasco, ItalySearch for more papers by this author, and M. L. Gullino†Corresponding author: M. L. Gullino; E-mail Address: [email protected]https://orcid.org/0000-0002-7706-1915Centre of Competence AGROINNOVA, University of Torino, 10095 Grugliasco, ItalySearch for more papers by this authorAffiliationsAuthors and Affiliations A. Garibaldi G. Tabone I. Luongo M. L. Gullino † Centre of Competence AGROINNOVA, University of Torino, 10095 Grugliasco, Italy Published Online:9 Jun 2023https://doi.org/10.1094/PDIS-08-22-1786-PDNAboutSectionsPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Native to the temperate and subtropical regions of Japan and China, Hydrangea macrophylla (big leaf hydrangea) is a shrubby plant in the Hydrangeaceae used to embellish parks, gardens, private terraces, and balconies. Punctiform brown leaf spots and stem necrosis appeared in the summer of 2021 on 10 of approximately 50 H. macrophylla plants. The 20-year-old plants were cultivated in pots and in mixed borders in a private garden located in Piedmont (Biella Province 45°36′00″N, 8°03′00″E). Spots enlarged to encompass the entire surface; eventually affected leaves abscised. Affected leaves at the early stage of disease were randomly collected and fragments, obtained from the margins of necrotic tissues, were disinfected in 1% sodium hypochlorite, rinsed in sterilized water, and distributed on potato dextrose agar (PDA) amended with 25 mg/liter of streptomycin sulfate. Plates were maintained at room temperature (22 to 26°C) and colonies, characterized by white, aerial, fluffy mycelium with a dark pigmentation at the center, appeared after 5 days. Mycelium of one representative pure culture grown on PDA for 20 days became gray with age and developed dark brown pycnidia and black stromata at maturity. Alpha conidia were hyaline, aseptate, fusiform to ellipsoidal, often biguttulate, and 5.1 to 10.2 (average 7.2) × 1.9 to 3.7 (average 3.0) μm (n = 50). Beta conidia were hyaline, aseptate, linear to curved, and 14.5 to 35.7 (average 24.9) × 0.6 to 2.9 (average 1.6) μm (n = 50). These morphological characteristics permitted identification of the fungus as Diaporthe sp. (Gomes et al. 2013). Polymerase chain reaction (PCR) and Sanger sequencing was carried out for ITS, calmodulin (cal) (Cal228F/Cal2Rd), tef1 (EF1-728/EF1-986R), and tub2 (T1/Bt2b) (Udayanga et al. 2014) of the representative isolate 22/12-1 (GenBank accessions OP080713, OP700046, OP14610, and OP700047, respectively). Sequences analyzed with BLASTn (Altschul et al. 1997) showed 99.5% identity for ITS (567/570 bp; KC343097), 99.8% for cal (414/415 bp; KC343339), 100% for tef1 (347/347 bp; KC343823), and 100% for tub2 (690/690 bp; KC344065) with sequences from strain CBS 841.84 of Diaporthe eres. Pathogenicity of the isolate 22/12-1 was verified by performing a pathogenicity test on five healthy 20-year-old H. macrophylla plants grown in pots and in borders. A conidial suspension at a concentration of 1 × 105 conidia/ml, obtained from a pure culture of the isolate grown for 20 days on potato dextrose agar (PDA), was sprayed on the leaves of the H. macrophylla plants (5 ml/plant). Three noninoculated plants sprayed with sterile deionized water were used as a control. Plants were covered with a transparent plastic bag and maintained in the garden at a temperature of 24 to 36°C. Leaf spots and stem necrosis developed on the inoculated plants after 7 to 10 days. Colonies with the same morphological characteristics of the original isolate were isolated from the margins of the necrotic leaf tissue. The pathogenicity test was repeated once, obtaining the same results. D. eres was previously reported on several species of Hydrangea (Farr and Rossman 2022). To our knowledge this is the first report of D. eres on H. macrophylla in Italy. Diaporthe (syn. Phomopsis) is the causal agent of severe disease and could represent a serious problem for the cultivation of H. macrophylla in gardens and a reduction of the ornamental value.The author(s) declare no conflict of interest.