Abstract
Species of Diaporthe are considered important plant pathogens, saprobes, and endophytes on a wide range of plant hosts. Several species are well-known on citrus, either as agents of pre- or post-harvest infections, such as dieback, melanose and stem-end rot on fruit. In this study we explored the occurrence, diversity and pathogenicity of Diaporthe species associated with Citrus and allied genera in European orchards, nurseries, and gardens. Surveys were carried out during 2015 and 2016 in Greece, Italy, Malta, Portugal, and Spain. A total of 79 Diaporthe strains were isolated from symptomatic twigs, branches and trunks. A multi-locus phylogeny was established based on five genomic loci (ITS, tef1, cal, his3 and tub2), and the morphological characters of the isolates determined. Preliminary pathogenicity tests were performed on lemon, lime, and orange plants with representative isolates. The most commonly isolated species were D. foeniculina and D. baccae, while only four isolates of D. novem were collected. Two new Diaporthe species, described here as D. limonicola and D. melitensis spp. nov. were found associated with a new devastating dieback disease of lemon plants. Furthermore, one cluster of sterile Diaporthe isolates was renamed as D. infertilis. Pathogenicity tests revealed most of the Citrus species as susceptible to D. baccae, D. foeniculina, and D. novem. Moreover, D. limonicola and D. melitensis caused serious cankers affecting all the Citrus species tested. This study is the first report of D. baccae and D. novem on citrus in Europe, and the first detection of a new Diaporthe canker disease of citrus in Europe. However, no isolates of D. citri were found. The study improves our understanding of the species associated with several disease symptoms on citrus plants, and provides useful information for effective disease management.
Highlights
Diaporthe species are present worldwide as plant pathogens and endophytes in healthy leaves, stems, seeds and roots, or as saprobes on decaying tissues of a wide range of hosts (Muralli et al 2006, Garcia-Reyne et al 2011, Udayanga et al 2011)
Several additional Diaporthe species have been reported associated with Citrus and have previously been considered as synonyms of D. citri, such as D. citrincola described from the Philippines, P. californica from California, P. caribaea from Cuba, and P. cytosporella from Italy (Rehm 1914, Fawcett 1922)
In particular the objectives of the present study were to: (1) conduct extensive surveys for sampling symptomatic plant materials; (2) cultivate as many Diaporthe isolates as possible; (3) subject those isolates to DNA sequence analyses combined with morphological characterization; (4) compare the obtained results with the data from other phylogenetic studies on the genus; (5) place three strains previously named as D. citri in the correct taxonomic context based on DNA sequence inference; and (6) evaluate the pathogenicity of the isolated Diaporthe species to citrus plants
Summary
Diaporthe species are present worldwide as plant pathogens and endophytes in healthy leaves, stems, seeds and roots, or as saprobes on decaying tissues of a wide range of hosts (Muralli et al 2006, Garcia-Reyne et al 2011, Udayanga et al 2011). Diaporthe species are well-known as the causal agents of many important plant diseases, including root and fruit rots, dieback, stem cankers, leaf spots, leaf and pod blights, and seed decay (Uecker 1988, Mostert et al 2001a, b, Van Rensburg et al 2006, Rehner & Uecker 1994, Santos et al 2011, Udayanga et al 2011, Diaz et al 2017). Diaporthe citri is a well-known pathogen causing melanose and stem-end rot disease of Citrus species in several regions (Timmer 2000, Mondal et al 2007). Several additional Diaporthe species have been reported associated with Citrus (often as Phomopsis) and have previously been considered as synonyms of D. citri, such as D. citrincola described from the Philippines, P. californica from California, P. caribaea from Cuba, and P. cytosporella from Italy (Rehm 1914, Fawcett 1922). Several additional Diaporthe species have been reported associated with Citrus (often as Phomopsis) and have previously been considered as synonyms of D. citri, such as D. citrincola described from the Philippines, P. californica from California, P. caribaea from Cuba, and P. cytosporella from Italy (Rehm 1914, Fawcett 1922). Wehmeyer (1933) considered D. medusaea, D
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