First Report of Leaf Spot of Alcea rosea Caused by Alternaria alternata in Italy

Abstract

HomePlant DiseaseVol. 104, No. 2First Report of Leaf Spot of Alcea rosea Caused by Alternaria alternata in Italy PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Leaf Spot of Alcea rosea Caused by Alternaria alternata in ItalyA. Garibaldi, G. Gilardi, S. Matic, I. Luongo, and M. L. GullinoA. GaribaldiCentre of Competence for Innovation in the Agro-Environmental Sector (AGROINNOVA), 10095 Grugliasco, ItalySearch for more papers by this author, G. GilardiCentre of Competence for Innovation in the Agro-Environmental Sector (AGROINNOVA), 10095 Grugliasco, ItalySearch for more papers by this author, S. MaticCentre of Competence for Innovation in the Agro-Environmental Sector (AGROINNOVA), 10095 Grugliasco, ItalySearch for more papers by this author, I. LuongoCentre of Competence for Innovation in the Agro-Environmental Sector (AGROINNOVA), 10095 Grugliasco, ItalyDISAFA, 10095 Grugliasco, ItalySearch for more papers by this author, and M. L. Gullino†Corresponding author: M. L. Gullino; E-mail Address: marialodovica.gullino@unito.ithttp://orcid.org/0000-0002-7706-1915Centre of Competence for Innovation in the Agro-Environmental Sector (AGROINNOVA), 10095 Grugliasco, ItalyDISAFA, 10095 Grugliasco, ItalySearch for more papers by this author AffiliationsAuthors and Affiliations A. Garibaldi1 G. Gilardi1 S. Matic1 I. Luongo1 2 M. L. Gullino1 2 † 1Centre of Competence for Innovation in the Agro-Environmental Sector (AGROINNOVA), 10095 Grugliasco, Italy 2DISAFA, 10095 Grugliasco, Italy Published Online:20 Nov 2019https://doi.org/10.1094/PDIS-08-19-1614-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Alcea rosea L. (common name hollyhock) is an ornamental plant in the family Malvaceae, typically grown in parks and gardens. During spring 2019, extensive necrosis was observed on leaves of plants grown in a private garden near Biella in northern Italy (45.6121660 latitude and 8.0562970 longitude). The disease affected approximately 50% of about 100 plants, grown under high relative humidity at temperatures between 15 and 25°C. The first symptoms were usually brown, circular lesions of 1 to 4 mm, surrounded by a yellow halo, frequently localized on basal leaves. As disease progressed, lesions coalesced and enlarged. The center of lesions generally fell out, giving the leaf spot a shot-hole appearance. Plants were sometimes defoliated. A fungus was consistently isolated from infected leaves tissues of five affected plants on potato dextrose agar (PDA), with streptomycin sulfate added at 25 mg/liter, resulting in 10 morphologically similar isolates collected. The representative monoconidial isolate 19-24, grown for 7 days at 20 to 22°C at 12 h dark/12 h light on potato carrot agar, produced a greyish to dark mycelium with brown conidia, alone or mostly in chain of seven to 15 elements, ovoid, elliptical, or obclavate in shape. Conidia showed one to five transverse and zero to three longitudinal septa and measured 10.3 to 41.4 µm (average 26.1 µm, n = 40) × 5.2 to 12.5 µm (average 9.5 µm, n = 40). The beaks when present measured 1.5 to 7.2 µm (n = 29). Based on its morphological characteristics, the pathogen was identified as Alternaria sp. (Simmons 2007). The extraction of the DNA from the 19-24 isolate was carried out with the E.Z.N.A. Fungal DNA Mini Kit (Omega Bio-Tek, Darmstadt, Germany), and a PCR reaction was performed using the internal transcribed spacer (ITS) (White et al. 1990), the rpb2, the endoPG, the tef1, and the OPA10-2 primers (Woudenberg et al. 2015). Sequences with 587 (ITS), 865 (rpb2), 460 (endoPG), 269 (tef1), and 633 bp (OPA10-2) (GenBank accession nos. MN249628, MN258022, MN258023, MN258024, and MN258025, respectively) were obtained. A BLASTn analysis of these sequences exhibited 100% identity with the ex-type CBS 916.96 of Alternaria alternata (Fries) Keissler in ITS, rpb2, and tef1 portions (accession nos. AF347031, KC584375, and KC584634, respectively) and 99% identity in OPA10-2 and endoPG portions (accession nos. KP124632 and JQ811978, respectively). Phylogenetic analysis based on endoPG sequences grouped the 19-24 isolate with reference isolates of A. alternata and not with those of other Alternaria species. A pathogenicity test was performed by spraying leaves of healthy 4- to 5-month-old hollyhock plants with a spore suspension (105 CFU/ml) obtained from cultures of two isolates (19-24.1 and 19-24.2) grown on PDA for 10 to 12 days, in light-dark, at 22 ± 1°C. Plants inoculated only with sterilized water served as controls. Three inoculated and three noninoculated plants were used in each trial. Plants were covered with plastic bags for 5 days after inoculation and maintained outdoors at temperatures ranging between 17 and 29°C. The first lesions developed on leaves 10 days after inoculation, whereas control plants remained healthy. A. alternata was consistently reisolated from the lesions of the inoculated plants only. Pathogenicity tests were carried out twice. This is, to our knowledge, the first report of A. alternata on A. rosea in Italy, although A. tenuissima and Alternaria sp. were reported in Cuba and Florida, respectively (Farr and Rossman 2019). The presence and importance of this disease is, at present, limited. A. alternata has a wide host range, and this report adds to the list a new host.