First Report of Leaf Necrosis of Salvia dorisiana Caused by Alternaria alternata in Italy

Abstract

Salvia dorisiana Standl. (fruit-scented sage), Lamiaceae, is a perennial plant producing scented leaves and magenta flowers. During the spring of 2018, we observed irregular, necrotic spots 1 to 5 mm in diameter on leaves of 6-month-old S. dorisiana growing in a greenhouse at the Centre of Competence Agroinnova, University of Torino, Grugliasco (Torino Province), northern Italy. The necrosis gradually enlarged until about 4 cm in diameter. Affected tissues dried, causing curling of the affected leaves that remained attached to the plants. About 30 of 100 plants were affected. Symptomatic leaves were washed in sterile water and small fragments from the margins of necrotic tissue plated on potato dextrose agar (PDA). Incubation at 23 ± 1°C, 16-h light and 8-h dark, produced olive green fungal colonies with light and dark concentric rings and conidia within 10 days. Conidia of isolate DB18MAG21 were catenulate, olivaceous, elliptical to ovoid or obclavate or irregular and measured 10 to 22 × 6 to 10 (average 14 × 7) μm (n = 50). Conidia had one to four transverse septa and zero to two longitudinal septa. The beak was 2 to 4 (average 2) µm long or absent. These morphological characteristics suggested the fungus isolated from S. dorisiana was a species of Alternaria (Simmons 2007). Isolate DB18MAG21 was grown on PDA and the DNA extracted using the E.Z.N.A.Fungal DNA Mini Kit (Omega Bio-Tek, Darmstadt, Germany). Polymerase chain reaction was performed using primers of β-tubulin genes, T1 (5′-AACATGCGTGAGATTGTAAGT-3′) (O’Donnell and Cigelnik 1997) and β-tub-2 (5′-ATCATGTTCTTGGGGTCGAA-3′) (Peever et al. 2004). A BLASTn search of the 990-bp sequence (GenBank accession no MH580297) showed 100% similarity with A. alternata (Fries) Keissler (KU512287). Potted, healthy 10-month-old plants of S. dorisiana grown in the open field were used to test the pathogenicity of A. alternata. Conidia and mycelium from a 15-day-old pure culture of isolate DB18MAG21 grown on PDA were macerated in sterile water to produce a spore suspension of 0.7 × 10⁵ spores/ml. Three plants were inoculated by spraying 15 ml of the spore suspension onto unwounded leaves; three control plants were sprayed with sterile water. All plants were kept in a moistened plastic bag for 7 days. The first symptoms were irregular necrotic spots 1 to 5 mm in diameter, which appeared on the inoculated plants after 8 to 10 days. A. alternata was reisolated and identified (GenBank accession no. MK050502) with the same molecular method described above. Control plants remained symptomless, and attempts to isolate the pathogen from these failed. A. alternata was previously reported on Salvia spp., such as S. farinacea in Japan (Negishi and Suyama 2002). To our knowledge, this is the first report of A. alternata on S. dorisiana in Italy. Current management options to prevent this disease include minimizing leaf wetness and reducing relative humidity. The importance of this disease on S. dorisiana is currently minor owing to its limited cultivation. However, because the production of species belonging to the Lamiaceae is increasing in Italy, both as ornamental and as aromatic plants, further work on chemical management options may be useful.