First Report of Alternaria macrospora Causing Leaf Spot of Solanum lycopersicum in China

Abstract

Tomato, Solanum lycopersicum L., Solanaceae is one of the most common vegetable grown in China. In mid-August 2014, a heavy leaf spot infection was observed on tomato near Hohhot, Inner Mongolia, China. Early symptoms appeared as small, water-soaked chlorotic spots on the leaves. Lesions later expanded to 7-12 mm in diameter and were pale brown, and irregular shape. The leaf spots affected more than 30% of the plants in one field. Conidiophores with conidia appeared on the surface of leaf spots in continuously wet or humid conditions. Conidiophores were ca 60-150 μm x 57 μm, medium brown, simple with 1 to 2 spore scars. Conidia were solitary or occasionally appeared as 2 conidia in chain. Conidia bodies were long-ovoid or long-ellipsoid, ranging from 40–88 μm × 11– 25 μm, with 6–12 transverse distosepta, occasionally having a euseptum and 2–7 longitudinal or oblique distosepta, tawny to dark brown. Essentially all conidia developed an apical filamentous beak ca 100–297 μm long and 1.5–2.5 μm in diameter throughout its length, colorless. The pathogen was identified as Alternaria macrospora on the basis of spore morphology (Simmons 2007; Zhang et al. 2003). Identification was confirmed by sequencing the internal transcribed spacer region (ITS) of rDNA with the primers ITS1 and ITS4, the elongation factor 1-alpha (EF-1a) with the primers EF1728F and EF1-986R) and the RNA polymerase subunit (RPB2) gene with the primers RPB2-5F2 and fRPB2-7cR (Woudenberg et al. 2014). The corresponding sequences (GenBank Accession Nos. KU877347, KU877348 and KU877349) shared complete sequence identity to the sequences of the type strain of Alternaria macrospora, CBS 117228 ( GenBank Accession Nos. KC584204, KC584668 and KC584410, respectively) (Woudenberg et al. 2014). The fungus strain (ZM140768=HHAUF140768 a preserved in the Henan Agriculture University fungal collection) The fungus was isolated on potato carrot agar (PCA) by picking a single conidium from symptomatic leaf tissue using a needle under a stereomicroscope. The colony was light yellow to tawny as it developed. Koch’s postulates were performed with the leaves of 70-day-old tomato cv. Jinfen 2 plants. Ten leaves of three plants were inoculated with mycelial plugs harvested from the periphery of a 7-day-old pure culture colony on PCA; an equal number of leaves were inoculated with plugs of sterile PCA as the negative control. All test plants were covered with plastic bags for 24 h to maintain high relative humidity and incubated at 24-28 o C with natural light. After 5 days, all inoculated leaves showed symptoms identical to those observed in natural conditions, whereas the negative controls remained symptom-free. A. macrospora was successfully isolated onto PCA from lesions on the inoculated leaves confirming that the causal agent was A. macrospora. A. solani and Alternaria linariae (=A. totatophila, A. cretica and A. subcylindrica) are two big spore species of Alternaria previously reported on tomato (Woudenberg et al. 2014). To our knowledge, this is the first report of leaf spots caused by A. macrospora on tomato in China. Considering tomato is widely produced in China, A. macrospora could be a significant threat to tomato production in the future.