First Report of Leaf Spot Caused by Alternaria alternata on Chard (Beta vulgaris var. cicla) in China

Chard (Beta vulgaris var. cicla L.) is popular vegetable in China. In June 2023, a leaf spot disease was observed on Chard plants in Hunan Province (27°46'10.99″N, 112°05'52.80″E), China. The disease incidence was 30% in a surveyed of about 500 plants. Symptoms began as many light brown round- to polygon-shaped spots on chard leaves, then developed and enlarged into grayish-white lesions, with the edge of the spots brown to dark brown. A total of 10 symptomatic samples were randomly collected. To identify the pathogen, symptomatic tissues (0.5 × 0.5 cm) from the lesion margin surface were sterilized with 75% ethanol for 30 s and 2% NaClO for 1 min, rinsed 3 times with sterile water, air dried. The sterile pieces were placed on potato dextrose agar (PDA) and incubated at 25°C. A total of nine isolates were obtained. Fungal colonies cultured on potato carrot agar (PCA) were almost the same as each other, and two representative isolates (TC0, TC10) were used for further identification. On PCA, the fungal hyphae were initially white and finally gray-brown with flocculent aerial mycelia. Conidia were solitary or in chains, with various shapes, mostly subglobose, the size was 13.2 to 28.0 μm long and 5.8 to 13.0 μm wide (n = 30). The cultural and morphological characteristics of isolates were similar to those of Alternaria sp (Simmons et al. 2007). For molecular identification, four loci, ITS (White et al. 1990), RPB2 (O'Donnell, 2022), H3 (Zheng et al. 2015), and GAPDH (Berbee et al. 1999), were sequenced from two representative isolates (TC0, TC10). Compared with a reference isolate, Alternaria alternata strain CBS 107.27, GenBank accession nos. KP124300.1 (ITS), KP124768.1 (RPB2), KP124157.1 (GAPDH). The ITS, RPB2, and GAPDH sequences of TC0 and TC10 showed 99% (502 of 504 bp ), 100% (753 of 753 bp), and 99% (560 of 561 bp) similarity, respectively. Compared with a reference isolate, A. alternata isolate 21-5, GenBank accession no. MN840996.1 (H3), H3 sequences of TC0 and TC10 showed 99% (399 of 401 bp) similarity. The sequences of two isolates (TC0, TC10) were deposited in GenBank with accession numbers PP837733.1, PP565404.1(ITS), PP839298.1, PP573905.1(RPB2), PP839299.1, PP573904.1 (GAPDH), and PP839297.1, PP573903.1(H3). Phylogenetic trees were constructed using the sequences and showed that isolates (TC0, TC10) were in the same clade with A. alternata strains. TC0 and TC10 were identified as A. alternata based on the morphological characteristics and molecular phylogeny. Pathogenicity testing was conducted on six-month-old healthy plants, (cv. Green Stalk), three plants were inoculated by spraying spore solution (1 × 106 conidia/mL), and three plants were sprayed with sterile water as a control. The pathogenicity test was performed 3 times. Plants were maintained at 28°C and >80% RH. Plants showed symptoms after 30 days, symptoms were observed similar to those of the original infected plants, control plants were asymptomatic. The fungus was reisolated, confirmed as A. alternata based on conidial characteristics, no pathogenic fungus was isolated from the control plants. A. alternata has previously been reported on beet (also Beta vulgaris) in China (Tai, F. L. 1979; Zhuang, W. Y. 2005). To our knowledge, this is the first report of leaf spot caused by A. alternata on chard in China. This result may expand the etiological study of A. alternata and the control strategy of Chard leaf spot.