First Report of Leaf Spot on water convolvulus Caused by Stemphylium solani in Yunnan Province, China.

Abstract

Water convolvulus (Ipomoea aquatica Forsk.), a member of the Convolvulaceae family, is an important tropical vegetable cultivated in China (Liu et al. 2017). From 2016 to 2020, dark-brown leaf spots were observed in major water convolvulus (cv. Large leaf) growing areas (2 ha) in Honghe City (24°12' N, 103°6' E), Yunnan Province, China. Field investigations showed that a leaf spot disease occurred on water convolvulus in four fields with 15% incidence (50 plants in each field were investigated) and resulted in up to a 10% decrease in its total production. Symptoms on water convolvulus plants appeared as small lesions, yellowish-green and circular on the leaves. Ten plants were selected randomly from the growing area, with three diseased leaves collected from each plant. Symptomatic tissues were excised, surface sterilized with 75% ethanol for 30 s, washed in sterile-distilled water three times, and placed on the Potato Dextrose Agar (PDA) followed by incubation at 25°C in the dark for 7 days. Colonies on PDA were gray to green in color and fuzzy in the middle, with irregular borders. Conidiophore morphology showed single, yellowish-brown or brown structures with 1~6 septa, and long 22~125 µm, wide 3.5~5.5 µm. Conidia were elliptical, black-brow, solitary, with a smooth surface, 1~6 longitudinal septa and 1~3 transverse septa, 20~30 µm in length, and 15~22 µm in width. The morphological characteristics of the fungus were consistent with the description of Stemphylium solani (Chai et al. 2014; Weber, 1930). To further confirm the identity of the 30 isolates, the partial gapdh (glyceraldehyde-3-phosphate dehydrogenase), tef1 (translation elongation factor 1-alpha), cmdA (Calmodulin) and ITS (intemal transcribed spacers) sequences were amplified by PCR with the primer pairs of gpd1/gpd2, EF1-728F/EF1-986R, CALDF1/CALDR2 and ITS1/ITS4, respectively (Berbee et al. 1999; Carbone & Kohn. 1999; Lawrence et al. 2013; White et al. 1990). Multiple sequence alignments were generated using MEGA7, and phylogenetic analysis was conducted with the neighbor-joining (NJ) method (Tamura et al. 2007), the results indicated that all sequences from the 30 isolates were identical. Thus, one representative isolate, KXC11033003 was chosen for further analysis. The ITS, gapdh, cmdA and tef1 sequences of this isolate were submitted to the NCBI GenBank database (accession nos. OL444947~OL444950). The strain KXC11033003 and S. solani (CBS-408.54) formed a clade with 82% bootstrap value (Figure S2). To fulfill Koch's postulates, 30 plants were inoculated for each of the thirty isolates. Conidia were sprayed on leaves of water convolvulus (8-true-leaf stage) in a suspension of 107 conidia/mL or water as a healthy control in a greenhouse at 15~18℃ (night) / 25~28℃ (day) with 95% humidity. Symptoms of dark brown spots appeared on the leaves after 7 days, whereas controls remained healthy The pathogens were reisolated from the lesions and confirmed identical to the original isolate by gene sequences. No pathogens were isolated from the control plants. To our knowledge, this is the first report of leaf spot caused by S. solani on water convolvulus in Yunnan Province, China. Further, Stemphylium leaf spot caused by S. solani has been reported previously on tomato, garlic, pepper (Zheng et al.2008; Nasehi et al.2018). This study stresses the need to identify appropriate management strategies for S. solani that help prevent quality and yield losses in water convolvulus in China.