First report of Nigrospora musae causing white leaf spot of Basella alba L in China.

Basella alba L, an edible annual twining herb of the genus Basella and the family Basella, has been widely introduced and cultivated in China. Basella alba L. as a leaf vegetable, is rich in vitamins A and C, iron, and calcium (FAO 1988). In May 2022, severe white leaf spots were observed in plantation located in Shuangfeng County (27°41'36" N, 111°56'60" E), Hunan Province, China. More than 50 Basella alba L plants were surveyed with over 80% disease incidence in an area of 300 square meters of greenhouse. The symptoms on leaves were initially small purplish-brown lesions from leaf margins or tips, with lesions expanded, the middle of the lesions was yellowish-white to yellowish-brown, slightly dented. The edge of lesions was purplish-brown, with obvious boundary between the diseased parts and the non-diseased ones. A total of 20 symptomatic samples were randomly collected. Lesion margins were surface sterilized in 2% sodium hypochlorite for 1 min, rinsed with sterile distilled water for three times, dried, placed on potato dextrose agar (PDA), and incubated at 25°C and 60% relative humidity in the dark for 3 days. Hyphal sections from colony edges were transferred to new PDA plates. Six isolates were obtained. Colonies were fast-growing, massive sparse aerial hyphae, initially white, turning gray and black after 7 days. Hyphae were branched, septa, and transparent. To induce sporulation, colonies were transferred to sodium carboxymethyl cellulose (CMC) plates (Z. M. Wen., & X. Y. Luo 1991). Conidia were single-celled, dark black, oblate, or nearly spherical, and measured 10.2 to 15.1 µm × 9.7 to 16.0 µm in diameter (n=50). For molecular identification, the rDNA internal transcribed spacer (ITS), the ß-tubulin gene (TUB), and the translation elongation factor 1-alpha gene (TEF1) were amplified from genomic DNA by primers ITS1/ITS4 (White et al. 1990), Bt2a/Bt2b (Glass & Donaldson. 1995), and EF1-728F/EF1-986R (Carbone & Kohn, 1999). The sequences of six isolates (L1, L2, L7, L10, L11, L12) were deposited in GenBank with accession numbers OP703335, OP703336, OP703337, OP703338, OP703339, OP703340 (ITS), OP784252, OP784157, OP784253, OP784254, OP784255, OP784256 (TEF-1α), and OP724156, OP724158, OP779771, OP779772, OP779773, OP779774 (TUB2). A blast search of sequences showed the ITS, TEF-1α, and TUB2 sequences had >98% identity with homologue sequences from Nigrospora musae isolates BRJ2 (OP451019.1), CBS 319.34 (KY019419.1) and LC6385 (KY019567.1), respectively. These morphological features and molecular identification indicated that the pathogen possessed identical characteristics as Nigrospora musae (Wang, 2017). Pathogenicity test was carried out in plants. Strains were cultured on CMC plates for 14 days, then the mycelium was scraped to make conidial suspension (1×106 conidia/mL). After 5-6 leaves of the Basella alba L were sprouted, conidial suspension was sprayed directly on the leaves, with leaves sprayed by sterile distilled water as the control. All plants were kept in the greenhouse with temperature at 25/30°C (night/day) and 75% relative humidity. After 7 days, symptoms were observed on inoculated leaves of plants, which were the same as previously described samples, while the control plants showed no symptoms. The test was repeated three times with similar results. The strains reisolated from the inoculated leaves were morphologically identical to Nigrospora musae, conforming to Koch's postulates. symptoms of Nigrospora musae is similar to that of the other leaf diseases of Basella alba L reported in China. (H. P. Jiang.2000; S. Tan.1996). To our knowledge, this is the first report of Nigrospora musae causing white leaf spot of Basella alba L in China. The pathogen may severely threat the production of Basella alba L. The information on identification of this fungus may be helpful to the control and prevention of the disease. References: 1. FAO. 1988. Page 103 in: Traditional Food Plants: A Resource Book for Promoting the Exploitation and Consumption of Food Plants in Arid, Semi-arid and Sub-humid Lands of Eastern Africa. FAO Food and Nutrition Paper 42. FAO, Rome, Italy. 2. Z. M. Wen., & X. Y. Luo. Fusarium graminearum spore production medium filtering [J]. Chinese journal of food hygiene, 1991 (04): 11-13. 3. White, T. J., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA. 4. Carbone, I., et al. 1999. Mycologia. 91: 553-556. 5. Glass, N. L., and Donaldson, G. C. 1995. Appl. Environ. Microbiol. 61: 1323. 6. Wang, 2017. Phylogenetic reassessment of Nigrospora: Ubiquitous endophytes, plant, and human pathogens. 7. H. P. Jiang., et al. Occurrence and comprehensive control of white leaf spot of Basella alba L [J]. Plant Protection Technology and Extension, 2000(02):19. 8. S. Tan. The symptoms and control measures of white leaf spot of Basella alba L [J]. Anhui Agricultural, 1996(08):15. *Indicates the corresponding author. Kaifa Guo, E-mail: andygkf@126.com.