First report of leaf blight on pumpkin caused by Nigrospora sphaerica in China.

Abstract

Pumpkin (Cucurbita moschata), which belongs to the gourd family (Cucurbitaceae), is widely planted throughout the world. In June 2023, many pumpkin plants (cv. Miben) displayed leaf blight and chlorosis in fields located in Suizhou (31.99°N, 113.02°E), Hubei Province, China. The disease incidence ranged from 30 to 40% in nine fields, 6.3 ha in total. The symptoms were irregularly shaped lesions that expanded along the mid-vein until the leaf turned brown and wilted. Fungal isolations were performed as described previously (Liu et al. 2023). Twenty pumpkin leaf samples with typical symptoms were collected and cut into 1 cm×1 cm pieces. The diseased tissue was surface-sterilized in 75% ethanol for 30 sec, plated on potato dextrose agar (PDA) medium and incubated at 25℃ for 3 days. Then, the emerging single fungal hyphal tip was transferred onto PDA plates to obtain purified isolates. A total of eighteen isolates on PDA plates were initially white and then developed to dark gray. The 5-day-old cultures growing on mung bean medium produced conidia that were black, single-celled, smooth, spherical or oblate, and ranged in size from 14.5 to 20.8 μm×13.3 to 20.5 μm (n=50). Therefore, the isolates were morphologically identified as Nigrospora sphaerica. Moreover, the genomic DNA of the isolates (HB-P1,HB-P2, and HB-P3) was extracted for amplification and sequencing of the regions of internal transcribed spacer (ITS) (White et al. 1990), nuclear large subunit rRNA (nLSU) (O'Donnell 1992; Rehner and Samuels 1994), and β-tubulin (TUB2) (Glass and Donaldson 1995), with primers ITS1/ITS4, LROR/LR3, and Bt2a/Bt2b, respectively. Sequences were submitted to GenBank under accession numbers PP348112, PP348113, PP348114 (ITS), PP411414, PP411415, PP411416 (nLSU), and PP357438, PP357439, PP357440 (TUB2). BLASTn showed that the sequences ITS, nLSU, and TUB2 of HB-P1, HB-P2, and HB-P3 had >99% nucleotide identities ((ITS: 100%, 508/508 bp, MF996488.1; 99.8%, 506/507, ON326588.1; 100%, 500/500 ,MK748317.1), (nLSU: 99.83%, 573/574, KT462720.1; 99.83% , 574/575 bp, KT462720.1; 99.65%, 575/577, KT462720.1), and (TUB2: 100%, 388/388, MN719407.1; 99.74%, 387/388, MN719407.1; 100%, 387/387, MN719407.1)) with Nigrospora sphaerica, respectively. A multilocus (ITS, nLSU and TUB2) phylogenetic analysis indicated that the isolates were Nigrospora sphaerica. Pathogenicity of three isolates were tested on pumpkin plants (cv. Miben). Fifteen pumpkin plants were inoculated by spraying the leaves (1×106 spores/ml), respectively, and 10 pumpkin plants were treated with sterile water as a negative control. All plants were incubated in an artificial climate box (LongYue, ShangHai) at 25℃ for 12 days. The experiment was repeated three times. Twelve days later, the inoculated pumpkin plants developed symptoms of leaf blight, while the control plants remained healthy. Then, pathogens were re-isolated from the each leaf of inoculated pumpkin plants and not from the control plants. Nigrospora sphaerica has been previously reported to cause leaf spot on watermelon in Malaysia (Ismail and Abd Razak 2021). To our knowledge, this is the first report of N. sphaerica causing leaf blight on pumpkin in China. This new disease can cause leaf blight, which may affect pumpkin productivity.