First Report of Gibellulopsis nigrescens Causing Yellow Wilt on Chinese Cabbage in China.

Abstract

Chinese cabbage (Brassica rapa L. ssp. pekinensis), in the family Brassicaceae, is a widely planted crop in China valued for its nutritional benefits. In May 2023, wilt symptoms on Chinese cabbage (cv. 'Dongtian118') were observed in several commercial fields located in Sheqi County, (32.47ºN, 112.46ºE), Nanyang, Henan Province, China. A disease survey noted that disease incidence on plants was approximately 20% to 50% within observed fields. Symptoms included yellowing and wilting leaves, and vascular discoloration of the stem bases. To isolate the pathogen, ten symptomatic leaves collected from different diseased cabbage in two field were cut into small pieces (5 × 5 mm), surface disinfected with 75% ethanol for 30 s, then washed three times in sterile water. After drying, tissues were transferred onto potato dextrose agar (PDA). Plates were incubated at 28℃ for 7 days in the dark. Twelve morphologically similar fungal isolates were obtained by single-spore subculture. The mycelia on PDA were originally white, later becoming dark gray due to the formation of masses of melanized chlamydospores after 15 days of culture. Conidiophores were hyaline and most had secondary branches. In addition, verticillate branches had three to four phialides in each whorl. The conidia were hyaline, elliptical or nearly circular, measuring from 3.2 to 9.5 × 2.6 to 3.8 μm (n=40). These morphological characteristics were similar to those described for Gibellulopsis nigrescens (Zare et al. 2007). The isolates were further identified based on PCR amplification. The ITS, GAPDH, and TEF1 genes were amplified using primers ITS1/ITS4, VGPDf2/VGPDr (Inderbitzin et al. 2011) and EF-2/EF1-728F (O'Donnell et al. 1998). BLAST analysis revealed 12 isolates were highly similar to G. nigrescens, with 99.82% similarity for ITS (OR818474, KJ534578), 93.17% similarity for GAPDH (JN188192.1, JN188166.1) and 91.07% similarity for TEF1 (EF543798.1, EF543804.1). Sequences of the representative isolate BC230515 were deposited into NCBI GenBank with accession nos. OR889646 for ITS and PP135039 for GAPDH. Pathogenicity of all 12 isolates was tested on potted Chinese cabbage plants (cv. 'Dongtian118'). Twenty-four healthy Chinese cabbage plants were inoculated by applying a 10 mL conidial suspension (1×107 conidial/mL) at the artificially wounded root region of each plant. Twenty-four control plants wounded similarly were treated with sterile distilled water. All plants were kept in a growth chamber at 22~25°C (day)/18~20°C (night) , 85% relative humidity and a photoperiod of 12 h per day. After 15 days, inoculated plants exhibited wilting symptoms similar to those observed in the field, whereas control plants remained healthy. The pathogenicity test was repeated three times. The associated fungus on the artificially inoculated plants was reisolated from the symptomatic leaves, and its identity was confirmed by PCR with the primers described above. Reisolated G. nigrescens had identical morphological and molecular characteristics to the original isolates, confirming Koch's postulates. To our knowledge, this is the first report of G. nigrescens causing yellowing and wilt of Chinese cabbage in China. G. nigrescens is a destructive pathogen with multiple hosts such as beet (Zhou et al. 2017), alfalfa (Hu et al. 2011), prevention and control measures should be taken in advance.