First Report of Radish Tuber Black Heart Rot Caused by Pectobacterium parvum in China

• Published in: Plant disease
• Main Authors: Wang, Hao, Li, Xin, Ben, Haiyan, Wang, Li, Fan, Tengfei, Xie, Xuewen, Shi, Yanxia, Li, Baoju, Chai, Ali
• Format: Journal Article
• Language: English
• Published: United States 25.07.2025
• Subjects: disease development and spread; Yield loss and economic impacts; Vegetables; Prokaryotes; Crop Type; Causal Agent; Epidemiology; Subject Areas
• ISSN: 0191-2917
• DOI: 10.1094/PDIS-10-24-2100-PDN

Radish (Raphanus sativus L.), which belongs to Brassicaceae, is widely cultivated throughout China. The 'Shawo' radish is renowned for its crispness, sweetness, and juiciness, and predominantly cultivated as a fruit in Tianjin. In November 2023, black rot on radish tubers was observed in several commercial fields on different farms (approximately 10 ha total, with distance of 2-3 km), with disease incidence of 40%-50% in Xiqing District (116.97°E, 39.09°N), Tianjin city, China. Infected plants were stunted and grew yellow leaves, the radish tubers turned blackish brown and emitted a foul odor, eventually rotting completely inside. No fungal mycelium or spores were found on infected tissues by microscopic examination. To identify the causal agent, four radish tubers with rot symptoms were collected from four distinct affected fields belonging to four different farms. Small pieces of tissue (5 × 5 mm) were excised from the margin of diseased and healthy tuber with sterile scalpel blades, soaked in 75% alcohol for 30 s, rinsed three times in sterile distilled water, then ground, suspended in sterile distilled water, spread onto nutrient agar (NA) plates and incubated at 28°C for 24 h. Individual colonies on NA plates appeared oval, slightly convex, translucent with a shiny surface and regular edges. Three independent isolates (LB2, LB3 and LB4), each derived from a tuber in three different fields, were identified by 16S rDNA sequencing using the universal primer pair 27F/1492R (Weisburg et al. 1991). Sequences (accession nos. PQ066487, PQ066488 and PQ066489) had 99.29% homology to that of Pectobacterium parvum (CP087392.1). Housekeeping genes icdA, mdh, pgi, and proA were amplified from all three strains with primer pairs of icdA400F/icdA977R, mdh86F/mdh628R, pgi815F/pgi1396R and proAF1/proAR1 (Ma et al. 2007), resulting in 100% sequence identity across all the strains. The sequences of representative strain LB4 were uploaded to GenBank with accession numbers PP066861, PP066862, PP066863, and PP066865, respectively. Phylogenetic analysis showed that LB2, LB3 and LB4 clustered with P. parvum type strain s0421T (Pasanen et al. 2020). All the three isolates were negative for utilization of citrate, and no color change on Simmons' citrate agar within 72 h inoculation. The pathogenicity test of the three strains was carried out by inoculating 20 µL of each bacterial suspension (108 CFU/mL), which was prepared in sterile distilled water, onto conical wounds. The conical wounds, with a depth of 3 cm and a top diameter of 4 mm, were created by using a sterile 200 μL pipette tip pricked on the surface of five healthy 'shawo' radish tubers. Sterile distilled water was used as a control. Tested radish tubers were incubated in a glass cabinet maintained at 28°C with 90% relative humidity with the inoculated area left open. The pathogenicity test was repeated three times per strain. All the suspension inoculated 'shawo' radish tubers exhibited softened epidermis, blackish brown and severe internal decay three days after inoculation, similar to the symptoms observed in the field, while control radish tubers without symptoms. P. parvum was reisolated from the infected 'shawo' radish tubers and confirmed by sequencing comparition described above, thus fulfilling Koch's postulates. Pectobacterium parvum was first isolated and described as a causal agent of Brassica rapa subsp. chinensis in China by Pasanen et al. 2020. Then, P. parvum was reported causing potato stem rot in Hebei and Inner Mongolia Autonomous Region, China (Wang et al. 2022; Wu et al. 2023). To our knowledge, this was the first report of P. parvum causing radish (shawo cv.) tuber rot in China as well as in the world. This pathogen of P.parvum requires further research on epidemiology and disease management options against its potential impact on local plantation of shawo radish.