First Report of Soft Rot Caused by Pectobacterium carotovorum on Pinellia ternata in China

First Report of Blight Caused by Rhizoctonia solani AG4-HGI on Pinellia ternata in Guizhou, China

Pinellia ternata (Thumb.) has been used for over 1000 years as a traditional Chinese herbal medicine (Ying et al. 2007) and is widely cultivated in Guizhou Province, China. It is cultivated over an area of 2000 hectares, and is of great value to underdeveloped regions. In April 2020, blight was observed in a field of P. ternatain Bijie County, Guizhou Province, China (27°30'N, 105°28'E). Around 20 hectares of P. ternata were surveyed and the disease incidence ranged from 10 to 12%. The disease symptoms included light brown lesions formed on the stems near the soil line. The color of the lesions became darker, and the stems became constricted around the lesions and broke, associated with the leaf blight. To identify the causal agent of this blight, 22 diseased plants (about 30 d-old） were collected, the margins of the infected parts were cut into small pieces (5 mm) and surface disinfested with 1% NaOCl for 10 min, 75% ethanol for 30 s, and rinsed three times in sterile distilled water. The pieces were blotted dry with sterile filter paper and placed on potato dextrose agar (PDA, Hopebio, China), incubated at 28℃ in darkness until fungal hyphae growth was visible. Sixteen cultures with different morphologies were recovered from the samples. When representative isolates of each culture type were inoculated onto plants, one produced similar blight symptoms. The representative isolate was called CD-1. The colony color was first white but turned light brown after grown on PDA for 6-7 d, and produced dark brown sclerotia. The hyphae were branched at right angles, with a slight constriction at the base of the branches and a septum near the junction where the branch separates from the main hyphae. Hyphal cells were stained with 0.5% Safranin O and 3% KOH and were observed to be multinucleate. These morphological features indicated that CD-1 likely is R. solani (Sneh et al. 1991). When paired with tester strains AG1 and AG4(provided by Dr. Genhua Yang, Yunnan Agricultural University). CD-1 showed anastomosis with isolate of AG4 (Fenille et al. 2002). Genomic DNA was extracted from the isolate (Thangaraj et al. 2018) using a fungal genomic DNA extraction kit (Tiangen, China). The internal transcribed spacer (ITS) regions were amplified using the primers ITS1/ITS4 (White et al. 1990). A 535 bp fragment was amplified that showed 99% coverage and 99.4% identity with an isolate of R. solani AG4-HGI (GenBank: HG934417). The gene sequence was deposited in GenBank as accession #OL518945. Pathogenicity tests were performed using 30 d-old plants planted in sterilized soil in pots. Cut mycelial discs (diameter 6 mm) from 3-day-old PDA cultures and placed beside stems of 21 healthy plants. Nine plants treated with agar plugs were control samples. Inoculated plants were maintained at 24 ± 5℃ in a green house and watered every two days with sterilized water. Typical blight symptoms developed on the inoculated plants at d 3-5 post inoculation, whereas the control plants remained healthy. The experiments were repeated three times, and the isolates was re-isolated from the inoculated plants and identified as R. solaniAG4 by morphological features and molecular method. R. solani has been reported to cause blight of many plants such as coffee (Ren et al. 2018) and sesame (Cochran et al. 2018). To the best of our knowledge, this is the first report of R. solani AG4-HGI causing disease on P. ternate, both in China and worldwide. This finding suggests that this pathogen may cause a threat to cultivation and production of P. terenata.

Host Specificity and Differential Pathogenicity of Pectobacterium Strains from Dicot and Monocot Hosts

Recent phylogenetic studies have transferred certain isolates from monocot plants previously included in the heterogeneous group of Pectobacteriumcarotovorum (Pc) to a species level termed Pectobacterium aroidearum. The specificity of Pectobacterium associated infections had received less attention, and may be of high scientific and economic importance. Here, we have characterized differential responses of Pectobacterium isolates from potato (WPP14) and calla lily (PC16) on two typical hosts: Brassica oleracea var. capitata (cabbage) a dicot host; and Zantedeschia aethiopica (calla lily) a monocot host. The results revealed clear host specific responses following infection with the two bacterial strains. This was demonstrated by differential production of volatile organic compounds (VOCs) and the expression of plant defense-related genes (pal, PR-1, lox2, ast). A related pattern was observed in bacterial responses to each of the host’s extract, with differential expression of virulence-related determinants and genes associated with quorum-sensing and plant cell wall-degrading enzymes. The differences were associated with each strain’s competence on its respective host.

The Changing Face of Bacterial Soft-Rot Diseases

Bacterial soft rot is a disease complex caused by multiple genera of gram-negative and gram-positive bacteria, with Dickeya and Pectobacterium being the most widely studied soft-rot bacterial pathogens. In addition to soft rot, these bacteria also cause blackleg of potato, foot rot of rice, and bleeding canker of pear. Multiple Dickeya and Pectobacterium species cause the same symptoms on potato, complicating epidemiology and disease resistance studies. The primary pathogen species present in potato-growing regions differs over time and space, further complicating disease management. Genomics technologies are providing new management possibilities, including improved detection and biocontrol methods that may finally allow effective disease management. The recent development of inbred diploid potato lines is also having a major impact on studying soft-rot pathogens because it is now possible to study soft-rot disease in model plant species that produce starchy vegetative storage organs. Together, these new discoveries have changed how we face diseases caused by these pathogens.

Differential pathogenicity and genetic diversity among Pectobacterium carotovorum ssp. carotovorum isolates from monocot and dicot hosts support early genomic divergence within this taxon

The capability of Pectobacterium carotovorum isolates to infect monocotyledonous plants has been previously reported; however, no full consideration was given to characterize the association between such isolates and their monocot hosts. To assess differences in aggressiveness among P. carotovorum ssp. carotovorum isolates originating from monocotyledonous or dicotyledonous plants, we used as model plants two susceptible monocot hosts, the ornamentals Zantedeschia aethiopica and Ornithogalum dubium, as well as two common dicot hosts, Solanum tuberosum and Brassica oleracea. Using virulence assays and different genetic analyses we characterized P. carotovorum ssp. carotovorum isolates from diverse geographical locations which originated from plants belonging to four unrelated orders of monocots and five orders of dicots. Invariably, isolates originating from monocots exhibited higher virulence towards the tested monocot plants than dicot isolates, independently of their geographical source. Moreover, monocot and dicot isolates were clearly differentiated by various genetic analyses, such as 16S rRNA sequence clustering, intergenic transcribed spacer-PCR (ITS-PCR) banding pattern and amplified fragment length polymorphism (AFLP). We propose that the observed relationship between pathogenicity and genetic diversity among P. carotovorum ssp. carotovorum isolates reveals a co-evolutionary specialization trend in the interaction between this pathogen and its hosts.