Phytopathogenic Curtobacterium flaccumfaciens Strains Circulating on Leguminous Plants, Alternative Hosts and Weeds in Russia

Many bacterial plant pathogens have a broad host range important for their life cycle. Alternate hosts from plant families other than the main (primary) host support the survival and dissemination of the pathogen population even in absence of main host plants. Metabolic peculiarities of main and alternative host plants can affect genetic diversity within and between the pathogen populations isolated from those plants. Strains of Gram-positive bacterium Curtobacterium flaccumfaciens were identified as being causal agents of bacterial spot and wilt diseases on leguminous plants, and other crop and weed plants, collected in different regions of Russia. Their biochemical properties and susceptibility to copper compounds have been found to be relatively uniform. According to conventional PCR assays, all of the isolates studied were categorised as pathovar Curtobacterim flaccumfaciens pv. flaccumfaciens, a pathogen of legumes. However, the strains demonstrated a substantial diversity in terms of virulence on several tested host plants and different phylogenetic relationships were revealed by BOX-PCR and alanine synthase gene (alaS) sequencing.

First report of Curtobacterium flaccumfaciens pv. flaccumfaciens causing bacterial tan spot of soybean in Russia

Curtobacterium flaccumfaciens pv. flaccumfaciens (H.) Collins & Jones is known as a pathogen of different legume crops, including soybean (Glycine max (L.) Merr.) (Hedges 1922; Dunleavy 1983). OEPP/EPPO (2011) considers C. flaccumfaciens pv. flaccumfaciens as present in Russia based on reports of the disease on common beans in two regions of Russia (North Caucasus and Far East) made without proper pathogen identification. During the summer of 2020 and the spring of 2021, soybean plants with tan spot disease (10-40% of plants) were reported during routine assays of several fields in Stavropol Krai (44.72°N, 43.29°E). After harvest in 2021, we inspected 48 soybean seed lots collected in different regions of Russia for the presence of C. flaccumfaciens pv. flaccumfaciens. Seed testing was performed using the OEPP/EPPO (2011) protocol. For bacteria isolation, seed extracts were spread on MSCFF agar plates (Maringoni et al. 2006). After 5 days of incubation at 28°C potential, C. flaccumfaciens pv. flaccumfaciens colonies were used for further tests on NSA and SSM agar (Tegli et al. 2017, Maringoni et al. 2016). Six seed lots produced in Stavropol, Ryazan (53.95°N, 40.62°E), Orel (52.39°N, 37.69°E) and Amur (51.31°N, 128.22°E) regions were suspect. Ten isolates (SB1 to SB4 from Stavropol, F-125-1 to F-125-3 from Ryazan, and F-30-1 to F-30-3 from Amur) were selected, and further identified by morphological, physiological, and biochemical properties, MALDI TOF MS, 16S rRNA sequences, and specific primers CffFOR2 and CffREV4 (Tegli et al. 2017). Isolates consistently formed yellow, circular, smooth colonies on agar, and were identical to C. flaccumfaciens pv. flaccumfaciens type strain DSM 20129T in diagnostic physiological properties (Tegli et al. 2017). DNA was isolated from the bacteria by the CytoSorb Kit (Sintol, Moscow). All tested strains were positive in the PCR assay (Fig. 1). 16S rRNA fragments were amplified using primers 27F/1492R (Marchesi et al. 1998) and PCR products were sequenced (Evrogen, Moscow, Russia). The obtained 16S rRNA sequences (1473 bp, Accession No. OL539808.1-OL539817.1) were 100% identical to DSM 20129T (AM410688.1) according to a BLAST NCBI search. A pathogenicity test was done by leaf-cutting with scissors wetted with inoculum (for soybeans) or by injecting 5 microliters of the bacterial suspension (108 CFU/ml) into the stem (for common beans). All ten isolates for the inoculum were grown on nutrient agar for 72 h at 28°C. Soybean cv. Kasatka plants (stage V1) were used for inoculation, and common bean (cv. Purpurnaya) plants were inoculated as well to confirm multi-host virulence. Sterile water served as a control. Ten plantlets were used as replicates for each treatment. The plants were incubated at 24°C, 80% RH, and a 14 hour light/10 hour dark cycle. Tan spots (soybean) and wilt (beans) have developed 7-21 d.p.i (Fig. 2.1-2.6). Control plants remained asymptomatic. Seed inoculation by soaking them in the same bacterial suspension repeatedly produced twisted primary root (Fig. 2.7-2.8), but typical disease symptoms on leaves developed in 4-5 weeks only. The pathogen was successfully reisolated from all infected plants and not from the controls, thus fulfilling Koch's postulates. The identity of the reisolated strains was confirmed using morphological and physiological characteristics and the DNA sequence data for the 16S rRNA. These results indicated that a causal agent of the tan spot is present on soybean in three important agricultural areas of Russia (South, Central, and the Far East). To the best of our knowledge, this is the first report of C. flaccumfaciens pv. flaccumfaciens causing a bacterial tan spot of soybean in Russia.

First Report of Curtobacterium flaccumfaciens pv. flaccumfaciens Causing Bacterial Wilt and Blight on Sunflower in Russia

In the summer of 2018, wilt and leaf spots were observed on sunflower (Helianthus annuus L.) plants in fields near Kursk (51.74°N, 36.02°E) in Russia. In the following years, incidence of this disease was 5 to 20% in the inspected fields. Marginal chlorosis on seedling leaves developed into wilt and necrosis about one week later (Fig. 1). Mature plants had leaves with blight and reduced height compared to symptomless plants. Pathogen isolation from seeds was done by the method of Tegli et al. (2002) with modifications. Bacteria from diseased plants were isolated by streaking inoculum from symptomatic tissues on nutrient dextrose agar (NDA) (Schaad et al. 1988). The plates were incubated at 30°C for 7 to 10 days. Isolates consistently formed slow-growing, yellow, circular, smooth colonies without soluble pigment. The isolated bacteria were aerobic, gram-positive, and rod-shaped. Eight strains, CF-20 to CF-26 from plants, and Curt1 and Curt3 from seeds, were identified by MALDI TOF MS analysis as Curtobacterium flaccumfaciens pv. flaccumfaciens or C. flaccumfaciens pv. poinsettiae. All strains had GENIII MicroPlate (BIOLOG) test results identical to C. flaccumfaciens pv. flaccumfaciens strain DSM20129T. Further analysis was done by specific PCR (Tegli et al. 2002) and 16S rDNA, gyrB, and atpD gene sequencing. For PCR amplification, DNA was extracted by the CitoSorb Kit (Syntol Co., Moscow). Primers 27F/1492R (16S rRNA) (Marchesi et al. 1998), 2F/6R (gyrB) (Richert et al. 2005), and aptD2F/aptD2R (Jacques et al. 2012) were used to amplify the target gene sequences. The PCR products were sequenced by Evrogen (Moscow). The 16S rRNA sequences (OL584192.1 to OL584199.1) were identical to that of C. flaccumfaciens pv. flaccumfaciens strain DSM20129T (AM410688.1; 1,477/1,477 bp). The phylogenetic tree of concatenated gyrB (560 bp) and atpD (716 bp) sequences (OL548915.1 to OL548922.1 and OL548923.1 to OL548930.1, respectively) clustered the strains from sunflower among C. flaccumfaciens pv. flaccumfaciens, C. flaccumfaciens pv. betae, and C. flaccumfaciens pv. oortii (Fig. 2) with high genetic similarity to other C. flaccumfaciens strains: 96.3 to 100% for atpD and 95 to 100% for gyrB. A pathogenicity test for each of the strains was performed by injecting 5 μl of a bacterial suspension (108 CFU/ml) grown for 72 h on NDA into the stems of five plantlets (four true leaf stage) of the sunflower cv. Tunka (Limagrain, France) and soybean cv. Kasatka (VIM, Russia). Strain DSM20129T was a positive control, while sterile water was a negative control. The plants were incubated at 24°C, 80% relative humidity, and 14-h light/day. Wilting and blight on sunflower (Fig. 3) and tan spots on soybean were observed in 15 to 20 days after inoculation for all sunflower strains and strain DSM20129T. The negative control plants were asymptomatic. The bacteria re-isolated from the inoculated plants exhibited the same morphological characteristics and 16S rDNA sequence as the original culture, thus fulfilling Koch's postulates. The presence of C. flaccumfaciens pv. flaccumfaciens in sunflower seeds indicated that the bacterium was transmitted via seed. Sunflower has been previously reported as a host for the pathogen (Harveson et al. 2015). The presence of C. flaccumfaciens pv. flaccumfaciens on beans in Russia was suggested from the disease symptoms (Nikitina and Korsakov 1978), but, to our knowledge, this is the first report of the pathogen affecting sunflower in Russia. Phytosanitary categorization placed C. flaccumfaciens pv. flaccumfaciens in the EPPO A2 list (EPPO 2011). Thus, sunflower seeds should be tested to protect pathogen-free areas from introduction of this pathogen.