Phenotypically and Genotypically Heterogeneous Strains of Pseudomonas syringae Associated With Alfalfa Leaf Spot Disease in Iran

Diversity of Strains in the Pseudomonas syringae Complex Causing Bacterial Stem Blight of Alfalfa (Medicago sativa) in the United States

Alfalfa growers in the Intermountain West of the United States have recently seen an increased incidence in bacterial stem blight (BSB), which can result in significant herbage yield losses from the first harvest. BSB has been attributed to Pseudomonas syringae pv. syringae and P. viridiflava; however, little is known about the genetic diversity and pathogenicity of these bacteria or their interaction with alfalfa plants. Here, we present a comprehensive phylogenetic and phenotypic analysis of P. syringae and P. viridiflava strains causing BSB on alfalfa. A multilocus sequence analysis found that they grouped exclusively with P. syringae PG2b and P. viridiflava PG7a. Alfalfa symptoms caused by both bacterial groups were indistinguishable, although there was a large range in mean disease scores for individual strains. Overall, PG2b strains incited significantly greater disease scores than those caused by PG7a strains. Inoculated plants showed browning in the xylem and collapse of epidermal and pith parenchyma cells. Inoculation with a mixture of PG2b and PG7a strains did not result in synergistic activity. The populations of PG2b and PG7a strains were genetically diverse within their clades and did not group by location or haplotype. The PG2b strains had genes for production of the phytotoxin coronatine, which is unusual in PG2b strains. The results indicate that both pathogens are well established on alfalfa across a wide geographic range and that a recent introduction or evolution of more aggressive strains as the basis for emergence of the disease is unlikely.

Bacterial Brown Pit, a New Disease of Edible Mushrooms Caused by Mycetocola sp

From September to December 2018, commercial button mushroom (Agaricus bisporus) farms in central Iran were surveyed to monitor the causal agent(s) of browning and blotch symptoms on mushroom caps. In addition to dozens of pseudomonads (i.e., Pseudomonas tolaasii and Pseudomonas reactans), six slow-growing gram-positive bacterial strains were isolated from blotched mushroom caps. These bacteria presented as creamy white, circular, smooth, nonfluorescent, and shiny colonies with whole margins resembling members of Microbacteriaceae (Actinobacteria). All of the actinobacterial strains were aggressively pathogenic on cut cap surface of two edible mushrooms (i.e., A. bisporus and Pleurotus eryngii), inducing brown pit symptoms 48 h postinoculation. The strains did not induce symptoms on the vegetables tested (i.e., carrot, cucumber, and potato), and they did not affect the growth of mycelium of tested plant-pathogenic fungi (i.e., Acremonium sp., Fusarium spp., and Phytopythium sp.). Phylogeny of 16S ribosomal RNA and multilocus sequence analysis of six housekeeping genes (i.e., atpD, dnaK, gyrB, ppK, recA, and rpoB) revealed that the bacterial strains belong to the actinobacterial genus Mycetocola spp., whereas the species status of the strains remains undetermined. Mushroom-associated Mycetocola species were previously reported to be capable of detoxifying tolaasin, a toxin produced by P. tolaasii, whereas the strains isolated in this study did not show tolaasin detoxification activities. Altogether, this is the first report of a mushroom disease caused by an actinobacterial species, and "bacterial brown pit" was assigned as the common name of the disease.

Two Novel Genomospecies in the Agrobacterium tumefaciens Species Complex Associated with Rose Crown Gall

In this study, we explored the pathogenicity and phylogenetic position of Agrobacterium spp. strains isolated from crown gall tissues on annual, perennial, and ornamental plants in Iran. Of the 43 strains studied, 10 strains were identified as Allorhizobium vitis (formerly Agrobacterium vitis) using the species-specific primer pair PGF/PGR. Thirty-three remaining strains were studied using multilocus sequence analysis of four housekeeping genes (i.e., atpD, gyrB, recA, and rpoB), from which seven strains were identified as A. larrymoorei and one strain was identified as A. rubi (Rer); the remaining 25 strains were scattered within the A. tumefaciens species complex. Two strains were identified as genomospecies 1 (G1), seven strains were identified as A. radiobacter (G4), seven strains were identified as A. deltaense (G7), two strains were identified as A. nepotum (G14), and one strain was identified as "A. viscosum" (G15). The strains Rnr, Rnw, and Rew as well as the two strains OT33 and R13 all isolated from rose and the strain Ap1 isolated from apple were clustered in three atypical clades within the A. tumefaciens species complex. All but eight strains (i.e., Nec10, Ph38, Ph49, fic9, Fic72, R13, OT33, and Ap1) were pathogenic on tomato and sunflower seedlings in greenhouse conditions, whereas all but three strains (i.e., fic9, Fic72, and OT33) showed tumorigenicity on carrot root discs. The phylogenetic analysis and nucleotide diversity statistics suggested the existence of two novel genomospecies within the A. tumefaciens species complex, which we named "G19" and "G20." Hence, we propose the strains Rew, Rnw, and Rnr as the members of "G19" and the strains R13 and OT33 as the members of G20, whereas the phylogenetic status of the atypical strain Ap1 remains undetermined.