Phenazines are secondary metabolites with broad spectrum antibiotic activity and thus show high potential in biological control of pathogens. In this study, we identified phenazine biosynthesis (phz) genes in two genome-completed plant pathogenic bacteria Pseudomonas syringae pv. tomato (Pst) DC3000 and Xanthomonas oryzae pv. oryzae (Xoo) PXO99^A. Unlike the phz genes in typical phenazine-producing pseudomonads, phz homologs in Pst DC3000 and Xoo PXO99^A consisted of phzC/D/E/F/G and phzC/E1/E2/F/G, respectively, and the both were not organized into an operon. Detection experiments demonstrated that phenazine-1-carboxylic acid (PCA) of Pst DC3000 accumulated to 13.4 μg L⁻¹, while that of Xoo PXO99^A was almost undetectable. Moreover, Pst DC3000 was resistant to 1 mg mL⁻¹ PCA, while Xoo PXO99^A was sensitive to 50 μg mL⁻¹ PCA. Furthermore, mutation of phzF blocked the PCA production and significantly reduced the pathogenicity of Pst DC3000 in tomato, while the complementary strains restored these phenotypes. These results revealed that Pst DC3000 produces low level of and is resistant to phenazines and thus is unable to be biologically controlled by phenazines. Additionally, phz-mediated PCA production is required for full pathogenicity of Pst DC3000. To our knowledge, this is the first report of PCA production and its function in pathogenicity of a plant pathogenic P. syringae strain.

Zinnia elegans (syn. Zinnia violacea), known as common zinnia, is one of the most spectacular ornamental plants in the family Asteraceae. Zinnia plants are widely cultivated in China for their impr...

Agrobacterium tumefaciens is a gram negative bacterial that can infect a range of plants and result in root crown gall. A total number of 10328 bacterial strains were isolated from rhizosphere of cherry tree. One strain of LWB10 showed clear inhibition zone around the bacterial colony in YEB media inoculated with A. tumefaciens C58. Morphological, physiological, and biochemical characterization indicated that LWB10 belongs to member of the genus Pseudomonas. Results from the high-throughput matrix-assisted laser desorption/ionization biotypersmart system indicated that this strain had a score value of 2.247 relative to Pseudomonas mosselii. Also, phylogenetic analysis based on 16S rRNA gene sequence showed that strain LWB10 shared the highest similarity with Pseudomonas mosselii CIP 105259T. The antagonist strains also exhibit well in growth inhibition of other five A. tumefaciens strains. Coinoculation of LWB10 and plant pathogenic strain of A. tumefaciens CFCC1369 showed strongly inhibition of tumor formation in tomato stems. All the results demonstrated that the isolated strain is P. mosselii LWB10 and its antibacterial ability to A. tumefaciens may offer new way for management of crown gall disease in the future. Keywords: Agrobacterium tumefaciens, Pseudomonas mosselii, antagonist, biological control

Sweet viburnum (Viburnum odoratissimum Ker-Gawl.) is a widely used ornamental plant, which has dense branches and leaves, and fast spreading and evergreen habit. In October 2022, leaf spot symptoms were observed in a hedge of sweet viburnum in Yuanshi Garden, Ningbo, China. Fungi were isolated from symptomatic leaves, and were identified using morphological characteristics and phylogenetic analyses of partial sequences of internal transcribed spacer (ITS), actin (act), and translation elongation factor 1-alpha (tef1-α), and were evaluated in pathogenicity tests. The causal agent of sweet viburnum leaf spot was identified to be Phyllosticta capitalensis. Effects of seven fungicides on P. capitalensis were assessed in vitro. Fungicide EC50s (mg L-1) against P. capitalensis were: 270.77 for 75% chlorothalonil (WP); 0.02 for 250 g L-1 azoxystrobin SC; 0.27 for 10% difenconazole WDG; 0.02 for 75% trifloxystrobin + tebuconazole WDG, 9.03 for 35% fluopyram + tebuconazole SC, 5.90 for 500 g L-1 fluazinam SC, and 89.11 for 10% prothioconazole SC. Among these, azoxystrobin SC and trifloxystrobin + tebuconazole WDG could be used for control of viburnum leaf spot. This is the first report of P. capitalensis causing leaf spot of sweet viburnum, and this study provides guidance for chemical management sweet viburnum leaf spot, and on other host plants.

Prunus rufoides is a deciduous wild tree, native to China, which is also used as an ornamental. From late March to December in the years 2018 to 2023, P. rufoides plants growing in Siming Mountain (29°71’08”N, 121°15’12”E), Ningbo, Zhejiang Province, were severely affected by powdery mildew. The disease initially emerged in late March each year, and was characterized as white, irregular mycelial patches on the adaxial surfaces of young leaves. Between July and August, the powdery mildew colonies on affected parts of leaves disappeared, leaving only irregular yellow-brown spots. The disease recurred in September and persisted until late December. Chasmothecia containing asci and ascospores were observed on the leaves in December. Morphological analyses of the chasmothecia indicated a Podosphaera sp. as the causative agent. Molecular identification, based on the internal transcribed spacer (ITS) region (primers ITS4/ITS5) confirmed the identity of the pathogen as Podosphaera prunigena. Koch’s postulates were fulfilled by inoculation tests in which the same pathogen was identified from the inoculated leaf tissue. This study represents the first report confirming the powdery mildew disease on P. rufoides in China to be caused by P. prunigena.