A Systematic Survey on Prevalence of Grapevine Trunk Disease Pathogens in Oregon Vineyards.

Grapevine trunk diseases (GTDs) are found in vineyards worldwide and can be caused by different fungal pathogens. To characterize types of GTDs in Oregon vineyards, and how the GTD pathogens' prevalence is affected by two geographical regions, a survey was conducted in which grapevine trunk samples were collected from 15 and 14 wine grape (Vitis vinifera) vineyards in southern and northern Oregon, respectively. Fungal species were identified through culture and PCR-based methods. GTD pathogens that were identified included Botryosphaeriaceae spp. and Phaeoacremonium spp. from 72 and 21% of the surveyed vineyards, respectively; Phaeomoniella chlamydospora, Cryptovalsa ampelina, Truncatella angustata, Seimatosporium lichenicola, Hormonema viticola from 7% of the surveyed vineyards; and Dactylonectria macrodidyma, and Pestaloptiopsis sp. from 3% of the surveyed vineyards. Pathogens were identified in both regions and in young and mature vineyards. The presence of GTD from the Botryosphaeria dieback complex was significantly affected by regions (P = 0.021), with pathogens being significantly more abundant in Willamette Valley (northern region) compared with Rogue Valley (southern region) vineyards. Some differences among other tested variables such as vineyard age, cultivars, rootstocks, and pruning methods were observed for all disease complexes; however, the differences were not statistically significant. Our study summarizes that Botryosphaeria dieback and Esca disease complexes are the most prevalent diseases infecting grapevines in Oregon vineyards and management practices need to be geared toward these economically important diseases. In addition, pathogens from other disease complexes are also present, suggesting a need for regular disease monitoring and following practices to limit the spread of these pathogens.

Contact-dependent traits in Pseudomonas syringae B728a.

Production of the biosurfactant syringafactin by the plant pathogen Pseudomonas syringae B728a is a surface contact-dependent trait. Expression of syfA, as measured using a gfp reporter gene fusion was low in planktonic cells in liquid cultures but over 4-fold higher in cells immobilized on surfaces as varied as glass, plastic, paper, parafilm, agar, membrane filters, and leaves. Induction of syfA as measured by GFP fluorescence was rapid, occurring within two hours after immobilization of cells on surfaces. Comparison of the global transcriptome by RNA sequencing of planktonic cells in a nutrient medium with that of cells immobilized for 2 hours on filters placed on this solidified medium revealed that, in addition to syfA, 3156 other genes were differentially expressed. Genes repressed in immobilized cells included those involved in quaternary ammonium compound (QAC) metabolism and transport, compatible solute production, carbohydrate metabolism and transport, organic acid metabolism and transport, phytotoxin synthesis and transport, amino acid metabolism and transport, and secondary metabolism. Genes induced in immobilized cells included syfA plus those involved in translation, siderophore synthesis and transport, nucleotide metabolism and transport, flagellar synthesis and motility, lipopolysaccharide (LPS) synthesis and transport, energy generation, transcription, chemosensing and chemotaxis, replication and DNA repair, iron-sulfur proteins, peptidoglycan/cell wall polymers, terpenoid backbone synthesis, iron metabolism and transport, and cell division. That many genes are rapidly differentially expressed upon transfer of cells from a planktonic to an immobilized state suggests that cells experience the two environments differently. It seems possible that surface contact initiates anticipatory changes in P. syringae gene expression, which enables rapid and appropriate physiological responses to the different environmental conditions such as might occur in a biofilm. Such responses could help cells survive transitions from aquatic habitats fostering planktonic traits to attachment on surfaces, conditions that alternatively occur on leaves.

Pseudomonas syringae Increases Water Availability in Leaf Microenvironments via Production of Hygroscopic Syringafactin.

The epiphytic bacterium Pseudomonas syringae strain B728a produces the biosurfactant syringafactin, which is hygroscopic. The water-absorbing potential of syringafactin is high. Syringafactin attracts 250% of its weight in water at high relative humidities but is less hygroscopic at lower relative humidities. This finding suggests that the benefit of syringafactin to the producing cells is strongly context dependent. The contribution of syringafactin to the water availability around cells on different matrices was assessed by examining the water stress exhibited by biosensor strains expressing gfp via the water-stress-activated proU promoter. Wild-type cells exhibited significantly less green fluorescent protein (GFP) fluorescence than a syringafactin-deficient strain on dry filters in atmospheres of high water saturation, as well as on leaf surfaces, indicating greater water availability. When infiltrated into the leaf apoplast, wild-type cells also subsequently exhibited less GFP fluorescence than the syringafactin-deficient strain. These results suggest that the apoplast is a dry but humid environment and that, just as on dry but humid leaf surfaces, syringafactin increases liquid water availability and reduces the water stress experienced by P. syringaeIMPORTANCE Many microorganisms, including the plant pathogen Pseudomonas syringae, produce amphiphilic compounds known as biosurfactants. While biosurfactants are known to disperse hydrophobic compounds and to reduce water tension, they have other properties that can benefit the cells that produce them. Leaf-colonizing bacteria experience frequent water stress, since liquid water is present only transiently on or in leaf sites that they colonize. The demonstration that syringafactin, a biosurfactant produced by P. syringae, is sufficiently hygroscopic to increase water availability to cells, thus relieving water stress, reveals that P. syringae can modify its local habitat both on leaf surfaces and in the leaf apoplast. Such habitat modification may be a common role for biosurfactants produced by other bacterial species that colonize habitats (such as soil) that are not always water saturated.