Pseudomonas syringae is a rod shaped, Gram-negative bacterium with polar flagella. It is a member of the Pseudomonas genus, and based on 16S rRNA analysis, P. syringae has been placed in the P. syringae group. It is a plant pathogen which can infect a wide range of plant species, and exists as over 50 different pathovars. Many of these pathovars were once considered to be individual species within the Pseudomonas genus, but molecular biology techniques such as DNA hybridization have shown these to in fact all be part of the P. syringae species. It is named after the lilac tree (Syringa vulgaris), from which it was first isolated.
P. syringae tests negative for arginine dihydrolase and oxidase activity, and forms the polymer levan on sucrose nutrient agar. It is known to secrete the lipodepsinonapeptide plant toxin syringomycin, and it owes its yellow fluorescent appearance when cultured in vitro on King's B medium to production of the siderophore pyoverdin.
Since the 1970's, P. syringae has been implicated as an atmospheric 'biological ice nucleator'. Recent evidence has suggested that the species plays a larger role than previously thought in making it rain and snow.
P. syringae, more than any mineral or other organism, is responsible for the surface frost damage in plants, exposed to the environment. P. syringae can cause water to freeze at temperatures as high as −1.8°C, but strains causing ice nucleation at lower temperatures (down to −8°C) are more common. The freezing causes injuries in the epithelia and makes the nutrients in the underlying plant tissues available to the bacteria. Artificial strains of P. syringae known as ice-minus bacteria have been created to reduce frost damage. P. syringae have ina (ice nucleation-active) genes that make Ina proteins which translocate to the outer bacterial cell wall on the surface of the bacteria where the Ina proteins act as nuclei for ice formation.
Disease by P. syringae tends to be favoured by wet, cool conditions - optimum temperatures for disease tend to be around 12–25°C, although this can vary according to the pathovar involved. The bacteria tend to be seed borne, and are dispersed between plants via rain splash..
Although it is a plant pathogen, it can also live as a saprotroph in the phyllosphere when conditions are not favourable for disease. Some saprotrophic strains of P. syringae have been used as biocontrol agents against post-harvest rots.
Following ribotypical analysis several pathovars of Pseudomonas syringae were incorporated into other species (see P. amygdali, 'P. tomato', P. coronafaciens, P. avellanae, 'P. helianthi', P. tremae, P. cannabina, and P. viridiflava). The remaining pathovars are as follows:
Pseudomonas syringae pv. aceris attacks maple Acer species.
Pseudomonas syringae pv. aptata attacks beets Beta vulgaris.
Pseudomonas syringae pv. atrofaciens attacks wheat Triticum aestivum.
Pseudomonas syringae pv. dysoxylis attacks the kohekohe tree Dysoxylum spectabile.
Pseudomonas syringae pv. japonica attacks barley Hordeum vulgare.
Pseudomonas syringae pv. lapsa attacks wheat Triticum aestivum.
Pseudomonas syringae pv. panici attacks Panicum grass species.
Pseudomonas syringae pv. papulans attacks crabapple Malus sylvestris species.
Pseudomonas syringae pv. pisi attacks peas Pisum sativum.
Note that Pseudomonas savastanoi was once considered a pathovar or sub-species of P. syringae, and in many places continues to be referred to as Pseudomonas syringae pv. savastanoi, although as a result of DNA-relatedness studies it has been instated as a new species. It itself has three host-specific pathovars, fraxini which causes ash canker, nerii which attacks oleander and oleae which causes olive knot.
Pseudomonas syringae pv. tomato DC3000 (Donors reference DC52) is a mutant generated from NCPPB 1106. The difference between 1106 and DC3000 is rifampicin resistance (it was generated as a spontaneous mutant). Both DC3000 and 1106 are available from the National Collection of Plant Pathogenic Bacteria (NCPPB)