Pseudomonas avellanae: From Specific Pathogens to a Species Complex

Bacteria; Proteobacteria, gamma subdivision; Order Pseudomonadales; Family Pseudomonadaceae; Genus Pseudomonas; Pseudomonas avellanae species complex, which includes different pathogenic lineages previously classified within the P. syringae complex, namely the pathovars avellanae, theae, actinidiae, and morsprunorum race 2.

As a whole, the P. avellanae species complex genome comprises 19.4% conserved genes and 80.6% flexible genome. Mobile genetic elements are present within the complex. Strains associated with bacterial canker of European hazelnut possess two plasmids, whereas multiple plasmids and megaplasmids can occur in the lineages associated with bacterial canker of kiwifruit and cherry. Fourteen type III secretion system effectors are shared by all lineages of the species, and each lineage apparently possesses specific effector proteins. Phytotoxins, such as phaseolotoxin, coronatine, and tabtoxin, can be produced by some lineages of P. avellanae. Siderophores are present in the P. avellanae species complex.

Corylus avellana, Camellia sinensis var. sinensis, Actinidia chinensis var. chinensis, A. chinensis var. deliciosa, Prunus avium, P. cerasus, P. domestica. A strain of P. avellanae has also been isolated from river epilithon.

Pseudomonas avellanae can cause severe symptoms, including bud necrosis (kiwifruit), leaf necrosis and wilting (hazelnut, tea, kiwifruit, cherry/plum), shoot and twig blight (hazelnut, tea, kiwifruit, cherry/plum), flower necrosis (kiwifruit), fruit necrosis (cherry/plum), trunk canker (hazelnut, kiwifruit, cherry/plum), and plant death (hazelnut, tea, kiwifruit).

Some P. avellanae lineages are favoured by air temperatures below 0°C. Penetration mainly occurs through stomata, leaf scars, and wounds induced by frost events. P. avellanae species complex exhibits both epiphytic and endophytic phases. During summer, very high temperatures can reduce the multiplication and dispersal of the pathogen. The global circulation of latently infected propagative plant material largely favours the spread of P. avellanae lineages across different cultivation areas.

Pathogens belonging to this complex can be detected in both symptomatic and asymptomatic plant tissues (e.g., twigs, buds, and cankers) using various conventional techniques and molecular tools (standard-, duplex-, TaqMan-, real-time PCR, LAMP, MLSA). Depending on the targeted symptoms, pathogenicity tests may be conducted on leaf, leaf scars or woody tissues.

Disease management is based on spray treatments to the crown using copper compounds applied in spring and autumn, as well as after harvest, during leaf fall, and following hail, frost events, and pruning. Balanced plant nutrition is important to reduce the risk of severe infection. Alternative promising strategies include the application of biocontrol agents, natural products, and bacteriophages.