Disease and frost damage of woody plants caused by Pseudomonas syringae: Seeing the forest for the trees

Pseudomonas syringae is a phytopathogenic bacterium that causes diseases of monocots, herbaceous dicots, and woody dicots, worldwide. On woody plants, reports of disease due to P. syringae have markedly increased in the last years and the diseases have been recognized as a major threat to the primary products of agroforestry practices. Detection in Italy of a new highly aggressive population of P. syringae in 2008 on kiwifruit, which caused severe epidemics in the following years throughout the kiwifruit-growing areas of Asia, Europe, Oceania, and South America, rendered the entire kiwifruit industry vulnerable to the disease. Similarly, occurrence of an aggressive population of P. syringae on horse chestnut in 2002 in the Netherlands has rapidly established itself as a major threat to horse chestnut throughout Northwest Europe. To better understand the origin of such disease epidemics, a thorough knowledge of the pathogen is needed in sensu lato. Here, we report the most important features of the pathogen and its hosts in an attempt to clarify some key aspects. In particular, the diseases and the economic losses they cause, disease epidemiology, pathogen diversity, and the possible means of disease control have been discussed throughout the manuscript. In addition to the ability to cause the disease, the damage caused to woody plants through the ice nucleation activity of this bacterium is discusse