Overview
Pseudomonas syringae is a Gram-negative bacterium and one of the most studied plant pathogens in the world. It is really a species complex, divided into more than 50 pathovars that each specialise on particular hosts. In fruit production its most damaging forms cause bacterial canker of cherry and other stone fruit, and — as pv. actinidiae (Psa) — the bacterial canker of kiwifruit that spread worldwide from around 2008 and reshaped the global kiwi industry.
Symptoms
Symptoms include sunken, gummy cankers on branches and trunks, blossom blast, dieback of shoots, and dark leaf spots often surrounded by yellow haloes. On kiwifruit, Psa produces reddish exudate and rapid cane collapse. Infection frequently begins in blossoms and through natural openings or pruning wounds.
How it spreads
Cool, wet conditions in autumn and spring strongly favour infection. Many strains are ice-nucleation active, promoting frost damage that opens further entry points. Rain, wind and contaminated tools spread the bacteria between plants and orchards.
The control challenge
Copper-based bactericides are the mainstay of management, but copper-resistant strains of P. syringae are now widely reported, steadily eroding their effectiveness. With few alternative chemistries and no cure once a tree is infected, growers are left with limited, mostly preventive options — a clear case for new, resistance-aware biological approaches.
Pseudomonas syringae is a primary research focus of Exacta's bacteriophage platform. Because bacteriophages target only their specific host bacterium, they offer a way to address P. syringae without the copper accumulation, phytotoxicity or resistance pressure of conventional bactericides. This is a research and development programme, not a commercially available product.