Pseudomonas savastanoi

Systematic position and type of pathogen: Pseudomonas savastanoi is a Gram-negative, motile, rod-shaped bacterium within the Pseudomonadaceae family. This phytopathogen is a highly specialized plant bacterium known for its ability to manipulate the host plant's hormonal balance. By secreting auxins and cytokinins, the bacteria induce abnormal cell proliferation in infected host tissues.

Diseases and host plants: The primary disease associated with this pathogen is the olive knot (or bacterial canker of olive trees). While primarily affecting the European olive, the pathogen also infects other woody species including oleander, ash, and lilac. The hallmark of the infection is the formation of galls (knots) on stems, branches, petioles, and leaves, which can lead to significant structural deformation of the plant.

Biology and life cycle: The bacterium gains entry into the host plant through natural openings or, more commonly, through physical wounds caused by pruning, mechanical harvesting, frost damage, or insect feeding. Once inside the apoplast, the bacteria multiply and cause the characteristic gall formation. The disease cycle is heavily dependent on moisture, as water helps disseminate the bacteria from the surface of the knots to new infection sites.

Developmental conditions and impact: Development of the disease is favored by high relative humidity and temperatures ranging from 20 to 25 degrees Celsius. The economic impact is significant; heavy infections reduce tree vigor, limit fruit production, and cause branch dieback. In severe cases, the entire tree can become stunted, leading to reduced orchard productivity and economic loss for growers.

Control and management measures: Managing Pseudomonas savastanoi relies on integrated pest management practices rather than curative treatments:

• Strict sanitation by removing and burning infected branches.
• Regular disinfection of all pruning tools using bactericidal solutions or bleach.
• Preventive spraying with copper-based bactericides following rainy weather or heavy pruning.
• Careful management of orchard health to minimize frost injury and wind-induced damage.
• Using pathogen-free nursery stock to ensure that new plantings remain uninfected from the start.