Pseudomonas savastanoi

Taxonomy and Nature of the Pathogen: Pseudomonas savastanoi is a Gram-negative, motile rod-shaped bacterium belonging to the kingdom Bacteria and the genus Pseudomonas. It is a specialized plant pathogen recognized for its ability to induce the development of galls (knots) on host plants. The pathogen achieves this by secreting specific phytohormones, such as indole-3-acetic acid (IAA) and cytokinins, which disrupt the normal growth regulation of the plant tissues.

Diseases and Host Crops: The most economically significant disease caused by this pathogen is "olive knot," which affects olive trees (Olea europaea). The disease leads to the formation of tumor-like growths on branches, stems, and sometimes leaves. Beyond olives, different pathovars of Pseudomonas savastanoi are known to infect other plants, such as oleander (Nerium oleander) and ash trees, creating similar symptomatic outgrowths.

Biology and Life Cycle: The bacterium survives within the established galls, which serve as the primary inoculum reservoir throughout the year. Transmission typically occurs through splashing rain, wind, and contaminated pruning tools. The bacteria enter the host plant through wounds caused by pruning, harvesting, frost damage, or hail, as they are incapable of penetrating intact, healthy plant surfaces on their own.

Environmental Conditions and Damage: Favorable conditions for the development and spread of the disease include high humidity and moderate temperatures. The damage caused by the pathogen includes reduced photosynthetic capacity, stunted growth, premature leaf fall, and the drying out of twigs and branches. In severe infestations, the systemic weakening of the tree leads to significant yield losses and potential death of younger specimens.

Measures of Protection and Control: Managing the disease relies on strict sanitation and preventive practices:

• Sourcing certified pathogen-free planting stock for new orchards.
• Rigorous disinfection of pruning tools between every single tree to prevent cross-contamination.
• Avoiding pruning activities during wet or humid weather conditions to minimize infection of fresh wounds.
• Application of copper-based bactericides immediately after pruning and following extreme weather events like hailstorms or frost to protect open tissue.