Anatomical pathogenesis and histological interaction between Pseudomonas savastanoi pv. savastanoi strain KT11 and Pseudomonas fluorescens strain PICF4 in olive knots

Abstract

The development of the olive knot disease, caused by Pseudomonas savastanoi pv. savastanoi was studied taking into consideration the anatomical changes at the inoculation sites. The genesis of knots was investigated in the absence and presence of Pseudomonas fluorescens bv. PICF4, a natural colonizer of olive roots and an effective biocontrol agent (BCA) of verticillium wilt of olive. This bacterium was investigated for its potential as a BCA against olive knot disease. Bioassays using 5-month-old olive plants (cv. Arbequina) were performed to assess the interaction between olive plant stems, Pseudomonas savastanoi KT11, isolated from active knots of naturally infected olive twigs, and PICF4. Thus, we studied whether strain PICF4 controlled knot development and anatomy compared with those observed in the absence of PICF4, and the population size of both bacteria when co-inoculated together in stems or separately injected at different intervals after inoculation. Results revealed that PICF4 was able to settle, colonize and persist in stem tissues upon artificial inoculation. While PICF4 was unable to suppress disease development, its presence affected the pathogen population size (reduction of ≈ 25% at 54 days post inoculation) and produced smaller knots. Histological sections (± 40 μm thicknesses) cut with a vibrotome and observed under a light microscope showed less tissue alterations and less cavities inside the knots, compared with those produced when the pathogen was inoculated without PICF4.