Some notes on the histology of knots produced by Pseudomonas savastanoi in micropropagated olive plants.

The histological examination of knots caused by Pseudomonas savastanoi in olive (Surico, 1977) and oleander (Wilson and Magie, 1964; Wilson, 1965) revealed differences in how they form in the two host species. In oleander, knot formation is a sequential process of the host cells adjacent to the centres of infection, beginning with hypertrophy and continuing with multiple divisions. In olive, knot formation is brought about by pre-existing (cambium) and newly differentiated generative tissues (diffuse cambial cells; phellogen). More recently in the course of an investigation on sources of resistance to olive knot, olive genotypes propagated in vitro were inoculated with a virulent strain of P. savastanoi. The histological examination of the knots and of stem tissue from near the inoculation site revealed that: i) P. savastanoi degrades the host cell walls; ii) the parenchymatous cells of the bark and of the medullary tissue undergo a process of multiple division; iii) meristem centres including xylem tissue differentiate in the bark parenchyma. In addition, P. savastanoi was seen for the first time in the xylem vessels.