HISTOLOGY OF PATHOGENESIS OF PSEUDOMONAS SAVASTANOI ON MYRTUS COMMUNIS
M. Temsah, L. Hanna, A.T. Saad
Myrtle (Myrtus communis L.) is an evergreen shrub widely grown in the Mediterranean region for its aromatic leaves and medicinal uses. The bacterium Pseudomonas savastanoi was recently reported to attack this shrub with characteristic symptoms of knot formation on stems and branches leading, in severe cases, to their death. The pathogenesis of P. savastanoi on myrtle was studied macroscopically and by light microscopy. Inoculation of myrtle shoots with P. savastanoi initiated swellings that developed into knots typical of the disease. Starting from the inoculation wound, the bacteria spread into the intercellular spaces, invading and colonizing host tissues by degrading cell walls. They multiplied in the resulting cavities inducing hypertrophy and hyperplasia in the contiguous cells to form tissue masses that grew into knots. The size of a knot depended on the depth of the inoculation wound and more precisely on the parenchyma tissues involved, including those of the cortex, vascular tissues, and/or pith. At an advanced stage of invasion, the knot was composed of an aggregate of tissue masses within which xylem elements had differentiated. The knot became surrounded by wound periderm. Lignification of the parenchyma cells of the knot finally led to its hardening and death. These findings on myrtle were compared to those reported to be caused by the same pathogen on ash, buckthorn, oleander, and olive.