Histological observations on tumours of olive knot.
The development and anatomical structure of olive knots were studied after inoculating the pathogen, Pseudomonas savastanoi, into slits through the stem bark of young plants in spring when the cambium was active, or in winter and summer when it was not. Spring, summer and winter knots did not differ substantially. Structural differences were found between knots in the inner cortex and those in the outer or those induced by bark inoculation with a hypodermic syringe. In all cases the inoculated tissues reacted either with a quick renewal (summer and winter) or an acceleration (spring) of cambial activity. Irregular cavities containing bacteria were formed in old knots. Fissures from these extended to the surface, enabling bacteria to reach intact knots. Decay appeared to set in at this stage.