Olive tree tuberculosis is a disease found on Olive trees in Spain that is caused by the bacteria Pseudomonas syringae. When this bacteria is introduced into a wound on the tree caused by hail, pruning or by the beating of the branches to harvest the Olives it results in the formation of a gall like formation that starts off round and light brown and then grows into an irregular wart shaped canker that ends up with a dark brown color and very hard.
There is no cure for Olive tree tuberculosis other than careful pruning with clean tools. Most often however the condition is not treated as it does limited harm to the tree. An infected tree can often continue producing for a number of years after becoming infected. The best prevention seems to be to use clean tools to avoid spreading the bacteria in the process of pruning and to try to avoid excessive damage to younger branches during harvest.
The dark objects in the tree below are not olives but rather cankers caused by the Pseudomonas syringae bacteria.