Black Spot Infestation in Olive Grove

My family recently bought a property with a small olive grove in Tasmania, Australia. We are converting it to organic production but the previous owners used various fertilizers and chemical herbicides to keep the grass down. The trees are currently pretty badly infested with black spot, which we plan to treat with an organic white oil product after the harvest.  Just wondering if anyone has any tips for preventing it?

Lucia, Welcome to permies!

Black spot is a fungal disease.  

These threads while maybe not relevant for now might help in the future:

https://permies.com/t/99739/Olive-Tree-Pests

https://permies.com/t/92459/wrong-daughter-Orcas-Pear-Orcas

This is what I feel might be the best advice:

Bryant said It appears to be either scab or black spot (nearly identical fungal diseases) the best (for non-chemical treatment) thing to use is a good compost tea sprayed all over the tree and soil beneath and surrounding the trees, this will help break the cycle via a protective coating.
As has been mentioned by James Freyr, you will want to gather up all infected leaves, take them away and burn them completely.
Adding more mycorrhizae around the root systems will also help as will using sea water (mineralization of the soil) to water with once a month for a few months.

I would also remove as much of the mulch from around the trees as is practical and dispose of that by fire too, it will harbor the fungi over the winter.

Feel free to ask any other questions you might have.

Great, thanks Anne for your help!