What’s going on at the base of my olive?

Hey everyone,

I have an old olive that seems to have some sort of superficial infection on the nebari. All the bark is dark greenish black; when it’s wet it feels spongy, when dry it flakes away effortlessly (to my dismay).

At a recent show where I displayed it, several knowledgeable senior members of our club said it was some type of photosynthetic bacteria I needed to spray with an antibiotic (one advised a beta-lactam the other recommended a fluoroquinolone, for any geeks that share my concern for antibiotic stewardship).

Can anyone verify that that’s what we’re seeing here? Does this organism have a name? Is there any concern for the tree with the use of antibiotics?
This tree is 40-60 years old and special.




if it is photosynthetic, could one not just black out the affected base?

It looks like wet rot to me. I would address the issue of the balance of water and oxygen rather than resort to chemicals. Olives dislike wet roots and need to breathe. The shallow pot is not really conducive to good drainage.
I live in North East Scotland, (which is dry by Scottish standards) and have a big old rugged deadwood olive bursting with health which lives outdoors in my bonsai courtyard on a low table with a slatted top for good air circulation. I put it in the greenhouse if necessary in winter to keep it dry but it does not mind being cold. We have long sunny summers and long dark winters!
Deeper pot, let it breathe and it will thrive. Good Luck.


I have an old olive and have seen it as well. Olives like periods of dryness between watering. Unless temperatures in my area get above 85F, I only water my olive bonsai every 3 days. This helps the balance of water and O2 and eventually the black fungus substance goes away. Just food for thought

Have you lightly scrubbed it with a toothbrush?
Maybe that will take some off.