"Black sooty residue" is probably the key here -- I suspect what you have is actually called Black Sooty Mould -- a mould which grows on honeydew, which is the sticky-sweet discharge from sucking insects like scale or aphids. I would look carefully at the branches and undersides of the leaves, I suspect you will find an infestation of either scale insects, mealybugs, aphids, or similar. If so, I would treat the trees, either with an oil-type spray, or with a systemic insecticide like Imadicloprid. Dennis SE Michigan

Olive trees really are neat looking plants, aren't they? I love the connection to people and civilizations in the past. I have a few friends in Seattle and in Portland growing them and they seem to be quite happy after a few years. My one friend in Crown Hill in Seattle has one potted on his south-facing patio and it grows vigorously and produces fruit each summer (they don't fully ripen as mentioned above, but they do form, so they add to the "oliveness" of the olive plant). I may give one a try as well. They're just really fun. Let us know if you get one and how it does for you. Updates from all of our olive-growers are encouraged. It's fun to hear who is growing what. Take care, Grant

Westes: Bay area has a lot of bonsai enthusiasts and several clubs. Google will bring up quite a few in your area. Contact them for advise. But you should be aware that the plant will need maintenance work to keep it in balance and looking good. A large pot will allow the plant to support more leaves and thus will be larger but at some point it will settle to a larger but fairly stable size. You can reduce the effective volume of the pot but by putting ballasts (bricks, large stones, etc). Every 3-4 years it will need to be repotted too. Ken: bonsai draws its inspiration from nature. They just happen to be large trees since there is enough soil mass to support that biomass over ground. Bonsai is a scaled down version where we balance the root mass to the plant size it can support. The biggest difference really is to use a very custom soil and specific root pruning to maximize the "tree" that a given volume of soil can support. Root pruning aims to remove all thick roots and replace them all with fine fibrous roots. The more fibrous roots you have the bigger the plant it can support. The thicker roots are purely structural. So the plant needs to be tied to the pot - all fibrous root, coarse soil and small soil mass cannot hold the relatively biggish plant in place.

I would like to plant another avocado tree, but the best location is where I have an old olive tree. The olive tree is over 35 years old and is slowly dying - just old I guess. If I remove the olive tree and most of the roots - would the soil and remaining decomposing roots be too harmful for growing an avocado tree?