Before bonsai my idea of a pest was my dog begging for table scraps. This brief article is no replacement for more detailed books on the subject. I’m only offering general advice.
There are several classes of pests; insects, diseases, fungus, soil pathogens, and others. We have a distinct advantage with bonsai, when the patient gets sick we can take it to the doctor. In this case the doctor can be your county ag agent, a university forestry department, or a local arborist.
Some pests are obvious such as insects or fungus. The quick solution is your local garden center’s selection of sprays. Insects feed on trees. If you make the tree toxic then the bug dies. I’ve found the best results come from broad spectrum systemic insecticides. Bayer, Ortho, and others offer sprays touted as “insect control”. As with any chemical you need to read the label. Because of the size of my collection I tend to spray prophylactically. Otherwise I often fail to see insect damage until it’s obvious. The hazard to this approach is developing resistant bugs which means changing chemicals every few years. Organic control is best whenever possible. You can do things like using Q-tips with alcohol to kill scale insects, blasting bugs off your trees with a pressure nozzle on your hose, using insecticidal soap, or just hand picking individuals off. There is one chemical that that combines the best of all worlds, neem oil. Neem is non-toxic to people and pets. It’s even used in cosmetics. It comes from the neem tree of India. Used as directed it’ll handle most insects and some fungi. Neem is not systemic and will need to be applied about three times at three day intervals to kill freshly hatched insects, Neem is temperature sensitive. I hardens in the bottle when cold. If that happens just put the bottle in a pan of hot water. It’s good stuff.
Fungus control is another issue. In this case chemical control is about the only treatment. There are an awfully lot of different fungi out there. If I notice the fuzzy kind on my fruit or flower buds I will remove the affected areas and treat with a broad spectrum systemic fungicide such as benomyl or Fore. These products seem to be banned and unbanned on a regular basis so they may not be readily available. Once again you can find commercially available controls labeled as “garden fungus control”. Things like slime mold and root rot are more difficult to deal with. Check with a local expert.
At this point I’m going to cop out. I have no expertise with bacterial or viral diseases. The take away from this article is to treat obvious problems with broad spectrum systemics. That’s kind of a shotgun approach, but, is usually pretty effective.
I've been doing bonsai since the late 70's. I'm passing on what I've learned in those 35+ years. These are real life techniques and tips that worked for me, but, may not for you. Remember, this is free advice and worth every cent of it.