Some products are not meant to work on the specific disease that you are trying to control. That means using it anyway and hoping it will help can sometimes cause damage to your prized bonsai. You should also find a very small place on your bonsai to test the product before treating the entire tree, just in case.
The best type of fungicide is one that is protecting. That means it is comprised of a shield that will protect your plants healthy tissue. If your bonsai already is infected with a fungus, then you want to eradicat, which will help kill it. Then to protect uninfected portions of the bonsai, you would use a systemic fungicide.
If the bonsai's root system has a fungus, it can usually be protected very effectively with Thiophanate methyl. Roots are obviously more difficult to treat. Therefore, you will need to apply the fungicide to the soil as a drench so it can then travel through the plant systematically. Another problem with roots is that because the soil stays moist, its loaded with diverse microbes. Beause of this, the fungicide will probably degrade quickly; meaning you will need to give more than one application.
The following are recommendations specifically for fungus and bonsai:
- leaf spots = benomyl, chlorothalonil, mancozebm propiconazole, triadimefon
- Powdery mildew - Triadimefon
- Rust - chlorothalonil, mancozeb, propiconazole
- Root rot (water mold) - fosetyl-A1, metalaxyl
- Root rot (fungi) - methyl, thiophanate
- Seed treatment - captan
