Now you need to understand a few terms: a PROTECTANT is a fungicide that will shield healthy tissue from invasion. An ERADICANT will kill fungi that have already invaded the plant. A SYSTEMIC fungicide spreads within the plant and can protect areas that weren't directly hit (for example, the undersides of leaves would be protected if the top surface of the leaf was hit by spray droplets.
Fungi are a diverse group of organisms, some of which are only distantly related to the others (for example, members of the Oomycetes, the water molds, are really more closely related to algae than to true fungi). Furthermore, they occupy different parts of the plant. For these reasons, not all fungicides will be equally effective on all fungi. If your bonsai shows symptoms of root rot, you need to treat the tree as quickly as possible, but without knowing what is causing the root rot, you may not be able to control the fungus in time to save the tree. Many root rot fungi can be controlled with Benomyl or PCNB, but Oomycetes require fungicides like Dexon or Truban. Most of these fungi will not be controlled with a protectant if rotting has already occurred, because the fungus can spread through root tissue; an eradicant is necessary. Because roots are hard to treat directly, either the fungicide must be applied as a SOIL DRENCH, or it has to travel systemically through the plant. Furthermore, because soil is wet and contains diverse microbes, a fungicide may degrade rapidly, requiring multiple applications.
Obviously, for a protectant to work most effectively, it must be sprayed before disease occurs and reapplied until there is no longer danger of infection. This, however, can be expensive, and the more fungi are exposed to a fungicide, the more likely they are to develop resistance to it. Common sense is required. If you live near an apple orchard and grow crabapple bonsai, you may need to spray your plants regularly to prevent infection by common apple pathogens.
The following is a list of some fungicides that can be used in New York State for diseases of trees and shrubs. Different fungicides can be sprayed if you grow bonsai in a greenhouse; confusing but true. To make absolutely sure you are spraying the right fungicide for your situation, call your local Cooperative Extension agent.
For powery mildew: Triadimefon (but some mildews are resistant).
For rusts: Mancozeb, Chlorothalonil, Propiconazole
For root rots caused by water molds (Phytophthora or Pythium): Fosetyl-A1, Metalaxyl
For root rots caused by other fungi: Benomyl
Seed treatments: Captan