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First, go out and buy the proper product: don't borrow your Uncle Ed's bottle of DDT that's been sitting in the garage for 25 years. It's no longer legal and it's past its expiration date.
Next, before using any pesticide, read the label. It will tell you whether the product will work for the disease you want to control. It will also tell you if it is REGISTERED for that use. Some chemicals may be effective for a use, but for whatever reason, have not been officially tested. You can get fined for misusing a pesticide. This is a particular problem for bonsai, since some plants (say, Osteomeles anthyllidifolia or Millettia reticulata) are unknown outside of the tiny community of bonsai growers, and no pesticide company is going to spend thousands of dollars to register a pesticide for a rare plant. You have to check the label for wording like "for greenhouse use" or "for use on landscape plantings". This means the product is broadly labeled (but a product labeled for indoor use should not be used in the backyard!). It's a good idea to test the product on a small portion of the plant or on a less valuable specimen of the same species to make sure it isn't PHYTOTOXIC, damaging or killing leaves.
The label will also give you information on how to safely apply the chemical. TOXICITY is the degree to which an insecticide is poisonous; HAZARD is the risk of being exposed. The label will instruct you on the use of protective clothing and precautions. In general it is a good idea to avoid skin contact with the pesticide, to avoid inhaling it, to wash skin and clothing after use, and to store it safely (in a cool, well-ventilated area away from children and pets).
Different fungi are ....different. They 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 Thiophanate methyl or PCNB, but Oomycetes require fungicides like Fosetyl-A1or Metalaxyl. Most of these fungi will not be controlled with a protectant if rotting has already occurred because once in tissue, the fungus can spread throughout the root system; 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. If you live above the timberline on Mt. Shasta, however, the use of protectants "just in case" are a waste of money.
Long Island Horticultural Research Lab
39 Sound Ave, Riverhead, NY 11901
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