Conventional wisdom says that all living things are either plants or animals. As is frequently the case conventional wisdom falls a little short. Sitting somewhere between plants and animals are a huge number of living things and fungi fall into this category. In fact, the kingdom of fungi is so big that it has many more species than either the plant or the animal kingdom.
The role of fungi in the large order of things is to break down tissue from either plants or animals into small segments that can be used as nutrients for the next generation. They are the ultimate recyclers or are garbagemen, depending on your point of view. Unfortunately, they sometimes try to break down things that we are not ready to have broken down and this leads to problems with or bonsai.
Many different members of the vast fungus kingdom can affect our bonsai and as a result the symptoms of fungus infections can be widely diverse. The most usual indication of a fungus infection problem is spots on the leaves of deciduous trees. This can range in severity from a spot on a single leaf to heavy spotting on all the leaves. When this occurs, it could be a prelude to loss of the plant. A related problem is powdery mildew, especially on maples.
A type of blight that attacks junipers and pines is active in the bay area. This appears to be spread by insects and will cause death of the tree if it is not treated promptly A symptom of this blight is dieback of individual branches, starting at the tips. The dead foliage has an orangey brown color.
The various organisms that cause root rot in bonsai are not technically fungi, but they miss the cut by a subtle difference that is only of interest to microbiologists who are interested in taxonomy. For our purposes they can be lumped with fungi.
Treatment of fungi differs from treatment of insect problems in at least three ways. The fungi are inside the tree whereas insects are for the most part outside the tree where they can be contacted by an insecticide. Because the fungi are living inside the tree, they are hard to reach with fungicides because the fungicides must penetrate the tree to be effective.
The biology of insects is pretty much the same for all species, so any insecticide will usually have at least some effect on any bug, ranging from effective control to at least making them angry. Fungi are a large family of beasties and each member of the family has a very different biology makeup and as a result any single fungicide may offer good control of your problem or may have no effect at all.
The third difference is that insects crawl or fly from branch to branch. Simple contact of a branch with bugs with an uninfected branch usually does not lead to an immediate spreading of the problem because the bugs must physically move from one branch to the other. In contrast, fungi are spread by spores and contact of a diseased leave with a fresh leaf is all it takes to spread the problem to the new leaf.
The most effective way to control fungal problems is to practice good hygiene. Promptly remove fallen leaves from your trees. The blight infecting conifers is particularly contagious. A great way to spread the fungus throughout the tree is to clip off contaminated dead branches and then to trim uninfected branches. The clippers will spread the fungus. If you are working on an infected tree, frequently disinfect your clippers by dipping them in alcohol or a weak peroxide solution. Powdery mildew can be caused by watering late in the day, allowing the foliage to remain wet overnight. Maples and oaks are particularly susceptible to powdery mildew.
In the case of insecticides, you have a choice of a very wide range of products starting with simply blasting them off your tree with a hose to chemicals that are very safe to harsh chemicals that must be handled with respect. In the case of fungal problems, the available choices are pretty much limited to harsh chemicals. Horticulture copper spays work and are probably the least toxic of the available products. Cleary 3336, available from Amazon, is usually more effective than the copper formulation and also more toxic. It is reputed to be systemic which means that it is absorbed by the tree and can potentially deal with the fungus inside the tree.
If you are only interested in nontoxic and environmentally gentle means of controlling fungus infections, the world of organic gardening offers a possible solution. The procedure has not been applied to bonsai but if you are an experimentalist at heart it might help to reduce fungal problems in general and leaf spotting in particular. The idea here is that on each living plant there is a balance between good and bad microbes. If you have a leaf spotting, it is caused by an imbalance of the microbes. You can restore the balance by adding good microbes to the tree. You do this by soaking compost in water which allegedly creates a lot of good microbes. The water is decanted and sprayed on the foliage. This procedure has been used at the University of California Botanical Garden with reduced leaf spotting and better apparent health of the trees. With no control group of unsprayed trees, it is not clear whether the apparent good results arose from the spray or something else.
Literature from England in the nineteenth century indicates that green house plants were regularly manured. This is a fertilization scheme which involves soaking manure in water, decanting the water and using it to fertilize plants in the green house. According to our organic gardening friends, this procedure would generate a lot of good microbes and would maintain a balance of microbes in the plant. Presumably, you could both fertilize your tree and maintain proper microbe balance by drenching your plants with a manure water concoction. If you do this and have good results let us know and, by the way, do not forget to compare your results with trees which are only fed with your current fertilizer.