Understanding Fungus 

Bacteria and Fungus are part of the natural life cycle of plants, providing mechanisms by which dead host plant tissues are broken down to components that can then be re-used by other plants for new growth. Fungi also interact with plants in some cases to support growth, as is the case with mycorrhiza, which lives symbiotically with plant roots and helps the plants uptake nutrients and water. 

Under certain conditions some fungus may infect actively growing plant tissue rather than just dead tissue, attacking the tissue and causing unsightly spots, weakness, or even death in bonsai trees. Managing fungal pathogens can help bonsai stay healthy and maintain their beautiful appearance.

How to Identify Fungal Infections

Typical foliar fungal infections that can be remedied by a bonsai hobbyist include powdery mildew, downy mildew, leaf spot, black spot, tip blights and rust infections. In addition, many common diseases can cause foliar symptoms when they are actually attacking other parts of the plant. Not all types of fungi can be effectively controlled using sprays or other treatments. 

Where leaf symptoms include margin burn (brown or dry tissue at the outer edges of the leaves), fungal pathogens are likely affecting either the vascular tissue (the wood and trunk) or the root system. If leaf symptoms are dispersed over the surface it is possible that the fungus is infecting the leaf tissue itself. In conifers like pines, different colors of bands on the needles may indicate the same as a spot on a broadleaf tree. Powdery mildew on Japanese maples is one of the more common foliar infections, as are black spots on trees like elms and members of the rose family like crabapple, quince, hawthorn, and others. 

In some cases environmental stress can mimic the symptoms of fungus, creating a challenge for diagnosing the cause correctly. Additionally, stress due to poor water quality can cause similar symptoms. 

Preventing Conditions that Encourage Fungal Problems

Generally, fungal pathogens thrive under specific environmental conditions, and are naturally suppressed in other conditions; the simplest disease control therefore is to naturally limit the conditions that allow it to grow. Typically, higher humidity and a specific spread of temperatures are required for each species of fungus; some thrive in hotter conditions while others thrive in mild conditions. The commonality among nearly all fungal pathogens is that constant moisture encourages more growth. 

Due to this, watering timing and habits and good air circulation can contribute to - or remedy - many fungal diseases. Overwatering soil can encourage root pathogens like Pythium and Fusarium, while watering foliage overhead late in the day can leave moisture on the surface for foliar pathogens like mildew on maples or phomopsis tip blight on junipers. Iif your climate is naturally humid during the growing season, you may not be able to limit foliar pathogens with only the timing of watering. Where this is the case, use of fungicide sprays can help mitigate the problem. 

Three Fungicides

1. Lime Sulfur

Lime sulfur is used in three different ways in bonsai: as a broad-spectrum fungicide, as a preservative fungicide for exposed deadwood, and as a dormancy spray to kill insects. Also known as Calcium polysulfide, this liquid is a chemical combination of lime (like in cement or garden lime) and elemental sulfur - and smells like rotten eggs.  

As a fungicide, lime sulfur can ONLY be used on dormant trees due to the caustic nature of the chemical. While trees are dormant, in the case of deciduous trees there is no tender tissue to damage, and in the case of many conifers, the adaptations that they have to winter temperatures protect them from damage from the lime sulfur. 

Lime sulfur is one of the best ways to treat for needle scale on pines and junipers and scale on other trees, since it kills the overwintering eggs of these insects.

Lime sulfur is applied at different rates for different reasons. If you are applying lime sulfur as a dormant spray in winter, 20:1 dilution with water to lime sulfur (equivalent to about 3/4 cup per gallon of water) is a dilution used by many hobbyists. Note that some species of conifers do not tolerate lime sulfur application, among which are notably many spruce species. Azaleas also do not tolerate lime sulfur. Application of Lime Sulfur without training and/or licensing is not allowed in some locations. 

Application of lime sulfur initially turns foliage yellow-orange, but then the sulfur evaporates, leaving the foliage coated in a white chalky film. This white residue is the calcium and will normally wash off in a few weeks time if you overhead water your plants regularly. On some deciduous tree species it can actually give the appearance of older bark.

Lime Sulfur is restricted for use in some parts of the United States, so check your local regulations and follow all directions on the product label. 

2. Copper

Copper spray, which is commonly sold as a topical fungicide, comes in a few different forms. The active ingredient may be copper sulfate, copper octanoate, or similar copper compounds.  Bordeaux mixture is a combination of copper sulfate and lime. Copper products can be effective treatment for preventing fungal infections from becoming established on bonsai foliage. Copper can be used as a winter dormant spray, but it can also be used during the growing season with little fear of plant damage. Copper in the fungicide interferes with many different aspects of fungal growth and reproduction, so resistance to copper is unlikely to develop. Copper leaves only a minimal visible residue so it can also be a good alternative to Lime Sulfur if you plan to show your trees in winter.  

Liquid copper fungicide is commonly sold "ready to use" in small spray bottles, which require you only to open the nozzle and apply the spray to all the surfaces of the plant. Copper spray concentrates are available; just follow the dilution instructions and spray all surfaces of leaves, twigs, and bark. Repeated application will be needed on new spring growth and as a result of rain.

Copper spray is not normally used as a root treatment.

3. Chlorothalonil

Chlorothalonil is sold as a number of different brands of garden fungal control, and commonly referred to as "Daconil". It provides good preventative control of fungal pathogens, but limited treatment of existing problems. Application as a preventive fungicide is basically a barrier to infection, and should be repeated regularly until conditions favoring fungal growth subside. Daconil leaves a white residue on leaf surfaces and needles. To provide effective control, avoid washing the residue off the leaves, and reapply after significant rainfall, the mode of action is mostly to prevent the establishment of fungal spores on the foliar surface.

Choosing a Fungicide: What the FRAC?!?

FRAC is a classification system for fungicides that groups chemicals by the way that they disrupt fungal growth or the fungal lifecycle. FRAC is an acronym for "Fungicide resistance action committee". Most fungicides that are available to consumers fit into the M1, M2, or M5 classification meaning that they have multiple modes of inhibiting fungal growth and reproduction. Fungii can develop resistance to the suppression that fungicides offer by developing new metabolic pathways that circumvent the action of the fungicide. The FRAC groups are a preventive measure intended to provide information that helps growers avoid resistance problems. 

In many cases a "fungicide" is actually just a fungal suppressant, limiting the growth of the fungus, but not actually killing it. Two primary strategies for limiting fungal growth are creating a barrier that prevents spores from establishing on a surface, or inhibiting the growth of hyphae.

So Which Fungicide Should I Use to Treat My Bonsai?

If you have a particular fungal problem, try to identify the culprit using symptoms. If you can identify the likely pathogen, then you are more likely to be able to control it. Each fungicide will list on the label whether or not it is rated for control of a particular disease, and how to apply it for control of the listed fungal pathogen. After following the directions to apply the fungicide, take careful note of the change in symptoms over a week or two, and then follow up with another assessment.

To control resistance to treatments, and to give yourself a backup, using two different fungicides typically provides best results. 

All three fungicides listed above are widely available and approved for residential use.  However, note that they are all only topical - none of them are curative fungicides or systemic fungicide. To control diseases you may want to add one or more systemic fungicides to your arsenal. Recommendations for these change annually and are based on observed resistance. In addition, many require licensing, although they are widely available on sites like DoMyOwn.com. For 2023, we are using this IMF guide as a starting recommendation.