Growing bonsai trees involves working with a variety of exotic species in sub-optimal growing conditions (the bonsai pot). This combination of often mal-adapted tree varieties and stress from the small growing container can lead to susceptibility to pests and pathogens. Plant pathology can be broken down into two categories, abiotic (non-living) and biotic (living) factors. It is much easier to blame a biological agent (biotic) for our plant ailments, but the true cause is usually abiotic. Some common abiotic factors are improper fertilization, heat stress, water stress, excessive pruning, repotting too often, wind stress etc.…. A healthy tree is one of the best preventative medicines you can provide against biotic pathogens. Biotic pathogens take many forms and often prey on compromised trees. Some of the most common biotic pathogens are bacteria, fungus, viruses, nematodes, and insect pests.

Bacterial pathogens can show themselves in different forms like leaf spots, galls, unexplainable wilt, and mushy rot. One of the most common way of spreading bacterial disease is from contaminated tools. To avoid bacterial contamination, it recommended to sterilize your tools before pruning each tree with “70% isopropyl alcohol and it can be used directly from the container” (Division of Plant Sciences University of Missouri, 2018). Below I provided a photo of leaf spot which is a common bacterial symptom.

Below is another photo of crown gall caused by an agrobacterium.

Fungal pathogens are the unholy grail of plant diseases with their tenacious survivability and adaptability to different climates and hosts. There are 5 main types of funguses called the zygomycetes, ascomycetes, basidiomycetes, mitosporic fungi, and oomycetes. 

Symptoms of fungal infection can manifest in countless forms like leaf spot, damping off, root rot, warts, galls, leaf curl, canker, anthracnose, etc.… One pathogen of particular interest is called phytophthora. Phytophthora is broad class of water molds under the phylum oomycetes that causes root rot for many different species of plants. Overwatering is a common theme for many bonsai practitioners, and this can create a breeding ground for water molds like phytophthora. Symptoms may look like yellowing or browning of leaves and eventual leaf drop. When a tree has a phytophthora infection the roots will look black and rotten. In order to avoid this, you must make sure your tree’s grow medium has adequate drainage for proper air flow. Below is an example of phytophthora root rot.

Viral pathogens will not usually kill your bonsai but can stunt and decrease vigor tremendously. Once a plant is infected with a viral pathogen there is no way to cure it. Prevention is the best defense so cleaning tools and checking bonsai or nursery stock for viral symptoms prior to purchase is a wise precautionary measure. Plant virus symptoms look like yellow streaks, mosaic patterns and deformities or distortions on the leaves. A viral pathogen I have personally encountered is called the “mosaic virus”. I purchased an infected Ficus carica tree before I knew about plant virus and unfortunately later had to dispose of the tree and take the loss. Below is a photo of the mosaic virus symptom on a Ficus carica. 

Parasitic Nematodes are small worm like creatures that feed off plant root tissues. There are several types of nematodes, but they generally make gall like structures on your trees roots. A common nematode I have seen is the root knot nematode. I personally have found some nurseries who are selling stock with infected trees. Many nurseries grow their trees on the ground and if the ground is infected with nematodes that is almost a sure way to transfer the disease to your trees. Unfortunately, nematodes are very difficult to treat once infection has occurred. I personally dispose of the tree immediately because I do not want to accidently spread nematodes to my other trees. Once again prevention is the best method of control. Proper prevention includes cleaning tools, never reusing old soil, and inspecting the roots of new trees before adding them to your collection. Not all species are susceptible to nematodes, but many trees with tender starchy roots are vulnerable. Therefore, you should buy trees from trusted sources only that are not negligent with their nursery stock because a nematode infestation can be a disaster. Below is a photograph of a root knot nematode infection.

Insect pests on a healthy tree is usually not a problem, but a compromised bonsai tree is a prime target. Some common pests include aphids, beetles, leafhoppers, mealybugs, thrips, and whiteflies. Insect pests can cause tissue and foliar damage to your tree. Insects can also act as vectors for nematodes, fungus, bacteria, and viruses. To avoid this dilemma, treat insect infestations during early stages. Most insects will become prevalent after the last frost when temperatures start to warm up. This is a good time to be on high alert. Below is a photo of the beginnings of an aphid infestation.    

References

CRC Press. (2017). Plant pathology: concepts and laboratory exercises (3rd ed.).

Division of Plant Sciences, plantsciweb@missouri.edu. (n.d.). Cleaning and Disinfecting Pruning Tools for Orchard Crops (Michele Warmund). Cleaning and Disinfecting Pruning Tools for Orchard Crops // Missouri Environment and Garden News Article // Integrated Pest Management, University of Missouri. https://ipm.missouri.edu/MEG/2018/1/cleaning_pruning_tools/.