Pests and Diseases – Part One

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I have been doing bonsai for some twenty-five years plus now. In that time, I have experienced and dealt with many problems concerning pests and diseases. Some of these come and go. Others seem perpetual. I notice this in the landscape, too. One year I am fighting a certain weed, and the next year, something totally different. Many insects and diseases wax and wane in severity with passing time due to many factors, some being their natural cycles, nearby host trees, what your neighbors are doing, etc… We don’t live in a closed environment, so we are subject to whatever the winds might blow in from next door. So it should be no surprise that we have new and different problems to deal with each season.

Today we are starting a multi-part series on Pest and Diseases – mostly how to treat them. I am not going to be able to cover all of the pests and diseases that your bonsai might encounter.  However, I do hope to cover quite a few and arm you with tools necessary to diagnose your problems and how to deal with them. This will not be a scholarly approach, but one based on experience, so be sure to double check what you can with your own library, web sources or expertise. This information is hard won, through triumphs and failures.  

I have collected information from club members, research, and through the many efforts of my teachers like Ryan Neil, Boon Manikativipart, and Mike Hagedorn. Each of these men maintain larger collections, both for themselves and others, and are able to experiment and adapt on a much larger scale than myself. So I am trying to do my part to make you successful by sharing what we have all gleaned out. In all fairness, I have to tell you that when it comes to bonsai, I am pretty much willing do what it takes to protect them.  

When I have the opportunity to use a more natural method, other than chemicals, I will do so and I hope to share a few that work very well. Otherwise, I tend to be on the side of nuking the suckers. If I have an old, beat up Pinto, I may not have insured more than the minimum, nor polished and waxed. But if I just acquired a brand new Mercedes, you can bet I am going to do whatever it takes to keep it in tip top shape. Conversely, we have to guard the health of our plants, and that also means using the least amount of chemicals possible. It can’t be good in any sense to just keep spraying away. It is also important to remember that once we eliminate the source of the problem, it will not fix the foliage immediately. Once it’s damaged, there is pretty much no repairing it. We have to wait and grow new foliage. For deciduous trees, that may mean this year, but likely next. But for conifers, that means that we are a good two years out, which can really affect your plans for showing a tree.  

By far the largest issue that I have had to deal with in the past few years is that of fungi, or diseases. We have had some perfect weather the past few springs for different fungi to really take hold and spread. That usually means wet weather with mid to warm temperatures in March, April, and May. Sounds like Portland, doesn’t it. I’ve noticed that there are two general classes of diseases – those that work on surfaces of foliage, and those that work on the interior,  or vascular system of the tree. Think about this when choosing a treatment. You can’t treat cancer with a band-aid, nor a cut with chemotherapy. Bingo, the light just went on for me.

How do you tell the difference? First, look and see if the damage you observe is in the foliage or in the trunk and/or branches. If there are chunks of leaves missing, you are probably dealing with an insect. Otherwise, we are dealing with either discoloration in the leaves, or desiccated and withered foliage. If the edges of leaves are turning brown, it might simply be a sign of sun burn. That means you need to alter your watering regimen, or move the tree to a shadier location. But if the new shoots from this year suddenly start to wither as the temperature warms, where they were fine before, you might be into some other systemic disease in which the vascular system and thus, moisture flow is interrupted. That makes the symptoms appear like leaf burn, but are vastly more devious. This means that you will need to treat with something that works systemically rather than the surface. These problems can be caused by Pythium, Phytophora, Verticillium, etc… Remember, conifers can burn too, but it is much less frequent. Do you see yellow, black, or red spots on the leaves? That’s likely a sign of rusts, or Anthracnose. These diseases are fairly easy to deal with via sprays, and many times the generic, broad spectrum garden sprays will work. What about powdery mildew? Same fix.

If you see cankers or lesions on the trunk or branches we are dealing with something much more insidious. Any kind of growth not natural to the tree is pointing to a systemic problem.
This usually involves the soil and/or root system and these are much more difficult to deal with. We will cover this more in depth in our next installment.

If you have colored bands on your conifer needles, you have a needle blight. Easy solution, Daconil. If you have tuffs of juniper foliage dying off, that is a bit harder to handle. It could be a few different diseases, including Phomopsis and Kabatina. We’ll cover those next time too.

So back to the insects. Take a look at your foliage and notice what is happening. Are there chunks missing out of the leaves? This is more likely to be insect damage than a disease. Most diseases do not remove tissue, but rather they discolor, desiccate, or otherwise alter the foliage, but do not remove it. Other causes for missing foliage might be hail stones, bird pecks, or other mechanical damage. Does the foliage look off color – sort of grayish, or have light colored stipling all over? This may be a sign of spider mites or lace bugs. To test for spider mites, hold your hand or a piece of paper under the foliage and give it a couple of taps. Small bits will drop onto the paper, which may be just about anything. But if you run your finger over them and they smear, it’s likely the spider mites. They can also make fine webs in the foliage.  

Are there chunks missing out of the side of the leaves? Shot hole bees make a perfectly round hole. Notched sides may indicate Weevils. I used beneficial nematodes for a couple of seasons and have not seen the weevils back for over ten years. Caterpillars are easy to take care of via BT, or Bacillus thuringiensis, a naturally occurring bacteria.  

Do you notice black mold on your trees, or sticky honeydew on the leaves? This is more than likely caused by some form of aphid. Usually green, but often black, sometimes wooly, covered by cottony tuffs. How about scales – these can also be wooly, or even armored – oval bumps along the trunks or branches. Yes, there are insects under those bumps. Use your fingernail to scrape them back and you will see their dark bodies. Most aphids are easy to remove – a blast of water will do, but they will be back. Sometimes with the help of ants that carry them into the trees to farm them for their honeydew. There are also insecticidal soaps that work well.  

Scales are different. That armored shell or tufty crust makes it almost impossible to penetrate with any kind of chemical, no matter the strength. For the last ten years or so, I have been using a chemical called Imidacloprid, which I have written about before. It is the most widely used pesticide in the world, at this point. Since I started using it I haven’t seen a single scale or aphid of any type. You can use it in liquid or granular form, under the name Merit or in product by Bayer, or others.  

Unfortunately, it is also part of a class of chemicals called Neonicotinoids. You have probably heard about them. Misuse of these chemicals, like a relative called Safari, have been responsible for bee population declines. The problem is using them on flowering trees, as this is what the bees feed from (All trees flower, but not all produce a pollen that the bees are interested in). You will have to make your own decisions about this and maybe do some more research if it is warranted. Use any chemical with caution and care. Protect yourself as well as the environment. Wear goggles or glasses when spraying and rubber gloves. Long sleeves are good – then wash the garment afterwards. Avoid spraying on windy days- which is just frustrating and wasteful anyway. Do not spray on days above 85 degrees as you may do more harm than good. And what cannot be emphasized enough, is READ THE LABEL. All of it that pertains.

When I first started bonsai, I wanted to make care and maintenance easier, so invested in all kinds of spray bottles, which I filled with mixed concentrations of each chemical I needed. The kind of sprayer that you squeeze the trigger to pump out the contents. They work great for occasional sprays and you can keep the chemical at the ready. The only problem being, as your collection grows, so must your arm strength. Spraying the entire collection in this manner requires mixing many refills and really gives your forearms a workout.  

What I have gone to now are the half-gallon sprayers that you hand pump prior to spraying. You can mix a larger quantity and it’s much easier on the arms. The units sell for between $510, and are available at most hardware or garden centers. I really prefer the ones with a metal nozzle and especially a metal pump shaft. I also label these like before, with each chemical, but I also mark the measured amount needed for that sprayer/chemical combination. When I need to spray the entire collection, I use a two-gallon tank sprayer. I use masking tape to make a quick label of what I have mixed inside if I don’t use it all in the first shot. I also have a second sprayer I use for weed killer so that I can never mix the two together. Be aware. Invest your money in the best sprayer that you can afford. If you buy the $20 model, you will be replacing it by years end, so better to spend the $30 the first time.  

I have never found the need for a full size backpack sprayer, but would certainly enjoy one if I had it. Now, the best tip of the whole article. Be sure to let out the pressure on all of these sprayers before returning them to the shelf. Many units have a valve for just this purpose. Doing so will save cleaning up a big chemical mess when the unit falls over or something bumps into the trigger and it sprays until all the pressure is gone. How do I know?.... I can’t really say happy spraying, but I do hope that your growing season is prosperous and I have been able to help a little.  

Next time: More in depth on fungi and how to treat them.


An array of Insecticides and Fungicides

An array of Insecticides and Fungicides

Keeping the chemicals in a leak-proof container

Keeping the chemicals in a leak-proof container

Chemicals are labeled with mixing instructions

Chemicals are labeled with mixing instructions

An assortment of pump sprayers

An assortment of pump sprayers