Chapter 46 Monitoring For Diseases, Pests, & Other Problems

When a bonsai tree shows signs of distress, we tend to try everything we can, as fast as we can, to help it rebound. Yet improper or over-treatment can stress a tree even more.

The only true emergencies that must be corrected immediately are overheating in full sun and lack of water. Other problems are less urgent, and often the problem that catches our attention masks other issues. Ultimately trees are better off when we slow down and analyze the problem fully so we can respond more appropriately.

46.1 Learn to Approach Tree Problems Like a Doctor

46.1.1 Be Systematic…

Physicians and veterinarians are trained to follow a standard exam plan that helps them identify the root cause(s) of big problems and ALSO uncover any clues that hint at other problems. They check each item in turn, and if there is no sign of a deeper issue, they treat the big obvious symptom or problem. If something is “not quite right” they look deeper or make a note to watch for changes over time. The added benefit to routine standard exams is you get a much better idea what “normal” looks like so you can spot subtle changes sooner.

46.1.2 …But Don’t Go Overboard!

Academic physicians have a saying: “when you see hoof prints, look for a horse, not a zebra.” It’s meant to remind their students not to over-complicate a diagnosis or treatment plan.

When diagnosing a tree problem, eliminate the most likely offenders before looking for something more unusual. Most of us will see a few common problems many times over. Nearly all of the diseases we encounter will be the same ones that affect landscape trees and shrubs in our area. Tropicals and exotic species (ficus or orange jessamine for example) will tend to have a couple problems or pests to which they are particularly susceptible. Learn what is common in the neighborhood or most frequently seen in the exotics you grow. These are your “horses.”

In my experience Piedmont NC tends to have an early spring and very humid summers. Excessively rainy periods intermingle with long semi-droughts. These are ideal conditions for these fungal and bacterial diseases that reappear year after year:

Powdery mildew
Cedar and other rusts
Leaf spot
Fire blight
Sooty molds (secondary to aphids or scale)
Several needle blights that affect pines and conifers

We also have certain Arthropod pests that show up every year:

Aphids (can attack almost anything)
Spider and other leaf mites (particular problem for junipers and conifers)
Army worms (will eat most foliage)
Oak worms (defoliate oaks, hornbeams)
Pine sawfly caterpillars (they defoliate pines VERY fast)
Hard scale (some tree species are more susceptible, but nearly any tree can become infested)
Mealybugs (more common on softer leafed deciduous trees and fleshy leaved tropicals)

Always check for and rule out these common diseases and pests before looking for something more exotic.

Tip: USDA publishes comprehensive lists of the diseases to which native and ornamental trees are susceptible. I DON’T recommend reading them. First, they make you think every tree and shrub is one bad day away from death, which is not true. Second, they make “zebras” look more common than they actually are; many of the diseases are restricted to particular areas of the country. Third, susceptibility lists are not meant for consumers; they are designed to inform horticultural professionals about POSSIBLE diseases affecting economically important species. These professionals may be cultivating a million seedlings for re-forestation projects. Others may be wholesale nursery growers who are devising a disease monitoring and management program for specific crops. The local and state Cooperative Extension Offices have better guides to what you are likely to encounter. They also have information about new pests that hav appeared in our area.

46.2 A “Physical Exam for Trees”

If you come across a tree that looks like it is not healthy, use this technique to diagnose the most common problems. Any order you prefer works; what is most important is to check a tree systematically. You will need a couple simple tools.

An unsharpened pencil or blunt dowel to serve as a general probe, and help push aside branches and foliage.
A metal awl with a blunted point, or a small narrow flat-bladed screwdriver. This will help you probe for soft spots in wood.
A small jar of 70% alcohol for sterilizing tools or killing insects and their eggs.
Some small pests are easier to see if you have a magnifier or hand lens. I like an 8X or 10X folding hand lens, but anything 5X or higher magnification should be fine.

Some steps have links out to additional information in the follow-up notes, or connect to related pages.

46.2.1 Pot, Soil, Roots

This part of the exam is longer because this is where many tree problems start. Also, we cannot see many soil and root problems directly so we must rely on indirect evidence.

Look at the soil surface. Does it have a crust or patches of off-white, gray, or green salt-like crystals? These salt crusts should disappear as soon as you water. If they return as soon as the soil dries, there is enough salt in the soil to damage the tree.
Water the tree normally, and wait 10 minutes for the pot to drain. If water stands on the top for more than a minute and does not penetrate, that suggests either the roots are too tightly packed, the soil particles are too small, or that the soil is not wetting evenly.
Alternatively if nearly all the water applied drains out of the pot almost immediately, there may be channels in the soil that let most of the water pass right through to the drain holes. This also could mean the soil is not wetting evenly.
If water soaked in, lift the pot so you can see underneath. Are the drain holes open? Are roots coming out of the drainage or wire holes? Did water drain out through all of the holes?
If the drain holes are wide enough, put your finger in and feel for moist soil. Dry drainage holes suggests the soil is not wetting evenly, or you are not providing enough water. The rule of thumb is, 1/5th to 1/4th of the pot’s total volume should run out of the bottom of the pot each time it is watered.
Sniff the drain holes. You should smell wet soil, but it should not smell rotten or foul.
Are there pill bugs or ants in the drain holes or under the pot?
Set the pot and tree on a table. Get close to the surface of the soil and give it a good sniff in several places. Again, it should not smell rotten or foul.
With the pot still on the bench, GENTLY rock the tree trunk back and forward, then left and right. Does the tree shift in the soil, or does the entire root ball move as a single mass? The latter suggests the tree may need repotting.
If the root ball lifts out intact, look underneath. Do you see small feeder roots, or mostly roots larger than the lead of a pencil? Is the root ball fully moistened, or do you see dry spots?
If you cannot lift the root ball out, push a dowel or unsharpened pencil into the soil about half way between the root mass and the container wall, all the way to the bottom of the pot. Wiggle it a bit, then remove. Did it move through the soil easily, or did you feel resistance or roots breaking? Sniff the pencil or dowel; again a foul odor suggests the presence of root rot.
Push down on the surface of the soil with two fingers together in several spots. Do any areas compress or sink very easily? If so, there may be a gap or space between roots, or spot where soil has washed out.
Look at the periphery of the pot or root ball. Do the roots circle around the edge of the container near the surface? This indicates that the tree needs repotting.

46.2.2 Surface Root Flare, Trunk, Main Branches

Use the dowel or pencil to gently poke into the spaces between the surface roots. Do any of the roots have soft spots? Is the bark on the roots spongy and wet, or does it shift or peel away from the rest of the root easily? Peeling bark on roots suggests the roots are staying wet for too long, or the soil is not draining well. Left long enough, these weak spots allow in pathogens. Water less often or repot.
Look at the roots and trunk. Also check all stumps and stubs of old branches. Are there any places where there is bare wood showing? They can be long splits in the bark, rounded dents where branches were removed, or open unsealed ends of pruned branches. Use the blunt awl or screwdriver to test the wood. If it is soft and easily scooped out, the tree needs cleanup and wood preservation. If the wood is sound but not protected by bark, consider applying wood preservative.
Does the bark look significantly different on one side versus the other of the tree? It may be thinner, a different color, or less textured on one side.

Small areas simply may be a difference in age or exposure, but large areas or changes in the bark that run down one entire side can mean there is a fungal or bacterial infection under the bark.

If you suspect this, scrape off a SMALL spot of discolored bark with a finger nail to show the cambium. It should be bright green like young leaves or fresh leaf lettuce. Discolored (dull gray-green, brown, or nearly black) cambium indicates a bacterial or fungal infection is moving through the vessels.

Are any branches coming off the trunk dead or seem to be failing? Are there significantly fewer leaves on one main branch than the others? Again, there are many reasons why some branches may have less foliage, but poor leaf-out combined with discolored bark (the previous step) suggests there is an active infection blocking water and nutrient flow through the cambium.
Look for cracks or pinholes that are weeping liquid or gummy material. These are signs of a canker or bacterial infection.
Look for small mushrooms or bracket fungi growing directly out of the trunk or branches. These fruiting bodies sprout from a network of mycelium growing beneath the bark. Usually they grow on dead wood, but there are a few species of mushrooms that start in deadwood then spread into living areas, killing them.

Be careful not to confuse mushrooms ON a tree with mushrooms emerging from the soil NEAR a tree. Mycelium in the soil is not feeding on the tree, but on the decaying bark and nutrients from fertilizers. Soil mushrooms are harmless; just break them off at soil level.

Look for small fuzzy lumps or masses stuck on the trunk, in cracks of the bark, or on the undersides of branches. These are egg cases of insects. Do not automatically destroy egg cases; some may be the eggs of beneficial insects. Take a few minutes to learn what beneficial insects’ egg cases look like, vs. the eggs of pests.

Scrape pest egg cases off with the screwdriver, and destroy them by soaking them in 70% alcohol.

46.2.3 Smaller Branches, Foliage

Use a hand lens to scan the branches and small twigs. Look for smooth round shield-like bumps (armored scale), white fluffy masses (cushiony scale, mealybugs, adelgids), or tiny red crawling dots (spider or red mites).
Check the twigs for a dusty black coating (sooty mold). If you are unsure whether the twigs are dark naturally, look at the undersides of several twigs. Mold rarely grows on the down-facing side of twigs.
Check the leaves or needles for signs of chewing insects. One or two damaged leaves or needles is normal, but several damaged needles or leaves (especially if they are near each other) suggests unwanted pests.
For conifers, use a pencil to push apart the needles near several branch tips. Use a hand lens to inspect the crevices around the bases of several needles. Look for aphids, scale, mites, and other insect larvae feeding or hiding in the needles. Look for tiny pale whitish spots on the scaly or needle foliage; these bleached spots are caused by mites feeding.
For deciduous trees, use a hand lens to look for aphids or scale insects feeding on the leaf stems, or congregating along the midrib and leaf veins. Look too for tiny pale or white spots on the leaves. These spots suggest mites are feeding on the leaves.
Turn over several leaves and check the undersides. Whiteflies, aphids, and many other pests will gather on the unexposed sides of leaves to avoid predators or being washed away by rain.
Check leaves on all sides of the tree for signs of leaf scorching, or leaf spot diseases. For leaf scorch, provide more shade. One or two leaves with leaf spot can be normal, or they can be a warning sign of a larger infection. Start watching these trees closely.
Look for the carcasses of dead aphids, scale, or other pests. These can indicate you have a healthy population of beneficial predators.
If you find an unfamiliar insect crawling on the foliage, try to identify whether is is a pest or a beneficial species.

46.3 Where to Learn More

NC State Plant Pathology Lab

Submit samples to get professional evaluation. They also publish guides that focus on individual pests and problems.

NC State Handbook of Plant Disorders

This is a chapter from NC State’s Gardeners’ Handbook, which is a great general resource for topics beyond just diseases.