What is that icky thing on my tree?

Bugs, chewed leaves, dead twigs and more--common plant pests and diseases and what to do about them

Plants get pests and diseases. Period. Many of these we may not even notice and aren't very damaging to the plant. A few are more serious and may require some time and attention to control. And some of the most serious ones we cannot control.

The first step is to understand what the problem is and to get expert help. Luckily, the University of California's excellent Integrated Pest Management website can help you find out what is affecting your plant and what to do about it.

Common plant pests

Aphids IMG_6899

Aphids are small, brown or black insects that suck up carb-rich plant sap, especially in the spring. They often excrete small drops of sap called honeydew. While this honeydew is water soluble, it creates a sticky mess. It can attract fungus, which results in black sooty mold growing on leaves. Treatment may be as simple as using a strong blast of water to blow them off the plant. If that is ineffective you can escalate your treatments. Use the UCIPM website for more information

Fertilizer can make this problem worse as it encourages weak, lush growth, so avoid fertilizing in the spring. And don’t fertilize Crape Myrtles; they will still grow and bloom beautifully without it.

Scale is a tiny insect that is usually seen on leaves or stems as tiny to small bumps . Scale infestations can be hard to notice until you look specifically for them. Most of the times, insecticides are ineffective because the critter is under the “scale” and is not in contact with the pesticide.


Leaf miners tunnel in between layers of the leaf causing minor damage. It can look dramatic, but is rarely serious so doesn’t generally need treatment.

Leaf eaters like these red-humped caterpillars eat leaves. They can munch just a little or defoliate an entire tree. Trees in good health will re-sprout leaves, but that takes energy away from growing.

These caterpillars feed close together so it is easy to remove the branch they are on and put it in your green waste can. Pest problem solved!

Some caterpillar infestations are more serious and may require pesticide. But use as little as possible--caterpillars hatch out into butterflies and moths

Redhumped Caterpillar on Western Redbud branch

Bark and boring beetles bore into tree trunks and can damage or kill a tree by disrupting the flow of water/sap. Climate, tree species, and beetle populations dictate how fast or slowly a beetle can reproduce and fungus can spread. You cannot spray to kill these pests as they are safe under the bark for most of their life. If you notice holes in the bark and the tree struggling, call in an expert (arborist or qualified applicator) to examine the tree and advise you.

Common plant diseases

Most tree diseases cannot be cured; but just like people who have colds or arthritis, the tree can survive. Keeping the tree healthy and vigorous gives your tree the best opportunity to fight off the disease. This means managing water, using mulch, and pest treatments if necessary.

Powdery mildew is a fungal disease that looks like someone spilled flour on the plant. It is common on roses, Crape Myrtle trees, and other landscape plants. Shade and moisture in the air promote powdery mildew. So be sure to plant your sun-loving plants in full sun and thin out dense growth in trees and shrubs, as well as dense plantings. Sometimes treatments are needed. Choose mildew resistant varieties when adding plants to your landscape


Verticillium wilt is first noticed when a branch of a plant seems to suddenly die. Maples are especially susceptible to this disease, but other trees and shrubs get it too. Plants with verticillium may decline or sometimes die, but some are much less affected. Depending on where the branch is, you may be able to cut out the diseased branches. Consult resources for more information on how to treat this disease

Fireblight is a common disease that affects members of the rose family that make apple-like fruits. Photinia, Pyracantha, flowering pear, apples, and pears are all susceptible. It is caused by bacteria and is moved around by insects during flowering. It appears in the late spring or summer as suddenly dead branches that look like they have been burned--rusty colored or blackened. This is another disease that can range from mild to severe. Cutting out the affected branches with a margin of healthy wood can control it. Look up how to sanitize pruning equipment to keep from spreading it to a healthy plant. When choosing new plants for your landscape, choose resistant varieties or non-apple-fruit species

Fireblight P1060475