I have discolored leaves on several plants and the leaves are starting to drop. I don’t see any insects. Is there a disease affecting them?

The symptoms you describe sound a lot like mite damage. Mites feed on a wide range of plants from flowers and vegetables to shrubs and trees. They are bad on needle evergreens, especially arborvitae. They feed on the bottoms of the leaves and are so tiny that they are difficult to see without using a hand lens. Shake the affected leaves over a sheet of white paper. If present you will see them running across the paper. With a hand lens you can see that they have 8 legs like spiders. That is why they are often called spider mites.

Insects and mites multiply rapidly in hot weather and can do a lot of damage in a short time. It pays to look for damaged, discolored or curled leaves. Always check the bottoms of the leaves because that is where bugs usually feed.

Sometimes chewing insects have already moved on and only damaged leaves are left behind. There is no need to spray if the pests are no longer present.

If you notice lady bugs, they are often feeding on aphids and other soft bodied insects. In most cases the lady bugs will take care of the problem.

A few years ago, I noticed aphids feeding on tree leaves. The leaves were sticky. Ants were feeding on the sticky exudate which they produce. I was in a hurry and made a mental note to spray the tree. Two days later when I had time to spray I found the aphids and ants were almost all gone. I noticed a few lady bugs feeding on the few that were left.

Since most pesticides do not kill the insect or mite eggs, it is important to make at least two applications spaced a week or two apart. Even if you kill all the adult pests, the eggs will often reinfect the plant. This is especially true for mites.

If you are not sure whether you have a pest problem, take a sample to a full service nursery or garden store or to the master gardener clinic at the agricultural extension office. If you live in Vancouver or Clark county, I am able to check on plant problems without any charge.

My two favorite insecticides are organics which are very safe to use. Neem oil is derived from the Neem tree and is effective on a wide range of pests and diseases such a powdery mildew. Spinosad is a recent organic pesticide which is usually only available in full service nurseries and garden stores or on line. You often have to check the label to find it listed under active ingredients. Spinosad is somewhat systemic in its action. It is taken into the plants circulation system and translocated to other parts of the plant. It is therefore effective on insects like borers and leaf miners which feed on the inside.


Allen Wilson

Allen has been writing about gardening for over 30 years. He is a retired professor of Horticulture.

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