Gardening with Allen: What’s up with discolored leaves?
By Allen Wilson
Published: June 11, 2019, 6:03am
Share:
Spider mite damage on the underside of tomato leaves. Allen Wilson
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, including mugho pine. 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 eight 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.
Snails and slugs feed on leaves at night, especially tender new growth, and hide in the daytime. Snail and slug bait applied in the evening is best for their control.
Sometimes chewing beetles eat holes in leaves and have already moved on by the time you notice the damage. There is no need to spray if the pests are no longer present.
If you notice ladybugs, they are often feeding on aphids and other soft-bodied insects. In most cases, the ladybugs 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 that 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 ladybugs 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.
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 and black spot. Spinosad is a recent organic pesticide that is usually only available in full-service nurseries and garden stores or online. 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 plant’s circulation system and translocated to other parts of the plant. It is therefore effective on insects such as borers and leaf miners, which feed on the inside.
