By Calvin Finch, Conservation Director, San Antonio Water System, and Horticulturist

SPIDER MITE, POWDERY MILDEW AND GRASSHOPPERS

Last weekend there were several calls on our KLUP radio show concerning spider mites on tomatoes. Spider mites are sucking insects that thrive in the heat and are especially fond of tomatoes.

Kelthane is the most effective miticide. You can find it in nurseries as red spider spray. The question is, “Is it worthwhile to spray the pests now when the tomatoes are so close to harvest?” If the damage is evident in the form of a blow torched-look to the plant and webs on the leaves, it is probably too late to spray. Early in an infestation of spider mites the symptoms will be a dusty look or yellow speckling. Verify spider mites by shaking or flicking a leaf over white paper. If you see pin-point-size little red organisms moving about, you have spider mites.

Kelthane is an effective miticide, but you may want to try some alternative strategies if your plants are less than a month from harvest. Spraying insecticidal soap or even plain water under the leaves every two or three days will physically remove a large number of the mites from the plant. Generation time is very short for spider mites when the weather is hot, so the spraying has to be done frequently. Tomatoes should be pulled from our vegetable gardens in San Antonio at the end of June, so the water spray is the best strategy for most of us. Hill Country folks and points north may want to use kelthane.

I am an advocate of composting plant material but when you finally pull a mite infested plant, it is best to put it in a garbage sack and let it go to the landfill. Spider mites are persistent creatures that can move from one plant to another. Relatives of this June population may end up on your fall tomatoes.

Powdery mildew is another problem appearing on many plants in our area. This fungus disease shows symptoms that resemble gray powder sugar on many plants, most noticeably crepe myrtles. The mildew invades new growth enough to cause leaf deformity and undersized leaves. If your crepe myrtle is a healthy, established specimen, the fungus will not cause any permanent damage. As soon as the temperatures increase and humidity drops the mildew will decline. There are some fungicides labeled for powdery mildew, but they are only useful to stop the spread of the disease on small or newly planted specimens. Powdery mildew will retard bloom start, but will not permanently affect the crepe myrtle.

The Indian named hybrid crepe myrtles such as Tuscarora, Catawba, Natchez, Sioux, Tonto, Acoma, and Hopi should be resisting powdery mildew.

One plant that does not survive powdery mildew is squash. If the disease appears on your plants expect blooms to drop and production to end. Summer squash does not like hot temperatures either. As temperatures increase, production drops off. If the mildew is not evident but the blooms are aborting, you can also check for squash bugs and/or vine borers. All of these problems are terminal, so be ready to remove the plants as soon as the last squash is harvested.

Grasshoppers are everywhere. It is a surprising phenomenon because populations of the pest usually do not build up when the weather is moist. Whatever the reason, large hungry specimens are munching away on leaves of every description. Many insecticides will kill grasshoppers if they are sprayed directly (malathion, carbaryl, diazinon, orthene), but the pests

are large enough that residue on plants won’t do the job. Protect small tender plants by frequent sprays (follow label instructions) and tolerate the damage on healthy established plants. A live oak stripped of foliage will re-leaf without any permanent damage. This may be one time when grackles might be welcome in your yard. They will eat grasshoppers.

Nolo Bait is a bacterial derivative product used to kill small grasshoppers. The folks at Garden-Ville say a mixture of two tablespoons of diatomaceous earth and two ounces of molasses diluted in one gallon of water and sprayed on the foliage will kill the adults. If you try it, let me know if it works.