Saturday, July 23, 2016 By: Ask A Master Gardener

Spider Mite Control on Tomatoes

Spider mites are difficult to control, problem in summer

Brian Jervis: Ask a Master Gardener

Saturday, July 23, 2016

Q: The leaves on my tomatoes are yellowing and dying. What should I do? R.R., Owasso
A: Far and away the most common cause of yellowing of tomato leaves this time of year is a spider mite infestation. Most Oklahoma gardeners will have some mites on tomatoes in the heat of the summer.
Adult mites are about 1/50th of an inch, and one needs a hand lens to see them. If you tap a leaf over a piece of white paper, sometimes they can be seen as tiny moving dots. With dense populations, some webbing of leaves may be seen as well.
There are many varieties of mites; tomatoes usually have the two-spotted mite. They are greenish and thrive when it is hot, dry and dusty. They can have a new generation every 5-6 days, and it doesn’t take long for a population to explode involving the whole plant.
Mites feed on a wide variety of vegetables, ornamentals, trees and houseplants. They feed on the underside of leaves where they suck out sap, producing yellow dots that develop into a bronzing of the leaves as if sandblasted. Eventually, leaves turn totally yellow, curl up and die.
Unfortunately, the diagnosis is the easy part. Control is difficult. Spider mites are arachnids, in the same family as spiders and ticks. Because they are arachnids and not insects, they do not respond well to standard insecticides.
A sharp stream of water will remove a significant number of mites, but beyond that, organic pesticides are the most effective. Man-made, nonorganic pesticides are not recommended because they kill the good insects helping to keep the mites under control.
Both of the organic pesticides — horticultural soaps and horticultural oils — are only effective if sprayed on mites directly. They are also the safest for your children, pets and the good insects. Because mites are commonly on the backside of leaves, they must be applied to both sides of leaves. Both preparations will need to be applied regularly, according to the labeled directions.
Horticultural oils come in two varieties — petroleum based, such as Sunspray, and others extracted from plant material. One of the best is neem oil, which is made from the seeds of the neem tree.
If you cannot control the mites with water, soap and oil, OSU suggests a trial of a short acting organic insecticide — pyrethrin — as long as you accept that it could harm some beneficial insects. It is available in several brands, some mixed with soaps and oils.
So the best strategy for homeowners is to monitor plants closely and to use what tools are available as early as possible. These tools may offer some control but are not likely to completely eliminate mites.
These measures may be unsuccessful. If spider mites cannot be controlled, it is best to pull up the plant and all its parts and send it to the landfill. You can then start with new plantings, looking toward a fall vegetable crop.

For more information or to ask a question about gardening, contact the Master Gardeners at 918-746-3701 from 9 a.m.-4 p.m.

Garden tips
• Make fall vegetable garden plantings in late July. Fact Sheet HLA-6009, “Fall Gardening,” gives planting recommendations for each vegetable as to when to plant and times to harvest. It also discusses strategies for dealing with the heat when planting a fall vegetable garden.
• Fescue lawns need 2 inches of water per week, Bermuda and Zoysia need 1 inch. Fescue usually will not survive the average summer without supplemental watering.
• Water plants deeply and early in the morning. Most plants need approximately 1-2½ inches of water per week.


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