Heavy dust coating will increase stress on plants

Everything around us is dusty during this drought. And we can’t use scarce water to clean off the dust.

When we sweep the front porch and the patio, however gently, we raise clouds of dust particles that drift onto neighboring plants. Leaf blowers send even more dust particles high into the trees.

Rainless, dusty summers are the norm in our area, but dust conditions this year are severe. Plants breathe through tiny pores (stoma) in their leaves. When the pores are blocked by dust particles, the leaves are unable to take in carbon dioxide and pollutants from the air and then release oxygen and water back into the atmosphere. Photosynthesis is reduced, lessening the amount of food available as the tree and leaf temperature rises. Our trees are already drought-stressed; the heavy dust coating will increase stress levels.

Some pest insects, especially red spider mites, thrive in hot dusty conditions. Spider mites are small sucking insects that create webs on the undersides of leaves. Signs of spider mite infestations include the webs and stippling or yellow spotting on leaf surfaces. Impatiens, when stressed by the summer heat, are very susceptible to spider mite infestations.

Control for spider mites, whiteflies, aphids and other sucking insects begins with washing dust off leaves and blasting the insects off the plants with a jet of water from the hose. But not this year.

We’re going to need to find ways to reduce the dust on plants and to control pest insects without using water if possible.

Try shaking or even sweeping as much dust as possible off plants. Wear a protective mask and eye goggles to prevent the dust from getting into lungs and eyes. A broom handle can be used to bang on higher tree branches to dislodge some of the dust.

Spiders are considered beneficial insects because they feed on many pest insects. Spider populations rise when hot, dry, dusty conditions stress plants so that they attract pest insects. Use long-handled brooms and brushes to knock down spider webs on a regular basis. Pay careful attention to removing webs near outdoor lights, windows, and foot traffic areas near the house. Wear long-sleeved shirts, long pants, gloves, and head coverings when going after spiders. The black widow (one-inch long, big shiny belly with an orange hourglass shape on the underside, aggressive when disturbed or protecting eggs) is the only poisonous spider in our area, but many other spiders have large enough jaws to bite through flesh.

Pesticides and herbicides are less effective when applied to dusty plants. Neem oil and insecticidal soaps will kill spider mites, aphids and whiteflies but must be applied to all surfaces to the point of runoff. (Neem oil and many other pesticides are toxic to bees on contact; use them only when bees are not foraging). Neem oil should not be applied to drought-stressed plants.

Glyphosate herbicides including Roundup are much less effective when applied to dusty weeds; knock or shake the dust off pest infested plants and weeds before applying pesticides or herbicides.

This story was originally published July 22, 2015 at 4:35 AM with the headline "Heavy dust coating will increase stress on plants."