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Monday, July 2, 2018

Whats Causing Leaf Damage on Creosote Bush

Q. Two creosote bushes in my landscape have distorted leaves and very thin foliage. Both bushes receive light, supplemental water and have for the past ten years. I noticed a Midge’s gall on each bush.  Can you give me recommendations on bringing them back to health?

A. It may be impossible to tell you exactly what caused these distorted leaves on your creosote bush but I put these types of problems into two categories; insects and environmental. I would rule out diseases.
            Judging from the pictures, most of the leaf distortion seems to be “cupping”. I see a few leaves with brown tips. I also saw in one picture some very light “webbing” and leaf tip dieback which makes me suspicious of spider mites.

Online reference to this article
            It’s a common misconception that native plants like creosote bush don’t have pest problems. They do. Many of our native plants have insects and other animals that coexist together. Some of them cause damage and others feed on these bad guys. That, and limited new growth, keep them in check. But, seldom does it get out of control.
            When leaves cup, some part of the leaf becomes damaged as it was growing. As the leaf continues to grow, the undamaged part of the leaf continues to grow normally while the damaged part doesn’t. This causes the leaf to “cup”.
            There are several native insects that already have a relationship with creosote bush and have the potential to cause this kind of damage; the Creosote Bush Stink Bug and the Creosote Bush Plant Bug. Both insects have piercing sucking mouthparts and can cause this type of damage which gets hard to find as leaves get bigger. Diseases can cause this type of damage, but probably less likely.

Creosote Bush Stink Bug
Creosote Bush Plant Bug

            If I follow the KISS principle when diagnosing problems. My best guess is probably insects because of their abundance with this plant. Check this plant for spider mites. Although they are not technically insects, they fall into a general “insect” category of damage.
            Spider mites are very tiny and hard to see. Take the stem of a plant still attached and slap it hard against white paper. Look for crawling dots the size of a period at the end of a sentence. If these tiny dots are moving, then you’ve nailed it.
            Brush your fingers across these dots and they will smear on the paper. Because I'm curious, I also look, with a hand lens, for tiny translucent balls on the leaves. These are their eggs. Plants oftentimes appear extra dusty when they have spider mites because of all the dead spider mite carcasses on the leaves.
             I think the succulence of this plant growing in a landscape was probably a huge magnet to plant feeders like a spider mites and other “bugs” looking for food. Plants like this with extra succulence are like a smorgasbord to plant feeders.
            What to use? Washing the plant with a stream of water helps to keep spider mites from getting established but once established they are a continuous problem during hot weather. There is a lot of anecdotal recommendations out there but most of them are untested in the field.
            Soap and water sprays, if you do it two or three times a week, will oftentimes get them under control. Make sure you spray the top side and bottom side of leaves. But it doesn't do much about the insects with piercing mouthparts. You've got to spray them directly with soap and water to kill them. And it will kill them but it must be a direct hit. Some organic sprays might work for you such as neem oil.
            Traditional pesticides used for insects oftentimes make a spider mite problems worse. Bad spider mites are usually kept under control by good mites. Spraying a traditional pesticides to control mites will wipe them both out. However, if it's really bad then go ahead and apply a miticide from your local nursery or garden center. But first try to wash them off with a stream of water and soap and water.

Link to some predatory mites

            Try irrigating your creosote bushes less often and creating less succulence. Water them like cacti after they get established. Watering them every three weeks once they are established should be enough.

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