Oleander Damage and Wooly Ash Aphids - June 29, 2011
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

A month or so ago, an article appeared in the Cottonwood Journal titled “Homeowners reporting dead, dying oleanders”. Since that article appeared, I have received many comments and questions regarding its content and recommendations. While much of the information presented was correct in a sense, some of the article may have been barking up the wrong tree – or shrub, in this case. On that note, I would like to clarify a few points and offer some recommendations.

Oleander (Nerium oleander) is a large, flowering shrub that thrives when planted in Arizona’s low desert areas. They are very heat- and drought-tolerant once established and produce abundant blooms during the summer growth period. Bloom colors include red, pink, white, and cream. The upright gray stems support dark green foliage. Mature size ranges from 3 to 20 feet tall with dwarf varieties being most desirable where short hedges are desired. They can also be purchased as standards (small trees), but these often produce abundant suckers from the base which must be pruned out to maintain the standard appearance.

Now for the bad news: most references to oleanders recommend they be planted in USDA Plant Hardiness Zones 8 through 10. This means that winter temperatures should not regularly go below 20 degrees F. In researching the some of the low temperatures from last winter, I found that they dipped between 12 and 13 degrees F in Cottonwood for two hours on the morning of February 3, dipped even lower at the Sedona Airport, and was even lower in the river and creek bottoms. These temperatures were certainly cold enough to damage oleanders and this is likely what people in the Verde Valley have observed.

Oleander leaf scorch (OLS) and cotton root rot (also called Texas root rot) were also mentioned in the article. OLS is a bacterial disease transmitted from infected plants by xylem feeding sharpshooter insects. OLS symptoms appear in the summer and start with yellowing. To date, the only cases of OLS in Arizona have been in Phoenix. Cotton root rot is a fungal disease that has been observed in many areas of the Verde Valley, but it also appears in the summer months and kills the plant relatively quickly. Since the recent damage seen in oleanders was observed in spring, it is very unlikely that these diseases were involved.

Young oleander could have been killed by the above described freezes, but older plants are likely still alive. Freeze-killed wood should be pruned out of damaged oleanders. When assessing larger specimens, you may decide to prune them all the way back to the ground. This technique, called rejuvenation, is often used on oleanders whether frozen or not. Buds below the pruning cuts should begin to grow with warming temperatures. If your mature plants were completely killed, then your location is likely too cold for oleanders.

Wooly ash aphids have been reported in many areas of Yavapai County. These small, sucking insects feed on young ash leaves in early summer causing them to “roll up”. They are sometimes mistaken for whiteflies (which we should not see outside of greenhouses at our elevation) because of the white, waxy material they excrete. The rolled leaves provide a secluded refuge for the insects to continue feeding and reproducing tucked away from predators such as ladybird beetles, lacewings, syrphid flies, spiders, and parasitic wasps.

Wooly ash aphids reproduce rapidly through asexual means (parthenogenesis). Some of the offspring are winged allowing them to disperse to other ash trees. The aphids also produce honeydew, a sweet residue which can attract ants and, when left alone, will also support growth of a black sooty mold. Initially, the foliar damage slows tree growth, but the heat of summer and natural enemies will usually keep wooly ash aphids in check. Usually, previously colonized trees recover and produce normal leaves for the remainder of the growing season.

When warranted, insecticidal soap and the systemic insecticide imidacloprid have been recommended for use against the wooly ash aphid. Begin control efforts with insecticidal soap drenching the tree and rolled leaves. Insecticidal soap residue does not harm non-target and beneficial insects. If aphids persist, imidacloprid can be applied using a soil drench application which is taken up by the roots and transported upward to the leaves. Applications of both insecticides are best done in spring, before massive leaf curling occurs.

Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: June 21, 2011
Content Questions/CJune 21, 2011g.arizona.edu

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