Artistic Arborist, Inc. - Complete Tree Health Management
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Artistic Arborist, Inc. - Complete Tree Health Management |
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(602) 263-8889 |
Oleander Leaf Scorch Confirmed in Phoenix, Arizona Andrew Backhaus, Ph.D. Oleander leaf scorch (OLS), one of the most lethal diseases known to oleanders (Nerium oleander L.), has now been discovered in the Phoenix metropolitan area. OLS which has existed in southern California since the early 1990’s was confirmed in May 2004 by enzyme-linked immuno assay (ELISA) tests performed at the University of Arizona on tissues taken from large, mature oleanders on two properties in central Phoenix. This particular infestation was severe but confined to adjoining properties and did not appear to infect oleanders in the remainder of the neighborhood. In this particular case the disease may have been spread two years earlier by chain saws that were used to clear these overgrown shrubs under power lines. This is noteworthy because OLS is thought to only be transmitted by sharpshooter insects that distribute the disease rapidly throughout an area. We have observed symptoms of OLS in a few other isolated locations in Scottsdale and Gilbert, Arizona but these have not been confirmed with ELISA tests. The threat of OLS in Arizona could have drastic consequences as occurred in the Coachella Valley of California where it took out the large stands of oleanders planted around the country clubs and golf courses of Palm Springs. According to Randy Myers of Randy Myers Nurseries, “It really wiped them out. There was nothing that could really stop them.” Whole 1-mile lengths of oleanders planted along the section line perimeters of properties had to be destroyed. Myers also noted that petit oleanders appeared to be resistant to OLS compared to standard, large-sized varieties. Oleander leaf scorch is caused by a tiny bacterium called Xylella fastidiosa, the same species but a different strain of the pathogen that causes Pierce’s disease in grapes. The symptoms of OLS begin with a chlorotic yellowing of leaves that give a mosaic, checkerboard appearance (see photo). This progresses to a scorching of leaf tips and margins and the eventual die-back of twigs and branches as the bacterium clogs the xylem vessels of plants. Left untreated, the oleander shrubs will eventually be killed about two years after initial onset of symptoms. Confirmation of OLS is done by ELISA, polymerase chain reaction or selective culture tests. Pierce’s disease has caused severe problems to the wine grape industry in California and Arizona. Many species of plants are also known to harbor the Xylella and some can act as symptomless carriers. Species susceptible to Xylella include citrus, ash, oak, periwinkle, crepe myrtle, almond, peach, plum and roses. A thorough review of this topic is available at: www.cnr.berkeley.edu/xylella. Possible Control Methods Although OLS is widely viewed as untreatable, there are a number of control measures that can be used that would ordinarily not be recommended for agricultural crops for economic reasons. For ornamental plants with higher values, more expensive treatments may be justified. In Palm Springs, for example, Myers noted that standard oleanders are rarely planted because of the widespread occurrence of the pathogen and its insect vectors. All of the original massive oleander wind breaks had to be replaced with Ficus or Carolina cherry. In Arizona, where OLS is new, it may be possible to curtail the disease with commonly acceptable treatments that are used for similar pests and bacterial diseases. Most experts agree that controlling the sharpshooter insect vectors with insecticides are useless. This was attempted in California and was ineffective. The two insect species that spread OLS, the glassy winged sharpshooter (Homalodisca coagulate) and the smoke tree sharpshooter (Homalodisca lacerta) are difficult to kill. These insects can transmit the disease with only a single feeding visit to a plant. The disease then spreads through the xylem where symptoms can appear after two weeks later, long after the insects are gone. However, there is a little-used technique that be that may be effective against sharpshooter visits. This was discovered by researchers at the USDA, who sprayed leaves with a thin coating of kaolinite clay to repel sharpshooter feedings on fruit trees. This simple, effective, non-toxic treatment should be effective for oleanders as well. Commercial formulations of kaolinite such as Redtop Snow are available in retail outlets, including Artistic Arborist. This material is commonly used as a reflective coating that acts as a sunscreen that lowers the leaf temperature and improves photosynthetic efficiency. Moreover, the layer of tiny, white clay particles deposited on leaf surfaces have been shown to physically repel sharpshooters. For oleanders that already show OLS symptoms, antibiotic treatments of oxytetracycline are approved for use against Xylella in ornamentals. To be effective, oxytetracycline must be injected into the soil or trunk of the diseased plant to be suitably distributed in the plant. For mature oleanders with basal trunk diameters over 4” we recommend using Mauget Mycoject that contains oxytetracycline. Low cost, water soluble forms of oxytetracycline for soil treatments are also available, but hard to find. Preliminary results on oleanders shows that oxytetracycline treatments may suppress and reduce the spread of the Xylella. However, more test results are required before recommending this procedure as an effective treatment method. The jury is still out as to whether Xylella can be spread by contaminated pruning tools. In theory this is plausible, as shown in lab tests where sap expressed from diseased plants transmitted infection to healthy ones. As a bacterium, OLS is capable of spread in this manner especially if contact with tools is rapid. However, it is unlikely that the disease organism can persist for longer than a few hours outside of the plant or insect vector. One local state agriculture inspector made the observation that over-watered, more succulent plants and weeds tend to attract more sharpshooters and consequently have higher titers of Xylella than less well-watered plants. Whether this is true or not should be examined. Experts from the Arizona Department of Agriculture have anticipated the arrival of OLS to the Phoenix area from California and felt that it was merely a matter of time. There are no rules for quarantine or destruction of oleanders in Arizona because the disease already existed in the state. It is vital that all diseases and insect pests be positively identified prior to beginning any treatments. We hope that some of the approaches suggested here for OLS can help avert the fate that befell the oleanders of Palm Springs. © 2003-2004 Artistic-arborist.com, Inc. All rights reserved. |
