Issue: May 27, 2006
Do we have new, exotic plant disease?
Question:
Our home was built in 1993 on a double lot in Las Cruces. It has an oleander hedge planted along the perimeters of the property adjacent to the rock walls as a privacy screen. This planting covers 320 linear feet. Over the years, it has filled in completely and is quite tall.
Late this spring we observed a great deal of leaf damage on at least half the plants. We assumed that we should have provided more water to these plants during the winter where, as you know, there has been essentially no rainfall. However, we are now reading information on the Internet from the University of California. There appears to be a problem with "oleander leaf scorch" caused by the bacterium Xylella fastidiosa. This bacterium is "vectored" by insects, primarily the "Glassy Winged Sharpshooter." We are informed that the problem first appeared in southern California but has since spread into Arizona and Texas. Someone apparently has not checked a map. The last time I looked, New Mexico lies between those last two named.
Accordingly, should we assume that the "oleander leaf scorch" has invaded our area? It appears from what we are reading that there is nothing we can do about it. We would greatly appreciate your comment.
Answer:
I think your first suspicion was correct. The dry winter is the most likely cause of the leaf dieback. Winter desiccation of oleander leaves is not uncommon in Las Cruces. The winter of 2004-2005 was wetter than normal. The previous winter, plants were much smaller and required less water. As plants become larger, their water requirements increase. Winter irrigation and mulch to conserve water in every season will help with this issue.
Dr. Natalie Goldberg, NMSU Extension Plant Pathologist reports that oleander leaf scorch has not been reported in New Mexico. She has recently tested some oleander samples from Las Cruces and says that oleander leaf scorch was not present. Dr. Carol Sutherland, NMSU Extension Entomologist, reports that one of its insect vectors, the smoke tree sharpshooter, has been reported in the state but not in Las Cruces. She says that the glassy winged sharpshooter has not yet been found in New Mexico. For these reasons, the best hypothesis is winter water deficit. The fact that New Mexico seems to be skipped over in the distribution of this disease is probably due to our lower population and reduced chance for importing the disease on infected plant material. The expanse of arid lands between us and the infested areas also provides some protection. The insects carrying the disease cannot easily cross the desert.
While it may be premature to assume that oleander leaf scorch is the cause, this hypothesis may be tested by sending a sample for analysis. If your plants have significantly different symptoms from other oleanders in your area, you may have them tested. Collect some stems that contain both healthy and dead leaves. Take those samples to your local Cooperative Extension Service office. The sample may then be delivered to the Extension Plant Pathologist for testing to determine the presence or absence of the disease organism if such testing is warranted. New Mexico gardeners hope that you will not be the first reported case of this disease in New Mexico. It will cause problems in plants other than oleander, and it will be a problem that is best avoided. If your plant samples are like those in many landscapes in Las Cruces, you won't need to send them in. With current information, winter desiccation is the most likely cause of leaf scorch of your oleanders.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.
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