Diseases of oleander (
Oleander leaf scorch (OLS)
Oleander leaf scorch (OLS) is caused by the oleander strain of Xylella fastidiosa, a bacterium that colonizes the xylem tissue of oleander. There are many different strains of X. fastidiosa that infect many different plants, and there is a large variation in the amount of disease expression. For example, the oleander strain is deadly in oleander but does not infect grape (the grape strain causes Pierce’s disease of grape and does not infect oleander).
Symptoms of OLS on oleander include an initial yellowing of leaves (photo 1) that is soon followed by the characteristic browning and necrosis of the tips and edges of leaves (photo 2). Symptoms often appear on one branch or part of the plant in the summer. Although the symptoms are often similar to drought stress and salt damage, infected plants continue to decline (photo 3).
The OLS strain of Xylella fastidiosa is transmitted to oleanders by xylem feeding sharpshooter insects. Presently, the most important vector of X. fastidiosa in oleander in Arizona is the smoke tree sharpshooter which is common in low desert areas, including Phoenix and Tucson. Another important vector, the glassy-winged sharpshooter, has not become established in Arizona. Although it has been reported in an isolated area of Sierra Vista, detection and eradication efforts by the Arizona Department of Agriculture continue to be effective. Both the glassy-winged sharpshooter and the smoke tree sharpshooter are much larger than other sharpshooters. They measure up to ¼ inch long (> 6 mm) and are easily visible but move quickly on leaves and stems. To date, X. fastidiosa has not been detected in the roots of oleander, and there is no data from controlled experiments showing that it is transmitted by pruning tools.
Symptoms of scorch diseases caused by Xylella fastidiosa may be easily confused with other problems that cause marginal necrosis of foliage. In Arizona, the symptoms may be very similar to salt burn or nutrient and water stress (photo 4). Disease can only be positively diagnosed in the laboratory using serological or molecular assays, ELISA and PCR.
OLS has been detected in Arizona at several locations; but as far as is known, it has become established and problematic only in north central Phoenix. In an area north of Camelback to about Greenway and between 15 Ave and 15 St, the disease has killed mature oleanders. Many diseased plants have been observed in hedges in landscapes with large areas of turfgrass that are flood irrigated. Both white and red flowered types of oleanders are affected.
There is no control for OLS. Antibiotics such as tetracycline may have some very short term benefit but are not an effective control. Reduction of the insect vectors may be effective in a large scale effort, but this is difficult or impossible in private landscape settings. Infected plants should be removed as soon as possible to reduce spread of the bacterium to new plants. Preliminary observations of an infected oleander hedge in Tucson indicate that infected plants cut close to the ground produce healthy new growth. In this location, where populations of sharpshooters are probably low and infected plant tissue is removed, new growth may remain healthy for some time. However, in areas with heavy infestations of X. fastidiosa and sharpshooter vectors, pruning away infected branches or cutting plants back entirely do not stop spread of the bacterium or disease development.
February 21, 2013