Your oleander may have a disease called leaf scorch. This disease is caused by a bacteria. It is spread from infected plants to healthy ones by a leaf hopper -- the glassy winged sharp shooter. Bacteria introduced into a plant’s stem by the sharp shooter’s feeding grow in the water-conducting tissue. This blocks the flow of water and nutrients from the root system to the plant's foliage. Leaves on the infected stem gradually turn yellow and die. Unfortunately there is no cure for the disease. You can prune out stems with dying leaves to temporarily improve the plant's appearance. The infection will eventually kill the entire plant. The bacteria does not survive in the soil, but replanting oleanders again is not recommended. All oleander varieties are susceptible to the disease and new plants could be infected if glassy winged sharp shooters feed on them.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August, 2012
For more information see UC IPM Pest Note 7480 Oleander Leaf Scorch
For a list of plants that could be substituted for oleander hedges, see the article “Good Bets for Hedges to Replace Dying Oleanders” in the January 2010 issue of our “Dig It” newsletter http://www.mastergardenerssandiego.org/newsletter/article.php?ID=1