Determining what causes the leaves of an oleander (Nerium oleander) to turn yellow could help save your shrub or small tree. Hardy in U.S. Department of Agriculture plant hardiness zones 9a through 11, the oleander's abundant green leaves form a perfect backdrop for its showy, colorful flowers. However, when disease, pests or improper care get the better of this poisonous but aesthetically pleasing evergreen, yellow leaves and plant decline could be an unpleasant consequence. (See References 1) Leaf Scorch Disease Leaf scorch is a disease caused by the bacterium Xylella fastidiosa. Primarily found in California, this disease spreads from infected plants to healthy ones by 1/2-inch-long insects known as glassy-winged sharpshooters. When an oleander becomes infected, the transport of nutrients and water from the roots to the leaves is blocked. This causes yellowing of the leaves from the tips inwards, followed by drooping and finally death. Although pruning affected plant parts can preserve the aesthetic interest of the oleander, there is no cure for this disease...