Why do oleander leaves turn yellow?

Oleanders are susceptible to the bacteria Xylella fastidiosa, which causes a yellowing of leaves called leaf scorch. The bacteria is mostly found in southern California and is spread by insects.

Leaf scorch is characterized by yellowing that starts at the tip of the leaf and works toward the branch. The bacteria that causes leaf scorch blocks the transportation of water and nutrients from the roots of the oleander to the leaves. Only some leaves are affected because the blockage is not uniform. Leaf scorch mimics the symptoms of drought. However, when an oleander experiences a drought, all of the leaves turn yellow.