There are more than 5,000 known species of rust on plants. It is most often found on mature plants where symptoms appear primarily on the surfaces of lower leaves. Early on, look for white, slightly raised spots on the undersides of leaves and on the stems. After a short period of time, these spots become covered with rust-colored spore masses. Later, leaf postules may turn yellow-green and eventually black. Severe infestations will deform and yellow leaves and cause leaf drop. Rust diseases are favored by 4 to 8 hours of low light intensity, warm temperatures and moisture, followed by 8 to 16 hours of high light intensity, high temperatures and slow drying of leaf surfaces. The spores are spread by wind or by water splashing back up onto the foliage.