Disease. Several different plant diseases lead to various parts of plants turning yellow, most commonly the leaves. Viral diseases such as yellow mosaic cause plant leaves or flowers to turn yellow. Some fungal diseases may also cause yellowing. Rust, for instance, might cause yellow powder to form on the leaves.isease. Several different plant diseases lead to various parts of plants turning yellow, most commonly the leaves. Viral diseases such as yellow mosaic cause plant leaves or flowers to turn yellow. Some fungal diseases may also cause yellowing. Rust, for instance, might cause yellow powder to form on the leaves.