When in leaf it's easier to spot because it seems as if the foliage has been damaged by a severe frost and turns limp and black.

BBC: The heartbreak of watching the slow death of ash trees

Invisible and, often odorless, green leaf volatiles emitted from vegetation act as a defense mechanism, communicating to other foliage that danger is near.

ENGADGET: Chemical sensors could detect plants' cries for help, reduce need for pesticides