Plant disease (6)
'plant disease' is the definition.
(blight is a kind of plant disease)
This is all the clue.
(Other definitions for blight that I've seen before include "Pestilence - state of decay" , "Disease that withers plants like potatoes" , "Shrivelling disease" , "Fungal disease - destructive force" , "Disease of plants etc" .)
(Other definitions for mildew that I've seen before include "Fungal disease in plants" , "Fungi growing on plants" , "Fungal plant disease" , "White coating on things exposed to damp" , "White powdery fungus" .)