Symptoms of white blister disease on leaves and bracts of sunflower and the presence of oospores in bracts. A–B. Head infection with involucral bracts showing pustules with sporangia (arrow). C. White blisters with asexually produced sporangia on the lower leaf sides (arrow) D. Blackish oospores (arrows) visible through the epidermis of the abaxial surface of a bract. E. Light micrograph of a fresh epidermal strip with black oospores still embedded in oogonium and antheridium (arrow). Bar = 50 µm.