AbstractCut flowers of rose cv. Scarlet Pimpernel were dusted with dry conidia of B. cinerea from 10- to 14-d-old cultures and incubated at 15?C in a range of humidities controlled to within 2% RH. Petals developed lesions within 24 h at, or above 94% RH of the air entering the controlled environment chamber containing rose flowers in the absence of water droplets, and necrosis was severe after 48 h. Petals were cleared, stained in aniline blue and viewed by fluorescence microscopy to show that conidia germinated to form only short germ tubes. They lacked appressoria or any other complex infection apparatus and colonized petals extensively within 48 h. Low temp. SEM confirmed that the conidia germinated in the absence of surface water and penetrated the epidermis without the formation of appressoria or evident external surface changes in the cuticle around germ tubes at the point of penetration of host cells. In freeze-fractures of infected petals, the apices of fungal hyphae caused no apparent deformation of host cell walls during penetration of, or emergence from, the mesophyll cells, indicating that cell wall-degrading enzymes are secreted and retained at the hyphal tip.