The infection of round-leaved mallow (Malva pusilla) leaves by Colletotrichum gloeosporioides f. sp. malvae was studied using light and confocal microscopy. Conidia germinated and produced appressoria within 24 h after inoculation. An infection peg arose from the base of the appressorium and directly penetrated an epidermal cell. An intracellular infection vesicle appeared beneath the penetration site by 48 h after inoculation. Large primary hyphae (LPH, approx. 4 μM diam.) emerged from the vesicle and grew intracellularly through several adjacent epidermal cells and then intercellularly between mesophyll cells. Epidermal cells infected by LPH maintained their viability, as shown by their ability to plasmolyse and accumulate neutral red stain. No visible disease symptoms appeared during this biotrophic stage of infection. Thin secondary hyphae (TSH, approx. 2 μM diam.) developed from LPH in 4–5 days after inoculation and were associated with the appearance of necrotic lesions. Host cell wall maceration was visible only during the necrotrophic stage. The duration of the biotrophic stage decreased as mallow leaves became older or were senesced by placing them in the dark. TSH and host cell necrosis developed as soon as 48 h after inoculation of the most senescent leaves, and TSH were observed directly emerging from the infection vesicle. Application of thiol reagents, such as glutathione and dithiothreitol, lengthened the biotrophic stage and delayed symptom development, whereas an antioxidant, ascorbate, enhanced infection and promoted symptom development.