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.