Immunoelectron microscopy using the anti-pectin monoclonal antibody JIM 7 confirmed earlier observations that pectin degradation is a primary event in the process of host cell wall breakdown during the development of chocolate spot disease (causal agent: Botrytis fabae (Sard.)) of broad bean. Close examination of infected and non-infected Vicia faba L. leaves indicated a loss of JIM 7-labelling, and therefore, methyl-esterified pectin, from swollen walls of infected and contiguous epidermal cells. Modified mesophyll walls also possessed less methyl-esterified pectin than healthy walls. Enzymes which attack methyl-esterified pectin appeared to be most active in regions of host tissue close to sites of fungal infection.

Ultrastructural studies using the enzyme, cellobiohydrolase conjugated to gold (CBH1-Au) revealed that the cellulose microfibrils of outer epidermal walls of non-infected V. faba leaf tissue were heavily masked by other components of the plant cell wall. Such material was most probably pectin because the cellulose microfibrils of swollen epidermal and modified mesophyll walls of infected host tissue were heavily labelled with CBH1-Au. These results were confirmed by double-labelling studies using JIM 7 and CBH1-Au. At early stages of the infection process, limited cellulose degradation was observed in infected leaf tissue.

Double-labelling experiments using the monoclonal antibody BC-KH4 directed against Botrytis matrices and a marker for the plant cell wall (JIM 7 or CBH1-Au) confirmed previous observations that the fungal matrices extended through modified host walls and degenerate cytoplasm. It is suggested that the wall-modifying action of the pectin-degrading enzymes produced during the infection process might facilitate pervasion of matrix material associated with the infection hyphae through host cell walls. Possible role(s) of such matrix material during the post-penetration processes of the V. faba–B. fabae relationship are discussed.