Changes caused by Septoria apiicola during the development of leaf spot disease in celery were quantified stereologically in susceptible cvs, a partially resistant parental wild celery line, fully resistant parsley and lovage. Thick sections, cut in vertical orientation, were stained and using a camera lucida, were superimposed on a lattice grid for stereological analysis. The volume of each component within the leaf space was calculated using the Cavalieri method.

In healthy tissue volume fractions of intercellular space, palisade layers and, to a lesser extent, collenchyma, vascular tissue and oil ducts were greater in the more resistant genotypes. This coincided with a reduction in the volume fraction of mesophyll tissue. Estimates of absolute volume fraction were similar. In both resistant and susceptible celery the principal response to infection was a substantial decrease in the volume fractions and absolute volumes of palisade and mesophyll tissue. Volume fractions of mesophyll and palisade tissues declined by 97% in susceptible lesions and by 98% in resistant lesions. The fungus was not observed in xylem or phloem cells. No appreciable changes were observed in either collenchyma or oil duct tissues in response to infection in resistant and susceptible genotypes. Infection had no effect on the epidermis or stomata and these remained intact until late in the infection cycle, when pycnidia erupted through the epidermis of the upper and lower leaf surfaces.

No hyphae were found in leaf sections of lovage or parsley, and fungal growth was severely restricted in all resistant material bred from wild celery. In wild celery lines, S. apiicola did not produce any, or produced very few, pycnidia and the volume fraction of vegetative hyphae was lower than that in susceptible cvs, where the fungus proliferated extensively within the lesions and many pycnidia were produced. This accounted for 30–50% of the fungal material in lesions on susceptible celery plants. The amount of fungus (21%) was less in lesions on resistant celery and in this material 96% of the fungus was present as vegetative hyphae. In lesions formed on susceptible celery 26% of the fungal tissue was reproductive and of that 11% was conidia whereas in resistant celery, where pycnidia were present, these structures were morphologically abnormal and did not contain conidia. Stereological analysis demonstrated clear differences in plant and pathogen response during infection of resistant and susceptible celery by S. apiicola.