The differential expression of 13 defence-related genes during Phoma koolunga infection of stems and leaves of susceptible versus resistant field pea (Pisum sativum) was determined using qRT-PCR. Expression, in terms of relative mRNA level ratios, of genes encoding ferredoxin NADP oxidoreductase, 6a-hydroxymaackiain methyltransferase (hmm6), chalcone synthase (PSCHS3) and ascorbate peroxidase in leaves and stems differed during 6-72 hours post-inoculation (hpi) and reflected known host resistance levels in leaves versus stems. In comparison to the susceptible genotype, at 24, 48 and 72 hpi, two genes, hmm6 (122.43-, 206.99- and 32.25-fold, respectively) and PSCHS3 (175.00-, 250.13- and 216.24-fold, respectively), were strongly up-regulated in leaves of the resistant genotype, highlighting that resistance against P. koolunga in field pea is governed by the early synthesis of pisatin. At 24 hpi, leaves infected by P. koolunga showed clear differences in expression of target genes. For example, the gene encoding a precursor of the defensin 'disease resistance response protein 39' was substantially down-regulated in leaves of both the susceptible and the resistant genotypes inoculated with P. koolunga. This contrasts with other studies on another pea black spot pathogen, Didymella pinodes, where this same gene is strongly up-regulated in leaves of resistant and susceptible genotypes. The current study provides the first understanding of defence-related genes involved in the resistance against P. koolunga, opening novel avenues to engineer new field pea cultivars with improved leaf and stem black spot disease resistance as the basis for developing more effective and sustainable management strategies.