Blackleg disease is one of the major diseases of Brassica napus (canola) worldwide resulting in yearly losses of 10 to 15% Australia-wide. This disease is caused by the pathogenic fungus, Leptosphaeria maculans whose lifecycle alternates between biotrophy and necrotrophy. When starting to infect canola leaves, this fungus exists in a biotrophic phase before it becomes necrotrophic and causes necrosis on the leaves. The fungus expresses different genes at each stage of its lifecycle and a comparison of gene expression levels between biotrophic and necrotrophic stages of L. maculans was conducted by Lowe et al. (PLoS ONE 9: e103098, 2014). In this dataset we identified a gene cluster which we refer to as the PKS5 cluster, containing a polyketide synthase gene and six adjacent genes which were upregulated during the period of biotrophic growth. To identify the compound produced by the PKS5 cluster, we activated the cluster artificially by introducing an extra copy of the transcription factor from the gene cluster into the genome under the control of a constitutive promoter. By this approach, we generated a strain of L. maculans that overexpressed the transcription factor in vitro and further examination revealed that the other six genes in the cluster (cytochrome p450, transferase, ABA1, ABA2, ABA3, and PKS5 genes) were also expressed in vitro. We discovered by using liquid chromatography quadrupole-time-of-flight mass spectrometry that the over-expression strain produced abscisic acid (ABA) in vitro while the wild-type strain did not. We hypothesise that the fungus produces the plant hormone ABA to promote its biotrophic stage of infection. Strains unable to produce abscisic acid are currently being generated and these will be tested for their ability to infect B. napus and to further define the role of ABA during the infection process.