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Background  The early phases of  Diaporthe helianthi  pathogenesis on sunflower are characterized by the production of phytotoxins that may play a role in host colonisation. In previous studies, phytotoxins of a polyketidic nature were isolated and purified from culture filtrates of virulent strains of  D. helianthi  isolated from sunflower. A highly aggressive isolate (7/96) from France contained a gene fragment of a putative nonaketide synthase ( lovB ) which was conserved in a virulent  D. helianthi  population.    Results  In order to investigate the role of polyketide synthases in  D. helianthi  7/96, a draft genome of this isolate was examined. We were able to find and phylogenetically analyse 40 genes putatively coding for polyketide synthases (PKSs). Analysis of their domains revealed that most PKS genes of  D. helianthi  are reducing PKSs, whereas only eight lacked reducing domains. Most of the identified PKSs have orthologs shown to be virulence factors or genetic determinants for toxin production in other pathogenic fungi. One of the genes ( DhPKS1 ) corresponded to the previously cloned  D. helianthi lov B gene fragment and clustered with a nonribosomal peptide synthetase (NRPS) -PKS hybrid/lovastatin nonaketide like  A. nidulans  LovB. We used  DhPKS1  as a case study and carried out its disruption through  Agrobacterium -mediated transformation in the isolate 7/96.  D. helianthi DhPKS1  deleted mutants were less virulent to sunflower compared to the wild type, indicating a role for this gene in the pathogenesis of the fungus.    Conclusion  The PKS sequences analysed and reported here constitute a new genomic resource that will be useful for further research on the biology, ecology and evolution of  D. helianthi  and generally of fungal plant pathogens.    Electronic supplementary material  The online version of this article (doi:10.1186/s12864-017-4405-z) contains supplementary material, which is available to authorized users.

Polyketide synthases of Diaporthe helianthi and involvement of DhPKS1 in virulence on sunflower