An Improved PCR Method for Rapid and Accurate Identification of Mating Types in the Late Blight Pathogen Phytophthora infestans

Mating type is a critical trait in heterothallic organisms. In plant pathogenic oomycetes, like the late blight pathogen Phytophthora infestans, it is usually identified through pairing between tester and candidate isolates, a method that is both laborious and applicable to live isolates only. Therefore, developing simple and fast PCR tests to reliably identify P. infestans mating types is of great interest for population genetic studies. A multiplex PCR assay combining the amplification of a locus diagnostic for P. infestans and of one diagnostic for the A1 mating type was developed and validated on a collection of 1,441 samples, covering the current and past diversity of European P. infestans populations. These samples were obtained from either freeze-dried mycelium or FTA cards on which diseased leaflets had been pressed. The multiplex assay correctly identified mating types in 97.4% of these samples. The main source of incorrect assignment was the lack of amplification of the A1 diagnostic allele, due to insufficient DNA quality and/or quantity in the reaction mix. This multiplex PCR, applicable to both live and stored material, thus constitutes a useful addition to the set of molecular tools available for population typing in P. infestans.