A PCR assay for the quantification of growth of the oomycete pathogen H yaloperonospora arabidopsidis in A rabidopsis thaliana

Summary The accurate quantification of disease severity is important for the assessment of host–pathogen interactions in laboratory or field settings. The interaction between A rabidopsis thaliana and its naturally occurring downy mildew pathogen, H yaloperonospora arabidopsidis ( H pa ), is a widely used reference pathosystem for plant–oomycete interactions. Current methods for the assessment of disease severity in the A rabidopsis – H pa interaction rely on measurements at the terminal stage of pathogen development; namely, visual counts of spore‐producing structures or the quantification of spore production with a haemocytometer. These assays are useful, but do not offer sensitivity for the robust quantification of small changes in virulence or the accurate quantification of pathogen growth prior to the reproductive stage. Here, we describe a quantitative real‐time polymerase chain reaction (q PCR ) assay for the monitoring of H pa growth in planta . The protocol is rapid, inexpensive and can robustly distinguish small changes in virulence. We used this assay to investigate the dynamics of early H pa mycelial growth and to demonstrate the proof of concept that this assay could be used in screens for novel oomycete growth inhibitors.