MATERIALS AND METHODS

Study system and plant and fungal material
Ribwort plantain, Plantago lanceola L., is a perennial monoecious plant that reproduces both sexually by wind pollination, and clonally by producing side rosettes (Sagar & Harper, 1964).  Podosphaerea plantaginis (Castagne; U. Braun and S. Takamatsu) (Erysiphales , Ascomycota) is a specialist obligate biotroph infecting P. lanceolata. As all powdery mildews, it requires living host tissue throughout its life cycle (Bushnell, 2002), and completes its life cycle as localized lesions on host leaves. Infected plants suffer significant stress, and infection may increase host mortality (Laine, 2004). The interaction between P. lanceolata and P. plantaginis is strain-specific suggesting gene-for-gene type control (Laine, 2004, 2007; Thompson & Burdon, 1992). In some cases the host can mitigate pathogen´s reproduction; the putative resistance mechanism includes two steps, where the host plant first recognizes the attacking pathogen and blocks its growth (Laine, 2004) and the following infection outcome depends on both host and pathogen genotypes (Laine, 2004, 2007).
In resistant interactions no infection develops, while in susceptible interactions there is considerable variation in pathogen development that is affected by both host and pathogen genotype (Laine, 2007). An inoculation protocol where conidia from small colonies or individual chains are placed on detached leaves or intact leaves of plants yields a robust characterization of resistance-susceptibility. A plant genotype is characterized as resistant when no pathogen growth is detected following inoculation, or when the test plant shows rapid cell death around inoculum source. A genotype is characterized as susceptible when infection is detected following inoculation. From an earlier large inoculation study consisting of 2944 host genotype–pathogen genotype combinations (Hockerstedt et al., 2018), we selected three genotypes (IDs 193_2, 2818_3 and 2818_6, named Res1, Res2 and Res3 here after) that were resistant against all tested pathogen strains, and two genotypes (IDs 313_6, 1553_5, named Sus1 and Sus2 here after) that were susceptible against all tested pathogen strains. Selected genotypes were cloned into six plants each as described in Laine (2004).