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).