Components of partial resistance to powdery mildew in wheat mutants

Components of resistance to powdery mildew (latent period, pustule density and conidium production) were analysed on glasshouse-grown artificially inoculated plants of wheat mutants (n = 11) derived from either induced mutagenesis or adventitious regeneration of cv. Guardian. These mutants had previously been shown to be partially resistant to the disease in the field over a two-year period. Analyses were carried out at three stages of development: seedling, tillering and heading. None of the mutants exhibited a latent period significantly different from that of the susceptible parent 'Guardian' at any stage of development tested. With respect to pustule density, one mutant (M61) was significantly more resistant than 'Guardian' at all stages of development, two mutants (M19 and SC100) were more resistant at tillering and heading, and two (M156 and SC267) were more resistant only at heading. Significantly reduced conidium production per pustule (as a measure of pustule size) over a 24 h period was observed on three mutants (M61, SC100 and SC231) at tillering and heading, while mutant SC68 exhibited this trait at heading only. On the basis of resistance components and relative groin yield in the presence of mildew, the eleven mutants were categorised into five categories. Tests for associations between field resistance score and components of resistance measured in the glasshouse, both measured at heading, revealed a significant positive correlation only between pustule density and field score. To assess the effects of combining different components (i.e. pyramiding different partial resistance genes), mutants were crossed. Transgressive segregation for at least one component was observed in the F₂ generation in crosses between mutants with complementary modes of action (i.e. involving different components of partial resistance), but not in cases where the parents exhibited the same component of resistance.