The histology of resistance to infection by conidia ofErysiphe graminisf.sp.hordeihas been studied in barley lines with various levels of partial resistance. Resistance reduced the proportion of conidia which formed haustoria and was expressed more strongly in fifth than in first formed leaves. Where attempted penetration from the first appressorial lobe failed, a second and sometimes a third lobe could be formed. Most penetrations were from first lobes, a few from second lobes and a negligible number from third lobes. Adult plant resistance decreased penetration success from both first and second lobes. An unidentified component of resistance appeared to impede the development of less vigorous conidia before they stimulated a host cell response. However, in all hosts the majority of failed penetration attempts stimulated a localized host response implicating this response in resistance. Some attempts caused hypersensitive death of epidermal cells in both resistant and susceptible hosts; the proportion of dead cells was higher in fifth than first leaves. Stomatal subsidiary cells were more susceptible to penetration than interstomatal cells and in resistant leaves the majority of successful penetrations were into subsidiary cells. The pathogen was also able to avoid resistance to penetration by establishing endophytic infection following entry of germ tubes through stomates.
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