Host responses to fungal penetration in Evysiphe graminis f.sp. hordei infections in barley

Resistance mechanisms restricting penetration and establishment were investigated in an incompatible interaction using an avirulent race CC/3 (BMV 1 + 4) of Erysiphe graminis f.sp hordei on barley cv. Athos (BMR 2 + 5) and a compatible interaction using the universal susceptible cv. Golden Promise (BMR 0). In both interactions: (i) auto‐fluorescence of the host cell wall occurred adjacent to the primary germ tube tip after 4 h, and by 10 h near to the appressorial germ tube; (ii) strongly fluorescing papillae developed after 12 h, being frequently associated with failed penetrations; and (iii) only 30% of the attempted penetrations from appressorial lobes resulted in an incipient haustorium at 16 h. Hypersensitive reactions occurred in cells with an incipient haustorium in 30% of appressorial penetrations on Athos, but only in 4% of those on Golden Promise. Twenty per cent of hypersensitively reacting cells in Athos appeared dead by 14 h using trypan blue (membrane exclusion test), compared with 5% for neutral red, suggesting that plasma membrane damage is an early event in the hypersensitive response. Haustorial death was associated with host cell death, but did not precede it; appressorial death occurred 2‐4 h after that of the host cell.