Light-mediated delay of primary haustorium formation by Erysiphe graminis f.sp avenae

Time-course studies showed that light delayed haustorium formation by Erysiphe graminis f.sp. avenae on oat leaves, although it did not affect germination or formation of the first appressorial lobe. Under continuous light at 150 μmol m −2 s −1, the first haustoria were not seen until 30 h, compared to 15 h under continuous darkness or a 12-h light/12-h dark photoperiod. Under a 12-h dark/12-h light photoperiod, haustorial formation was delayed until 24 h showing that light had a greater influence if applied after appressorial lobes had been formed. Under all treatments similar numbers of haustoria were formed eventually, but maximum numbers were attained by 21 h in continuous darkness and under the 12-h light/12-h dark photoperiod, by 36 h under the 12-h dark/12-h light photoperiod, but not until 36–48 h in continuous light. Thus light delayed, but did not prevent, haustorium formation. Similarly, light delayed but did not reduce or augment host response in the form of localized autofluorescence of host cell walls in the region of contact with appressoria. X-ray microanalysis also clearly showed that at 30 h after inoculation the silicon content of host cell walls beneath appressoria was substantially elevated in dark-treated leaves but not in leaves from continuous light. Effects on haustorium formation were inversely related to light intensity. Haustorium formation was affected by incubation under continuous red, green or blue light, but green and blue light were more effective than red. Four days after inoculation, colonies on plants grown under photoperiodic treatments were better developed than on plants kept under either continuous light or darkness. Macroscopic symptoms also developed faster under the photoperiodic treatments but leaves from continuous light eventually developed a similar level of disease; plants kept under continuous darkness became chlorotic and developed no macroscopic symptoms. The evidence suggests that light-mediated delay of haustorium formation is due to delayed maturation of appressoria, probably through direct effects on the pathogen. Thus pre-inoculation treatment of leaves, likely to affect their metabolic status at the time of fungal attack, had no detectable effect; effects were not transmissible within leaf laminae; light did not enhance host resistance since the same numbers of appressoria eventually formed haustoria. It seems the fungus may possess more than one photoreceptor system influencing appressorium maturat