LIGHT AFFECTS THE FORMATION AND DEVELOPMENT OF PRIMARY HAUSTORIA OF ERYSIPHE-GRAMINIS-HORDEI IN LEAF EPIDERMAL-CELLS OF HORDEUM-VULGARE

Conidia of the powdery mildew fungus (Erysiphe graminis f. sp. hordei, race CR3), when inoculated onto susceptible barley (isogonic line of cv Manchuria containing the ml-a gene) intact primary leaves or leaf segments floating on water, germinated and within 24 h formed primary germ tubes, an appressorial germ tube, an appressorium, an infection peg and a bulbous primary haustorium in a 12 h light followed by a 12-h dark (12L/12D) regime at 18-24 °C. Similar development occurred in a 24L regime. In 12D/12L or 24D regimes there was accelerated development, with the haustorium forming many fingers accompanied by up to three elongating secondary hyphal cells within 24 h. The light or dark treatment was most effective at regulating development when administered 4-8 b after inoculation. In 12D/12L and 24D regimes, development proceeded more slowly in subsidiary cells than in other types of epidermal cells, except guard cells which were never infected. Percent penetration (germinated conidia possessing appressoria that formed haustoria) varied from 5·8% in intact leaves in 24L to 38·1% in leaf segments in 24D. There was a significant difference in fungal development and percentage penetration between 24L leaf segments and 24L intact leaves, lending further support to the inference that the light effect resides in the host and not the fungus. © 1993 Academic Press Limited.