Development of appressoria of Erysiphe graminis f.sp. hordei in response to host plant age and water stress

SUMMARY Appressoria were observed to develop in sequence up to three lobes, each of which may establish a penetration of the underlying host cell and, ultimately, give rise to a haustorium and elongating secondary hyphae (ESH). Upon fully susceptible, well‐watered seedling plants increased appressorium development in terms of lobe number led to reduced penetration success, as judged by ESH formation. A concomitant reduction in the initiation of further penetration attempts at lobes newly formed after earlier unsuccessful penetration attempts was also observed. The effects of increased plant age and water stress upon this interaction were examined.