Optical and electron microscopy of a slowly developing major incompatibility in bean rust infections

Water-soaking and then necrosis occurred after 3–5 days in leaves of Phaseolus vulgaris cv. Epicure inoculated with race adhi of Uromyces appendiculatus. Changes in the appearance of host plasmalemmae and cytoplasm around haustoria did not occur until 56 h and preceded deterioration in adjacent uninfected cells. The penetrated cells remained alive until after 72 h when necrosis had occurred one or a few cells ahead of the intercellular hyphae and in rings around the infection sites. The number of necrotic cells in the rings increased and reached a maximum 7 days after inoculation when growth ceased having given rise to uredinial initials at some sites.