THE EXPRESSION OF RESISTANCE TO LATENT STEM INFECTION BY DIAPORTHE TOXICA IN NARROW-LEAFED LUPIN

Resistance to latent infection by Diaporthe toxica was examined in res istant acid susceptible cultivars and breeding lines of Lupinus angust ifolius (narrow-leafed lupin or blue lupine). Conidial germination (91 +/- 1% at 3 days after inoculation) or penetration of the cuticle (14 +/- 1% at 3 days and 24 +/- 1% at 5 days) by D. toxica were not affec ted by host resistance. After 5 days, the relative size and number of subcuticular coralloid infection hyphae differed on resistant and susc eptible hosts. On susceptible hosts, the majority of coralloid hyphae were large (>100 mu m long at 21 days). Resistant hosts had the same d ensity of coralloid hyphae (approximately 300 hyphae per cm(2)), but t he majority were small (<100 mu m long). Breeding line CE2:435, which has intermediate resistance, had an equal density (approximately 150 h yphae per cm(2)) of both types of coralloid hyphae. In excised stems, saprophytic growth from subcuticular latent infections was faster from large coralloid hyphae in susceptible hosts than from small coralloid hyphae in resistant hosts. Small coralloid hyphae often failed to pro duce saprophytic mycelia and were apparently nonviable. Resistance to latent infection by D. toxica in narrow-leafed lupin is expressed as a reduction in the frequency of large coralloid hyphae, an increase in the frequency of smaller and apparently nonviable coralloid hyphae, an d slower saprophytic colonization of host tissue. The type of resistan ce described, in which the host appears to actively suppress the estab lishment of ''saprophytically competent'' latent infection structures, is a new phenomenon in plant disease resistance.