VIEW ARTICLE

Resistance

The Expression of Resistance to Latent Stem Infection by Diaporthe toxica in Narrow-Leafed Lupin. M. Shankar, Agriculture Western Australia, South Perth, WA 6151, Australia, also Co-operative Research Centre for Legumes in Mediterranean Agriculture, University of Western Australia, Nedlands, WA 6009, Australia; W. A. Cowling, and M. W. Sweetingham. Agriculture Western Australia, South Perth, WA 6151, Australia, also Co-operative Research Centre for Legumes in Mediterranean Agriculture, University of Western Australia, Nedlands, WA 6009, Australia. Phytopathology 86:692-697. Accepted for publication 21 March 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-692.

Resistance to latent infection by Diaporthe toxica was examined in resistant and susceptible cultivars and breeding lines of Lupinus angustifolius (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 affected by host resistance. After 5 days, the relative size and number of subcuticular coralloid infection hyphae differed on resistant and susceptible hosts. On susceptible hosts, the majority of coralloid hyphae were large (>100 µm long at 21 days). Resistant hosts had the same density of coralloid hyphae (approximately 300 hyphae per cm²), but the majority were small (<100 µm long). Breeding line CE2:435, which has intermediate resistance, had an equal density (approximately 150 hyphae per cm²) 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 produce 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, and slower saprophytic colonization of host tissue. The type of resistance described, in which the host appears to actively suppress the establishment of “saprophytically competent” latent infection structures, is a new phenomenon in plant disease resistance.