Histological observations of latent infection and tissue colonization by Diaporthe toxica in resistant and susceptible narrow-leafed lupins

Latent infection and tissue colonization by Diaporthe toxica was examined b y light, scanning, and transmission electron microscopy in stems, leaves, a nd petioles of resistant and-susceptible narrow-leafed lupins (Lupinus angu stifolius). Resistance was observed during the latent phase of the disease as an incompatible reaction between the host and pathogen that appeared to occur after penetration of the cuticle. Conidia were attached firmly to the cuticle by an extracellular substance presumably exuded from the conidia. Conidia penetrated the cuticle directly via an infection peg and formed sub cuticular coralloid hyphae. The frequency of subcuticular coralloid hyphae was similar on stems, leaves, and petioles of each Line. At 14 days after i noculation, resistant plants had a high frequency of small coralloid hyphae (10-80 mu m length). The epidermal cells beneath these small coralloid hyp hae appeared necrotic and collapsed with accumulation of polyphenolics and electron-dense substances and a loss of internal organisation in the cytopl asm. Necrosis was occasionally observed in small coralloid hyphae as well. Susceptible plants had a high frequency of large coralloid hyphae (80-400 m u m length) in which intrahyphal hyphae were observed, and host epidermal c ells beneath large coralloid hyphae appeared normal. Colonization of tissue s below the cuticle began immediately after excision of stems from suscepti ble plants, but was delayed in resistant plants. At 8 days after excision, hyphae had invaded all stem tissues and initiated the formation of pycnidia in susceptible plants, but few hyphae were observed in stems of resistant plants.