Ultrastructure of wilt syndrome caused byVerticillium dahliae. III. Chronological symptom development in sunflower leaves

摘要

When sunflowers are infected by the vascular pathogenVerticillium dahliae, the first visible foliar symptom is the appearance of tiny chlorotic flecks. An attempt was made to determine the temporal order in which early ultrastructural changes leading to symptom expression occur. The evidence indicates that ultrastructural changes occur in the following order: (1) degradation of xylem vessel walls, (2) initiation of the necrotic band, (3) degeneration of the mesophyll tissue remote from the vein, and (4) degeneration of the phloem. The possible importance of xylem vessel plugs and of phenolic compounds to the initiation of symptom development is also discussed.