Ultrastructure of the vascular responses of tobacco to Fusarium oxysporum f sp nicotianae

The reactions of a broadleaf tobacco variety polygenically resistant to Fusarium wilt were compared with a susceptible variety. Symptoms of wilting, yellowing, and eventual death of leaves in the susceptible variety were associated with rapid movement of the fungus through the plant, whereas in the resistant variety the fungus was confined to the lower portion of the plant and symptoms occurred only on the lowermost leaves. The responses in the vascular tissue were similar in both varieties but appeared more intense in the resistant variety. Lipoidal wall coatings and occlusions occurred in the initially infected vessels. Apposition layers containing callose were deposited in the contact cells within 1 day of inoculation. Lipoidal occlusions occurred in the secondarily infected vessels, but apposition layers were not detected in the contact cells. Tyloses were not observed. Resistance is associated with the localization and slower movement of the fungus that is reflected in the observable differences in symptom development.