Ultrastructure of wilt syndrome caused by <i>Verticillium dahliae</i>. II. In sunflower leaves

Abstract

Ultrastructural changes occurred in sunflower leaves that were infected by Verticillium dahliae Kleb. during the appearance of foliar symptoms. Early in symptom development major degenerative changes took place in the leaf mesophyll tissue particularly in the vicinity of the xylem of the major veins. During these early stages vessel wall breakdown occurred in the vicinity of the fungus, but this degradation was not accompanied by swelling. Occlusion of the vessel elements by fungus, fibrillar material, or electron-dense plugs was not significant until late in disease development. Comparison of these observations with those that were made in a previous parallel study of Verticillium-infected chrysanthemum suggested that the ultrastructural changes induced by the fungus in the two hosts are similar but that the timing is different. In chrysanthemum, degenerative changes in the mesophyll are not extensive until after chlorosis, but vessel blockage is advanced before wilting is noticeable. The significance of this difference in the development of foliar symptoms is discussed in relation to the plugging and toxin theories of wilt induction in fungus-caused vascular diseases.