Histological comparison of Cladosporium fulvum race 1 on immune, resistant, and susceptible tomato varieties

Abstract

Histological comparisons were carried out on three varieties of tomato, Potentate (Cf0), V-121 (Cf3), and Vinequeen (Cf2, Cf4), inoculated with race 1 of Cladosporium fulvum. Examination of leaves harvested at regular intervals indicated that under optimal conditions for disease development, spore germination and penetration were similar on the leaves of the three varieties. The susceptible Potentate became extensively colonized by intercellular mycelium, but relatively little visible damage occurred in the affected cells until sporulation was abundant. In V-121, resistance was expressed by slightly slower growth of the mycelium in the leaf tissue, but colonization was still extensive. Host cell damage became prominent just before the formation of aerial mycelium. Cells in the lesion area on V-121 became necrotic or showed various changes, including reduced starch content in the chloroplasts and the association of extracellular material with the cell walls. Histochemical tests indicated that polyphenols were present in the lesions. The mycelia in V-121 became highly vacuolated and sporulation was absent or greatly reduced. In the highly resistant (immune) Vinequeen variety, fungal development was restricted to a few cells in the mesophyll region. Host cells adjacent to and some distance from the fungus showed extensive deposition of material that was at least partly composed of callose.