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Cytology and Histology

Infection Processes of Pestalotia subcuticularis on Leaves of Hymenaea courbaril. G. L. Fail, Biology Department, University of California, Santa Cruz 95064; J. H. Langenheim, Biology Department, University of California, Santa Cruz 95064. Phytopathology 80:1259-1265. Accepted for publication 12 June 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1259.

Pestalotia subcuticularis causes serious leaf blighting in some species of tropical trees, such as the leguminous species Hymenaea courbaril. P. subcuticularis forms latent (symptomless) infections as well as leaf spots. Processes of entry, intracellular ramification, latent infection, and lesion formation by P. subcuticularis in leaves of H. courbaril were followed with light, scanning electron, and transmission electron microscopy. Spores of P. subcuticularis germinated within 6 to 12 hr after inoculation onto leaves of H. courbaril from greenhouse trees, and epidermal penetration occurred within 12 to 24 hr after germination. Hyphae entered leaves directly without formation of appressoria and did not enter through stomata. During latent infections, hyphae grew beneath leaf cuticles and between mesophyll cells without any apparent damage to plant tissues other than localized cuticular degradation. Inoculated unwounded leaves from greenhouse trees often had symptomless infections for several weeks. Active infections formed within 1 wk if leaves were wounded by cutting or scraping. In active infections, hyphae grew through tracheary elements, causing vein discoloration. Cells were killed before hyphal entry. Affected cells had distorted chloroplast membranes, decreased numbers of starch grains, and cell walls that were degraded and flocculent in appearance. Inhibitory secondary compounds (resins and tannins) in leaves of H. courbaril may act to maintain infections in the latent phase.