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Cytology and Histology

Cytochemical Study of the Antagonistic Activity of Sporothrix flocculosa on Rose Powdery Mildew, Sphaerotheca pannosa var. rosae. M. R. Hajlaoui, Département de Phytologie, Centre de Recherche en Horticulture, Faculté des Sciences de l’Agriculture et de l’Alimentation, Université Laval, Québec, (Qc), Canada G1K 7P4; N. Benhamou, and R. R. Bélanger. Département de Phytologie, Centre de Recherche en Horticulture, Faculté des Sciences de l’Agriculture et de l’Alimentation, Université Laval, Québec, (Qc), Canada G1K 7P4. Phytopathology 82:583-589. Accepted for publication 3 December 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-583.

By using a wheat germ agglutinin-ovomucoid gold complex as a specific probe for localizing chitin distribution in cells of the rose powdery mildew fungus, Sphaerotheca pannosa var. rosae, and a reported antagonist fungus, Sporothrix flocculosa, we revealed a difference in cell wall composition of the two fungi as judged by the intensity of the labeling and the cytoplasm compactness. The same probe was used for indirectly determining whether or not chitinolytic activity was involved in the antagonistic action of S. flocculosa against S. p. rosae. Within 12 h after inoculation, the fungal host started to suffer some damage, characterized by a local retraction of the plasmalemma and an increase in cytoplasm compactness. Twenty-four hours after inoculation, close contact between the two fungi was sometimes associated with the penetration of the host cells by the antagonist that caused considerable changes in the cytoplasm but no discernible alteration in the chitin labeling distribution over the cell walls. At this advanced stage of antagonism, S. flocculosa caused complete plasmolysis of host hyphae, which were reduced to only cell walls. Close contact between the interacting fungi did not appear necessary before the antagonistic effect was induced. Our results, based on ultrastructural observations and cytochemical localization of N-acetylglucosamine, suggest that antibiosis rather than chitinolytic activity is involved in the antagonistic process of S. flocculosa against S. p. rosae.