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Title: Colonization of cashew plants by Lasiodiplodia theobromae: Microscopical features

Author
item MUNIZ, CELLI - Embrapa
item FREIRE, FRANCISCO - Embrapa
item VIANA, FRANCISCO - Embrapa
item CARDOSO, JOSE - Embrapa
item COOKE, PETER - New Mexico State University
item Wood, Delilah - De
item GUEDES, IZABEL - Universidade Estadual Do Norte Fluminense

Submitted to: Micron
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2010
Publication Date: 12/15/2010
Citation: Muniz, C.R., Freire, F.O., Viana, F.P., Cardoso, J.E., Cooke, P.H., Wood, D.F., Guedes, I.F. 2010. Colonization of cashew plants by Lasiodiplodia theobromae: Microscopical features. Micron. 42: 419-428.

Interpretive Summary: Cashew nut is one of the most economically important crops of northeastern Brazil and provides about $200 million of annual revenue. Lasiodiplodia theobromae is a phytopathogenic fungus causing gummosis, a disease which threatens the cashew crop in Brazil. No practical methods have been developed for the control of the disease, especially under epidemic situations. An immunofluorescent assay was successfully used to identify and locate the pathogen in various tissues of asymptomatic and symptomatic cashew tissues. This study provided a more comprehensive understanding of fungal colonization of cashew showing that the fungus begins growth in the xylem vessels (the water-conducting elements) and appeared in some plants not showing disease symptoms, thus the fungus can the fungus is able to inhabit the plant without causing disease symptoms. In plants that presented the disease, the fungus had spread to the tissues outside the xylem vessels into the surrounding xylem tissues. These studies are essential to provide basic information to gummosis resistance studies.

Technical Abstract: Lasiodiplodia theobromae is a phytopathogenic fungus causing gummosis, a threatening disease for cashew plants in Brazil. In an attempt to investigate the ultrastructural features of the pathogen colonization and its response to immunofluorescence labeling, light, confocal and electron microscope studies were conducted on symptomatic and asymptomatic cashew plants samples. Optical microscopy analysis revealed a longitudinally-sectioned hypha located within the xylem vessels, showing an extensive hyphal development in the secondary xylem tissue. Scanning Electron Microscopy (SEM) demonstrated that the fungus was found in some asymptomatic samples, particularly within the xylem vessels as confirmed by the optical images. Symptomatic samples images showed an extensive distribution of the fungus along the secondary xylem, within the vessels, but also infecting cells of the xylem parenchyma. A closer look in the cells from the secondary xylem parenchyma reveals a heavy and profuse invasion of the cells with a distinguishable cell wall disintegration and fully hyphae dispersal. Following incubation of sections with the polyclonal antisera, the fungus hyphae was intensely and regularly labeled. Rays, vessels and parenchyma cells were the preferred pathway for L. theobromae colonization. Immunofluorescence assay employed in situ was successfully applied and the polyclonal antisera produced was able to recognize the fungus and proved to be a sensitive technique to detect it.