Histopathology of Fusarium wilt of staghorn sumac (Rhus typhina) caused by Fusarium oxysporum f. sp. callistephi race 3. I. Modes of tissue colonization and pathogen peculiarities

Abstract

Light and transmission electron microscope studies of naturally infected or inoculated staghorn sumac plants by Fusarium oxysporum f. sp. callistephi race 3 are reported. Diverse extrinsic material (including latex in some instances) or elements occurred in vessel lumina. Some of this material labelled for pectin, often in association with tyloses, as did other opaque matter in paratracheal cells, related to alterations of their protective layer. Pronounced alterations of pit membranes of bordered pits occurred, with their outer portions disrupted into bodies of opaque matter, strongly labelled for cellulose, and their middle portions as unlabelled shreds. Similarly labelled opaque bodies occasionally occurred on vessel walls and lumina. Direct penetration of host cell secondary walls by the pathogen occurred, but these were degraded to any extent only following intramural invasion. Vessel walls, at all stages of infection, were lined with variously structured matter: in their thinnest forms, by single or paired, equidistant or widely spaced opaque bands, and in their thickest forms as alternating opaque and less opaque layers. Other thin elements, often enclosing opaque material, vesicular structures, or occasionally particles of ribosomal appearance were also delineated by similar but frequently infolded bands. These elements were sometimes observed to be confluent with fungal cells and to label for chitin. Many fungal elements were bound by only a thin or defective lucent wall layer, practically unlabelled for chitin, or by a locally thickened, labelled one; labelling for this substrate was also frequently associated with the fungal cell outer opaque wall layer or with some outer extracellular matter. Fine filamentous structures, connected to fungal cells, to the vessel lining matter, and to these other elements, extended into host walls. The lining itself generally did not label for cellulose or chitin. These observations are discussed in comparison with similar observations made regarding other wilt diseases that we have studied.

Histopathology of Fusarium wilt of staghorn sumac (Rhus typhina) caused by Fusarium oxysporum f. sp. callistephi race 3. II. Characteristics of opaque matter associated with extensive host cell and cell wall alterations

“…The study material, the sampling data, methods of tissue treatments, and techniques of cytochemical tests have been described in detail in Ouellette et al (2005). Present data also concern inoculated and naturally infected material.…”

“…To determine the kind of pathogen elements that could, in already colonized tissue, resume or continue their development in a less restricted manner, some samples were incubated for 1 to 4 d on a 2% potato dextrose agar medium before fixing. In addition to the cytochemical tests for chitin, as described in Ouellette et al (2005), tests for localizing DNA were conducted with a colloidal gold-complexed monoclonal antibody to this nucleic acid. The anti-DNA monoclonal antibody (mab) was a mouse mab IgM antibody against single and double-stranded DNA (Boehringer, Mannheim, Germany).…”

“…Hans. has been recognized only recently in the province of Québec, Canada (Ouellette et al 2005). Although the pathogen is likely soil-borne and ingress plants through roots, external symptoms appear only on the smaller branches and twigs, and this seemingly only under certain weather conditions, as observed since 1993 (the year of its first observations in Québec).…”

“…The symptoms are typical of other fungal wilt diseases, that is, internally, as discolorations of colonized wood, and externally, as browning and drooping of leaves in the acute disease stages and leaf yellowing and reddening in the less pronounced cases. An increased production of latex is also associated with infection (Ouellette et al 2005).…”

“…Studies concerning the general histopathology of the disease, based on correlative light and electron microscope observations and cytochemical tests, were reported in Ouellette et al (2005). In addition to mentioning particularities of pathogens cells, these observations dealt mainly with characteristics of extrinsic material in vessel lumina, including those apposed to vessel walls, and some aspects of tissue invasion and alterations.…”