An ultrastructural study of vascular colonization in three vascular wilt diseases I. Colonization of susceptible cultivars

Abstract

Transmission electron microscopy was used to study vascular colonization in susceptible cultivars of tomato infected with Fusarium oxysporum f. sp. lycopersici or Verticillium albo-atrum and of pea infected with F. oxysporum fsp. pisi. There were many similarities in colonization by all pathogens. Hyphae were normally in close proximity to vessel walls and aligned approximately parallel to the long axes ofvessels. Vessel to vessel colonization was uncommon but occasionally occurred by penetration of intertracheary pit membranes. Pit penetration was achieved by constricted hyphae and localized membrane degradation. Penetration of xylem parenchyma pit membranes was not observed in either host but colonization of xylem parenchyma cells occurred early in disease development in pea but only when the host plant was moribund in tomato. Fusarium oxysporum f. sp. lycopersici and V. albo-atrum caused extensive erosion of tomato vessel secondary walls, both where hyphae were in direct contact with walls and in areas remote from invading hyphae. Penetration of secondary walls was common but its extent was limited by the primary wall/middle lamella. There was little secondary wall erosion in pea although the primary wall/middle lamella of pit fields was often extensively degraded. Many vessel and xylem parenchyma cell secondary walls in both species contained, irregular electron-opaque areas. Vessel walls and colonizing hyphae were often coated in an amorphous electron-opaque material. The material occluded intertracheary and xylem parenchyma pit cavities and encrusted intertracheary pit membranes. Its appearance varied from smooth to rough and “bubbly”. A distinctive fibrillar coating was found much less frequently. Infection was accompanied by the gradual death of xylem parenchyma cells throughout infected plants. Xylem parenchyma cells adjacent to infected vessels often contained well developed “protective” layers. Some also contained wall appositions but their role was unclear. In young infected tomato plants xylem parenchyma cells apparently underwent uncontrolled division resulting in “whorls” of cells and crushing of vessels. The features observed are discussed in relation to symptom development.