Transmission electron microscopy was used to study vascular colonization in a resistant cultivar of tomato infected with Fusarium oxysporum f.sp. lycopersici or Verticillium albo-atrum and of pea infected with Fusarium oxysporum f.sp. pisi.
In both host species the pathogens were restricted to the vascular tissue of the roots and lower stems. Restriction was associated with, and possibly due to, the occlusion of vessels in tap roots and hypocotyls.
In tomato the principal occluding reaction was the formation of tyloses and this was normally associated with extensive accumulations of electron-opaque material in the vacuoles of both the xylem parenchyma cells and the tyloses themselves. Tyloses were absent in pea but vessels were occluded by gels. The formation of both tyloses and gels could be induced by wounding.
The ontogeny of tyloses and gels and the possible involvement of these structures in resistance to wilt disease is discussed.