A light microscope study of resistant and susceptible carnations infected with Fusarium oxysporum f.sp. dianthi

Abstract

An anatomical study of infection by Fusarium oxysporum Schlecht. f.sp. dianthi (Prill, et Del.) Snyd. and Hans, in the stems of a resistant carnation (Dianthus caryophyllus L.) cultivar, and a susceptible cultivar, was carried out to determine host structural responses to infection which might be important in resistance to the fungus. Histological examination of stems at 2–24 days following inoculation revealed certain features of infection which were common to both the resistant and the susceptible cultivars. These were colonization of the xylem vessels and surrounding tissues, hyperplasia, hypertrophy and altered cytoplasmic activity of the xylem parenchyma, blockage of vessels with gels, and swelling of the vessel pit membranes between pit pairs. Gel formation was associated with paratracheal parenchyma cells. However, the extent and timing of some of these features were different between the two cultivars. The susceptible cultivar showed extensive colonization and destruction of xylem vessels and surrounding tissues, limited hyperplasia, and limited vascular gelation which was also slow to occur. In contrast, the resistant cultivar had less colonization and destruction of xylem, extensive areas of hyperplastic xylem parenchyma which sometimes surrounded groups of infected vessels, and rapid and extensive gelation of the xylem vessels in advance of fungal colonization. The rapid gelation response, together with the hyperplasia, were considered to be responsible for the physical containment of the fungus in the resistant cultivar and are therefore an important part of the defence mechanism in this host against the pathogen.