Histopathological Response of Resistant and Susceptible Cultivars of Cucumber to Fusarium oxysporum f.sp. radicis-cucumerinum, the Causal Agent of Fusarium Stem and Root Rot

Two greenhouse-cucumber cultivars, Festival and Negeen that are resistant and susceptible to Fusarium oxysporum f.sp. radicis-cucumerinum, respectively, were used for histopathological studies. On days 3, 6, 10 and 18 after artificially inoculation with fungal plugs that were placed as close as possible to the crown and root of seedlings, samples were prepared from crown and roots of infected plants. After sectioning, fixation and staining, samples were investigated microscopically. Cortex tissue discoloration, tissue disintegration, development of mycelium in cortex of root and increasing in production of pectic compounds occurred in infected tissues. Pathogen growth, disintegration and discoloration in resistant cultivar restricted to the superficial layers of cortex, while in susceptible cultivar these phenomena developed in whole root system and reached to the vascular stele. The pectic compounds production in infected tissues started a few days after inoculation and the level of these compounds in resistant cultivar was higher than susceptible cultivar at day 18 after inoculation. According to these results, cell wall rigidity and level of cell wall pectic compounds can be considered as two important components of resistance in breeding programs for development of resistant cultivars to the disease.