Disease severity and relationship with the amount of fungal DNA and structural changes in resistant and susceptible bean cultivars infected with Fusarium wilt
Fusarium wilt is an important soil-borne disease that affects the common bean. The disease is caused by Fusarium oxysporum f. sp. phaseoli (Fop), a fungus that invades the plant mainly through the roots and colonizes the xylem, causing wilting, vascular discoloration, chlorosis, stunting, and premature plant death. The objective of this study was to analyze Fop disease severity in the Mortiño (tolerant), BAT477 (resistant), and A211 (susceptible) differentiating cultivars of common bean. Another goal was to examine the relationship of the disease severity with the amount of DNA of the pathogen in the vascular system in the stem as well as structural changes in plants affected by Fop infection. Assessment of the level of xylem colonization by Fop isolated in these cultivars grown in an experimental field (plot size of 30 m2) artificially infested with Fop showed in all the cultivars, a gradual increase in the fungus population over time, mainly during the maturation of the plants. The qPCR technique with the diseased genotypes collected at five different times starting in V2 stage (second trifoliolate developed) in the biological replicates proved to be efficient in quantifying the relative amount of DNA of the fungus. In the scanning electronic microscopy and in the evaluation of the disease severity, the structures of the fungus and of the plant were different. In the A211 cultivar, formation of hyphae and microconidia of the fungus and tylose and amyloplasts of the plant were observed. In cv. Mortiño found hyphae of the fungus and tyloses of the plant. In BAT477, no pathogen structure was observed. The disease severity in the differentiating cultivars was correlated with the amount of fungal DNA and plant structures formed in reaction to the presence of the fungus detected mainly in the last collection at the R6 stage of the bean plants.