Standardization of Screening Methodology for Assessing the Susceptibility of Sweet Basil Cultivars and Lines to Fusarium oxysporum f. sp. basilici

Abstract

Fusarium wilt (FOB), caused by Fusarium oxysporum f. sp. basilici, remains an important disease of basil because of its persistence in the soil and seed transmission. Standardization of the methodology for investigating FOB resistance of basil cultivars is necessary for meaningful results. In a ‘Nufar’ seed source experiment, six commercial seed sources were tested in the greenhouse for their response to a single isolate of FOB at four inoculum concentrations (0, 102, 104, 106) at the 6-leaf stage of growth. Differential susceptibility of the ‘Nufar’ seed source lines was only revealed at a concentration of 104, but not at the lowest (102) or highest (106) concentrations. To understand the effects of inoculum concentration (0, 104, 105, or 106), cultivar (‘Caesar’, ‘Nufar’, ‘RU172’), and leaf stage (2, 4, 6) on FOB incidence and severity, a split-split plot experiment was conducted. There was a highly significant (P < 0.0001) cultivar with inoculum concentration interaction effect on AUDPC, final plant height, and mortality. There were significant leaf stage with inoculum concentration interaction effects on AUDPC values (P = 0.0093) and percentage of live plants (P <0.0001). There was a significant cultivar by leaf stage interaction effect (P = 0.0006) on the final plant height. These results demonstrate that inoculum concentration and leaf stage interact to influence FOB incidence and severity. Based on our results, we recommend (i) screening at an intermediate inoculum concentration of 104 and a range of leaf stages; and (ii) culturing stem tissue from asymptomatic plants to detect latent infections.