Improving <i>Fusarium</i> Basal Rot Resistance of Onion Cultivars through Artificial Inoculation and Selection of Mature Bulbs

Fusarium basal rot (FBR) of onion, caused by the soil-borne fungus Fusarium oxysporum f.sp. cepae (FOC), results in a substantial storage loss of marketable bulbs. Seedling and field screening methods, which were used to generate FBR resistant long-day and intermediate-day onion cultivars, were found to be ineffective at increasing FBR resistance in short-day onion cultivars. This study attempted to improve the FBR resistance of seven commercial short-day onion cultivars and two check cultivars when evaluated at their mature bulb stage. Mature bulbs were artificially inoculated with 1 cm diameter potato dextrose agar plugs containing conidia of a virulent FOC isolate, CSC-515, at a concentration of 3 × 104 spore mL−1, after transversely cutting the basal plate tissue. Incubated bulbs, which had few or no FBR symptoms, were selected after 20 d using visual scoring, from 1 (no disease) to 9 (>70% basal plate is infected), and combined in seed production cages to produce the selected generation of a cultivar. Multiple cycles of phenotypic recurrent selection were conducted, and the resultant populations were compared with their respective original populations for FBR severity and incidence, from 2016 to 2019, using the same conidial inoculation method. A variable amount of progress was achieved in reducing FBR severity and incidence in the seven cultivars, with maximum improvement in the most advanced selected populations. FBR development in the advanced selected populations differed between mature bulbs of each entry and was influenced by yearly environmental variation. The progress of FOC infections was slower in resistant bulbs when compared to susceptible bulbs. These results indicated a partial or quantitative resistance against FBR. The partial FBR resistant cultivar populations could be used to develop synthetic short-day FBR resistant cultivars after multi-locational and multi-seasonal field trials. These populations could also be used to study the mechanism(s) of FBR resistance in onion, which has yet to be determined