Selection Progress for Resistance to Fusarium Basal Rot in Short-Day Onions Using Artificial Inoculation Mature Bulb Screening

Fusarium basal rot (FBR), caused by a soil-borne fungus, Fusarium oxysporum f. sp. cepae (FOC), is a major disease hindering onion production worldwide. This study was conducted to evaluate the initial and the most advanced selected populations of seven open-pollinated short-day onion cultivars for FBR susceptibility, along with two check cultivars using the conidial inoculation of mature bulbs for two consecutive years. The artificial inoculation of mature bulbs was carried out by applying a virulent FOC isolate 'CSC 515' at a final concentration of 3.0 x 10(4) spores mL(-1) to the transversely cut basal plates of onion bulbs. The basal plates of 20 arbitrarily chosen bulbs per plot were recut after 20 days of incubation and then were rated for FBR severity using a rating scale of 1 (no disease) to 9 (>= 70% of the basal plate is infected). The bulbs with a rating of 1 were saved and then bulked to form the seeds for the next generation. The selected populations exhibited a variable response for FBR severity when evaluated over two years, with an improvement in the most advanced selections observed for a majority of the cultivars. For example, the advanced selections of 'NuMex Sweetpak' exceeded the partially resistant check 'Serrana' in their levels of resistance when both were evaluated in the second year. A conidial inoculation can be effective in the development of FBR-resistant cultivars. In addition, this inoculation method can accelerate breeding efforts by determining the genetic mechanism(s) responsible for FBR resistance, locating quantitative trait loci, and facilitating marker-assisted selection.