Selection Progress for Fusarium Basal Rot Resistance in Onions.

Fusarium basal rot (FBR) of onion (Allium cepa L.), caused by Fusarium oxysporum f. sp. cepae, (FOC) results in severe crop damage in all the onion producing regions of the world. Using resistant cultivars is the best alternative to controlling this disease. However, FBR-resistant cultivars have yet to be developed, and effective screening methods are needed. The objectives of this study were to measure the selection progress in onion against FBR utilizing an artificial inoculation mature bulb screening while also improving the screening procedure. Three generations (original, first-generation selected, second-generation selected) of seven autumn-sown, overwintering cultivars and two checks were evaluated for their FBR severity and incidence by artificially inoculating transversely-cut basal plates with 12 or 3 x 10⁵ sporesŸml^-1 of FOC isolate ‘CSC-515’ during the summers of 2014 and 2015, respectively. After 20 days of incubation, the basal plates of 20 bulbs from each plot were cut transversely again and FBR severity and incidence were calculated. The improved screening procedure successfully created a high degree of disease severity and incidence across the seven cultivar populations and the checks. Evaluations revealed a lack of progress for reducing FBR in the two selected populations of each cultivar thus suggesting the initial selection method was ineffective. Further evaluations of the current and the newly-selected populations of the seven cultivars and checks will be conducted to access their level of FBR disease resistance using a lower inoculum concentration of FOC and an improved inoculation procedure.