Maximizing Spanish Onion Bulb Size and Quality Through Irrigation and Cultivar Selection in Organic Soils of Eastern Canada, Poster Board #315

American onion is a traditional crop produced in the organic soils of Eastern Canada. The relatively pungent bulb is small to medium in size, and stores well. However, Spanish onion is increasingly chosen by consumers for its sweeter and less pungent taste as well as its color and larger size. Growers are therefore interested in developing methods for producing a high-quality Spanish onion that keeps well and could fill some of the import market. The effect of irrigation and cultivars on yield, bulb size, quality, and storability in Spanish onion was investigated in 2010. Two experiments were arranged in a split-plot design with three irrigation treatments as main plots and nine yellow, red, and white onion cultivars as subplots, and with four replicates. The irrigation treatments were as follows: control with no irrigation (IO); irrigation throughout the growing season (IC); and irrigation during the early- to mid-bulbing stages (IB). Irrigation was initiated when the average soil water tension reading at a 15‑cm depth reached −20 kPa. The first site, characterized by an old organic soil, received four and three irrigations with 418 mm (IC) and 404 mm (IB) of rainfall (IO: 364 mm) and irrigation water. Both irrigation treatments increased total yield by 11% and marketable yield by 12%, mainly owing to increases of 98% in jumbo and colossal (> 89 mm) onion yield and 8% in average bulb weight (large, jumbo, and colossal). The second site, characterized by a young organic soil, received two and three irrigations with 432 mm (IC) and 439 mm (IB) of rainfall (IO: 403 mm) and irrigation water. Total and marketable yields, bulb size, and percentage of culls during storage were unaffected by irrigation because of the high rate of local rainfall. At both sites, significant variations in total and marketable yields, bulb size, and cull weight at harvest and after 12 weeks of storage were measured among the cultivars. ‘Condor’, ‘Highlander’, and ‘Mercury’ had more (> 45 t/ha) marketable bulbs larger than 3.5 inches in diameter. Pungency level (pyruvic acid concentration) and sugar content in ‘Mercury’ onions were not affected by irrigation. The average pyruvic acid concentration in the cultivars was 4.3 µmol/g fresh weight at harvest and 4.0 µmol/g after nine weeks of storage, well within the acceptable range for mild onions. Sugar content varied among the cultivars, with ‘Orizaba’ white onion being one of the sweetest.