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Clinton C. Shock, Erik Feibert, and Lamont D. Saunders

Onion (Allium cepa) cultivars for commercial production in eastern Oregon and southwestern Idaho are evaluated annually in replicated yield trials conducted at the Malheur Experiment Station, Oregon State University, Ontario. Market demand has progressively called for larger bulb size and bulbs with single centers. At harvest onions were evaluated for maturity, number of bolters, and single centeredness. Cultivars showed a wide range of bulbs with only one growing point or “bullet” single centers, ranging from 1% to 57% in 2000, from 7% to 70% in 2001, and from 1% to 74% in 2002. The percentages of bulbs functionally single-centered for processing uses ranged from 18% to 88% in 2000, from 24.7% to 91.3% in 2001, and from 14.4% to 92% in 2002. Bulb yield and market grade were evaluated out of storage. Marketable yield after 4 months of storage varied significantly by cultivar from 643 to 1196 cwt/acre (72.1 to 134.1 Mg·ha–1) in 2000, from 538 to 980 cwt/acre (60.3 to 109.8 Mg·ha–1) in 2001, and from 583 to 1119 cwt/acre (65.3 to125.4 Mg·ha–1) in 2002. Averaging over cultivars, super colossal bulb size averaged 26%, 14%, and 10% in 2000, 2001, and 2002, respectively.

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Marisa M. Wall, Ayaz Mohammad, and Joe N. Corgan

Heritabilities of the pungency and single-center traits were estimated in onion breeding lines using selection response and half-sib family analyses. Pungency was determined by measuring enzymatically produced pyruvic acid in individual bulbs. After one generation of selection, pungency was lowered by 0.37 and 0.42 μmol pyruvic acid/gram fresh weight in the breeding lines 90-61-1 and 89-69-8, respectively, and realized heritabilities of 0.21 and 0.51 were estimated. Heritability estimates calculated through half-sib progeny analysis were 0.53, 0.48, and 0.25 for pungency in the breeding lines 90-61-1, 90-62, and 89-69-8, respectively. The number of single-centered onions was increased by 19% and 22% in the lines 90-62 and 89-69-8, respectively, after one generation of selection, and the realized heritability estimates were 0.37 and 0.34, respectively.

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Clinton C. Shock, Erik B.G. Feibert, and Lamont D. Saunders

ring manufacturing efficiency is reduced when onions are multiple centered. Onion ring manufacturers have a target of 85% of the bulbs being either single centered or having a multiple center no larger than 3.8 cm diameter (“functionally single centered

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Christopher S. Cramer

Realized heritability estimates of bolting percentage, pink root and fusarium basal rot severities and incidences, and percentage of single-centered bulbs were estimated for half-sib families of an intermediate-day, open-pollinated onion (Allium cepa L.) population using selection response analysis. Half-sib families were selected based upon an index that equally weighted bolting percentage, pink root and fusarium basal rot severities and incidences, percentage of single-centered bulbs, and bulb quality. Families were subjected to one cycle of half-sib family recurrent selection. Pink root and fusarium basal rot severity was reduced by 17% and 7%, respectively, with realized heritability estimates of 1.28 and 0.65, respectively. More progress for pink root severity was made than was selected. Disease incidence was reduced by 18% and 12%, respectively, with heritability estimates of 0.65 and 0.60, respectively. Very little progress was made for the percentage of single-centered bulbs and this was reflected in a heritability estimate of 0.17. Selection based upon multiple characters at the same time may reduce the effectiveness of making improvements in a single trait. However even with low to moderate heritability, improvements were made, and suggest that further improvements can be made through selection.

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Christopher S. Cramer

As fall-sown onions are seeded earlier, an increase in premature seedstalk (bolting) incidence is observed for bolting-susceptible cultivars. The mechanism of resistance for bolting-resistant cultivars is not well known. Four fall-sown cultivars (Daybreak, NuMex Mesa, NuMex Sweetpak, Texas Early White), that differed in their bolting susceptibility, were seeded on four separate dates in September, each one week apart, in Las Cruces, N.M., to observe their growth and performance at each seeding date. Plant height and leaf number were measured monthly throughout the growing season from 10 plants in each plot. Prior to harvest, the number of plants that bolted were counted. When 80% of the plants in a plot were mature, the bulbs were harvested and the maturity date, disease resistance, bulb yield, and percentage of single centers were recorded. In general, earlier seeding dates resulted in larger plants with more leaves than later seeding dates when compared early in the growing season. By harvest time, plants from later seeding dates were comparable in height and had produced more leaves than earlier seeded plants. Plant height of `NuMex Mesa' (bolting resistant) was less than the plant height of bolting-susceptible cultivars. Bolting-resistant and bolting-susceptible cultivars produced similar numbers of leaves throughout the season. The mechanism of bolting resistance for `NuMex Mesa' may be a smaller plant size and/or a greater plant size required for receptivity to bolting-inducing temperatures as compared to bolting-susceptible cultivars. In general, cultivars exhibited less bolting, later maturity dates, and an increase in bulb yield with a delay in seeding.

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Christopher S. Cramer and Joe N. Corgan

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Clinton C. Shock, Erik Feibert, Lynn Jensen, S. Krishna Mohan, and Lamont D. Saunders

2000, two additional variety characteristics were evaluated: super colossal size bulbs (>4¼ inches diameter) and single-centered bulbs ( Shock et al., 2005a ). An onion bulb is single-centered when all concentric rings in the bulb end in one center

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Erik Feibert, Clint Shock, Stuart Reitz, Alicia Rivera, and Kyle Wieland

. Single centeredness is a heritable trait that allows for the development of cultivars with a high frequency of single centers ( Gamie et al., 1995 ; Wall et al., 1996 ). Seed companies are continuously releasing new cultivars bred to tolerate new pest

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Carrie H. Wohleb and Timothy D. Waters

cultivars that produce large bulbs with single centers (i.e., have a single growing point), because small onions and those with large multiple centers result in fewer usable rings. The WSU Onion Cultivar Trial is conducted in a different grower

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Jagtar Singh and Christopher S. Cramer

Onion growers in New Mexico often withhold irrigation for overwintered onion varieties during the months of December and January. This study was initiated to determine if this deficit irrigation program is detrimental to onion bulb quality. Twelve short- and intermediate-day onion cultivars, which differed in their maturity, were seeded in Sept. 2004 in Las Cruces, N.M. Once plants were established, 12 plots of each cultivar were not irrigated during the months of December and January (dry treatment), while the same number of plots was irrigated during these months (wet treatment). Once a plot had 80% of the plants with tops down, all bulbs were harvested, cured, and data on date of harvesting, bulb diameter, bulb height, firmness rating, number of centers, scale number, and scale thickness of first and third fleshy layers were collected. For most of the bulb traits measured, there was no difference between the two irrigation treatments for the cultivars tested. For the earliest-maturing cultivars, bulbs grown in the dry treatment had on average more fleshy scale layers than the bulbs grown in the wet treatment. For later-maturing cultivars, bulbs grown in the dry treatment had more growing points (centers) per bulb than the bulbs grown in the wet treatment. For the latest-maturing cultivars, average fleshy scale layer thickness was greater for bulbs grown in the dry treatment. From this work, a winter deficit irrigation program appears to be detrimental to the percentage of single-center bulbs for later-maturing, autumn-sown onion cultivars.