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Neel Kamal and Christopher S. Cramer

Onion thrips (Thrips tabaci Lindeman)–vectored Iris yellow spot virus (IYSV) causes the disease Iris yellow spot (IYS), which is a major threat to the sustainability of onion production worldwide. An increase in thrips resistance to various insecticides, high costs, and the limited efficacy of insecticides under hot and drier conditions found in various onion-growing regions restrict grower’s options for effective control of thrips and spread of IYSV. Because cultivars resistant to thrips and IYS are lacking, this study was undertaken to measure selection progress for IYS resistance after one selection cycle. In 2009, selections were performed on previously evaluated New Mexico State University (NMSU) breeding lines that showed some reduced IYS disease symptoms, and the selected plants self-pollinated the following year. In 2011 and 2012, plants from the original and selected populations along with a susceptible check, ‘Rumba’, were evaluated under field conditions when onion thrips and IYSV were present. Plants were rated for IYS disease severity and the number of thrips per plant was recorded three times during the study in each year. First-generation material, NMSU 10-776, NMSU 10-782, NMSU 10-785, NMSU 10-807, and NMSU 10-813, had fewer thrips number per plant, lower disease severity, and disease incidence than their original breeding lines on at least one or two rating times in both years. Some first-generation breeding lines performed better with a lower thrips number and disease severity than their original population in 1 year or the other. Most entries exhibited fewer thrips, lower IYS disease severity, and less incidence than the susceptible check ‘Rumba’ at most rating times. Overall, some progress was observed in this first-generation material for reduced IYS disease symptom expression when compared with their original populations.

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Neel Kamal and Christopher S. Cramer

Onions grown in New Mexico are currently harvested manually at 80% tops down (TD). Mechanical harvesting is a matter of urgency for growers in order to remain competitive and to reduce their cost and time. The objective of this study was to find the effect of different harvest stages on bulb quality. Twelve different onion cultivars were sown in Feb. 2004 in Las Cruces, N.M. The experiment was laid out in split-plot design with four harvest treatments based on physiological maturity—20% TD, 80% TD, 1 week after 80% TD, and 2 weeks after 80% TD as whole plots, with cultivars as sub-plots. After curing, data on harvest date, bulb diameter, height, firmness, number of growing points, average center diameter, fleshy scale number, and scale thickness were collected. Maximum number of scales was observed when bulbs were harvested 2 weeks after 80% TD, while average scale thickness was greatest when bulbs were harvested 1 week after 80% TD. Significant treatment by cultivar interaction was observed for bulb firmness. Cultivars Cimarron, Sierra Blanca and NMSU 04-52-2 produced firmer bulbs in all treatments, while NuMex Casper, NuMex Jose Fernandez and NuMex Centric produced firmer bulbs than others, only at 20% TD. Maximum bulb firmness was observed in NMSU 04-28 and NMSU 03-52-1 than others, when harvested 1 or 2 weeks after 80% TD. Overall, bulbs harvested 1 to 2 weeks after 80% TD exhibited firmer bulbs with more scales and greater scale thickness.

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Neel Kamal and Christopher S. Cramer

Onions grown in New Mexico currently are hand-harvested. In order to remain competitive and to lower production costs, growers will need to harvest onions mechanically. The current recommendation for harvest time is when 80% of onion tops have fallen. The objective of this study was to measure several bulb quality traits when bulbs were harvested at four different stages. Twelve short- and intermediate-day onion cultivars of different maturities were sown during Sept. 2004 in Las Cruces, N.M. Bulbs were harvested at four stages of physiological maturity: 20% tops down (TD), 80% TD, 1 week after 80% TD, and 2 weeks after 80% TD. After curing, data on harvest date, bulb diameter, height, firmness, number of growing points, average center diameter, fleshy scale number, and thickness were collected. For most traits, no differences existed among the different treatments. For the earliest-maturing cultivars, the maximum bulb firmness and number of scales were observed when bulbs were harvested 2 weeks after 80% TD. For later-maturing cultivars, the maximum number of scales was observed 1 week after 80% TD, while the maximum bulb firmness was observed at 2 weeks after 80% TD. For latest-maturing cultivars, bulbs harvested at 1 week after 80% TD were firmer than bulbs harvested at other times. For later-maturing cultivars, average scale thickness was greatest when bulbs were harvested 2 weeks after 80% TD. From this work, a delayed harvest of 1 to 2 weeks after 80% TD resulted in firmer bulbs with more scales.

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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.

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Christopher S. Cramer, Neel Kamal and Narinder Singh

Iris yellow spot (IYS) disease, caused by Iris yellow spot virus (IYSV), results in irregular and diamond-shaped, chlorotic, and necrotic lesions on the leaves and seedstalks of onions (Allium cepa L.). These lesions reduce leaf photosynthetic area and ultimately reduce onion bulb size and yield from larger bulb classes. IYSV is vectored by onion thrips (Thrips tabaci L.) that are difficult to control under certain environmental conditions. Currently, no onion cultivar is resistant to the disease symptoms, virus, and/or thrips. Twenty-one cultivars and 17 germplasm lines were evaluated in the field for IYS disease severity and thrips densities at multiple times during the season as well as leaf color, waxiness, and axil openness of these entries. Plants were grown under conditions that favored thrips populations (high temperatures, low moisture, and no insecticidal spray applications), IYSV presence and distribution, and IYS development. Plants of New Mexico State University (NMSU) 07-10-1 had fewer thrips than several entries later in the season in both 2009 and 2010. Several entries exhibited a lower number of thrips per plant early or later in the season; however, these results were not consistent across years and were not associated with a particular foliage characteristic. Lighter leaf color and/or a lesser amount of epicuticular wax did not always result in the fewest number of thrips per plant as has been reported in the literature. Plants of NMSU 09-58 tended to exhibit fewer and less severe IYS symptoms early in the season as compared with plants of other entries.

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Christopher S. Cramer and Joe N. Corgan

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Christopher S. Cramer and Todd C. Wehner

The combining ability (hybrid performance) of breeding lines is often determined to measure selection progress for yield. Plant breeders utilize this information to develop breeding lines with higher combining ability. The objectives of this study were to measure the specific combining ability for yield traits over three selection cycles from four pickling cucumber populations with Gy 14, a popular pickling cucumber inbred; and to determine the change in specific combining ability for yield traits in four populations improved through recurrent selection. Four pickling cucumber populations, North Carolina wide base pickle (NCWBP), medium base pickle (NCMBP), elite pickle 1 (NCEP1), and hardwickii 1 (NCH1), were developed and improved through modified half-sib selection from 1983 to 1992 to improve yield per se and fruit quality in each population. Eleven families were randomly selected from each of 3 selection cycles (early, intermediate, advanced) from each populations and were hybridized to Gy 14. Plants were sprayed with Paraquat to defoliate them and to simulate once-over harvest. The experiment was a randomized complete-block design with 22 replications per population arranged in a split plot with the four populations as whole plots and the three cycles as subplots. The combining ability for fruit quality rating of NCWBP and NCMBP increased as the number of selection cycles increased. Conversely, selection for higher yield per se decreased the combining ability of the NCEP1 population for improved fruit quality. In most instances, the combining ability of each population exhibited a constant response over selection cycles for each measured yield trait.

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Theodore J. Kisha and Christopher S. Cramer

The U.S. National Plant Germplasm System is one of the world's largest national genebank networks focusing on preserving the genetic diversity of plants by acquiring, preserving, evaluating, documenting, and distributing crop-related germplasm to researchers worldwide. Maintaining viable germplasm collections is essential to world food security but comes at a cost. Redundancy within the collection can incur needless expense and occurs as a result of donations of similar material under different names from different donors. Alternatively, similarly named accessions from different donors can actually be genetically distinct. We evaluated 35 short-day onion (Allium cepa) accessions using microsatellite and targeted region amplified polymorphic (TRAP) molecular markers to compare newly acquired germplasm with existing accessions in the collection to determine differences and redundancies and to compare the use of each marker type in distinguishing the onion accessions. Both marker types distinguished differences and found similarities, but the results did not always agree. TRAP markers found one of the Italian Torpedo entries to be different, whereas the 10 microsatellite loci analyzed found no differences. In contrast, microsatellite analysis found all three Red Grano entries to be different, whereas TRAP analysis distinguished only one accession. The eight White Grano entries were separated into four groups by microsatellite markers and five groups by the TRAP markers. Discriminating among closely related accessions using molecular markers can require a large number of random marker loci, especially when differences may be limited to a single trait. TRAP markers were more efficient, uncovering ≈10 random polymorphic loci per primer pair, whereas microsatellite markers each uncovered differences at a single locus.

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Troy A. Larsen* and Christopher S. Cramer

New Mexico onion production will begin using mechanical harvesters in the near future in order to stay competitive in today's market. Past onion breeding objectives have focused on improving onions for hand harvesting instead of mechanical harvesting. Our breeding program is starting to evaluate germplasm for bulb firmness. The objectives of this study were to evaluate hybrid lines for their bulb firmness, to compare two methods of measuring bulb firmness, and to compare bulb firmness using two different production schemes. Bulb firmness of spring-transplanted and spring-seeded intermediate-day hybrid breeding lines was measured using a digital FFF-series durometer and a subjective rating of firmness achieved by squeezing bulbs. Bulbs were rated on a scale of 1 (soft) to 9 (hard). In general, these hybrid lines produced very firm to hard onions whether the lines were transplanted or direct-seeded. Bulb firmness of these lines measured with the durometer was greater when the lines were direct-seeded (74.9) than when transplanted (73.5). Conversely, when firmness was measured with our subjective rating, transplanted onions exhibited slightly greater firmness (8.9) than direct-seeded onions (8.8). For both transplanted and direct-seeded onions, durometer readings were weakly correlated in a positive fashion with our subjective rating. In general, durometer readings gave a greater spread in firmness measurements with a range of 69.6 to 77.8 in firmness values. Subjective ratings of bulb firmness ranged from 8.5 to 9.0. Depending on the firmness of evaluated breeding lines, our subjective rating system should be adjusted to better distinguish firmness differences between bulbs.

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Jessica A. Gutierrez and Christopher S. Cramer

Fusarium basal rot (FBR), caused by Fusarium oxysporum Schlechtend.:Fr. f. sp. cepae (H.N. Hans.) W.C. Snyder & H.N. Hans, is a soilborne fungal disease that affects bulb onions (Allium cepa L.) worldwide. Short-day onion cultivars that are resistant to FBR are lacking. The goal of this project was to screen fall-sown onion germplasm for FBR resistance using a mature bulb field screening at harvest and after 4 weeks in storage. The project was conducted for 2 years, and in each year, 26 fall-sown onion lines were grown in a field known to produce a high incidence of fusarium-basal-rot-infected bulbs. When all the bulbs in a plot were mature, the basal plates of 20 bulbs were cut transversely and each plate was rated for disease severity on a scale of one (no diseased tissue) to nine (70% or more diseased). Bulbs were stored and rerated at 2 and 4 weeks after harvest. Disease severity and incidence were higher in the first year than in the second year. Both severity and incidence increased as bulbs were stored for 4 weeks. NMSU 00-25 exhibited the lowest disease severity and incidence in both years at harvest time and after storage. `Buffalo' and `Cardinal' exhibited the highest severities and incidences across both years and at harvest time and after storage. Other entries exhibited high or low disease severity and incidence but not consistently across years and between harvest time and after storage. In the development of FBR resistant cultivars, breeding lines should be evaluated over multiple years and bulbs should be stored for 4 weeks before being screened.