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  • Author or Editor: Yong-Biao Liu x
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Fumigation with cylindered pure phosphine free of ammonia has been used commercially at low temperatures in recent years to control pests on harvested fresh fruit and vegetables. However, long fumigation treatments cause injuries to lettuce (Lactuca sativa) and prevent its commercial use on lettuce. We evaluated whether absorbents for carbon dioxide (CO2) and ethylene can prevent injuries to lettuce in phosphine fumigation, hypothesizing that an accumulation of CO2 or ethylene be responsible for the injuries. Head and romaine lettuce were fumigated in chambers in the presence or absence of CO2 and ethylene absorbents for 3 days at 2 °C. The use of absorbents prevented lettuce injury associated with fumigation and resulted in higher lettuce quality. In the absence of the absorbents, both head and romaine lettuce sustained significant injuries in the form of brown stain, a typical symptom of CO2 injuries, and significantly reduced quality as compared with fumigation in the presence of absorbents. The injuries were likely caused by CO2 based on the facts that injuries were identical to CO2 injuries and the prevention of the injuries by the use of CO2 absorbent. The findings of this study have important implications for developing safe and effective phosphine fumigation protocols at low temperature for controlling insect pests on fresh commodities, especially when a long treatment time is required.

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The lettuce aphid, Nasonovia ribisnigri Mosley (Hemiptera: Aphididae), is a major insect pest of lettuce, Lactuca sativa L, in many commercial lettuce production areas around the world. Resistance to lettuce aphid biotype 0 (Nr:0) was first reported in Lactuca virosa L. accession IVT 280 and characterized as complete, i.e., virtually no aphids survived, and genetically dominant to partial resistance in L. virosa accession IVT 273. Complete and partial resistances to Nr:0 were conditioned by two alleles, Nr (complete resistance) and nr (partial resistance), but the genetic relationship to susceptibility was not reported. We previously reported two new potential sources of unique genes for resistance to Nr:0 in Lactuca serriola L. accession PI 491093 and L. virosa PI 274378. We report on the genetic and phenotypic nature of resistance to Nr:0 in these two wild lettuce accessions. Resistance to Nr:0 in PI 274378 is complete and allelic to complete resistance in IVT 280. Resistance to Nr:0 in PI 491093 was partial, recessive to complete resistance in ‘Barcelona’ that was derived from IVT 280, but dominant to susceptibility in ‘Salinas’. We propose the revised gene symbols for resistance to Nr:0: Nr:0C for complete resistance and Nr:0P for partial resistance, which was originally designated as nr but may now be regarded as the symbol for susceptibility to all strains of lettuce aphid. The dominance relationships among these three alleles are Nr:0C (in IVT 280, ‘Barcelona’) > Nr:0P (in PI 491093) > nr (in susceptible genotypes). Expression of partial resistance in PI 491093 was variable in controlled infestation tests, but in a naturally infested field test provided a potentially useful level of resistance to Nr:0. Partial resistance, where complete resistance has not been widely deployed, may either alone or as a component of integrated pest management delay or prevent emergence of genotypes that overcome complete resistance controlled by Nr:0C .

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Leafminer (Liriomyza spp.) is a major insect pest of many important agricultural crops including lettuce (Lactuca sativa L.). The goals of this study were to evaluate lettuce genotypes for resistance to leafminers and to estimate the heritabilities of leafminer-resistant traits in the field, to examine the association among different resistant traits, and to study the mechanism of leafminer resistance in lettuce. Seventy-eight lettuce accessions and 232 F2 plants of crosses were evaluated for leafminer stings and the production of pupae and flies in the field in 2001 and 2002, and resistant genotypes were subjected to no-choice test. Wild species (Lactuca serriola L., L. saligna L., and L. virosa L.) had significantly fewer stings than cultivated lettuces. Among cultivated lettuces, sting densities were lowest on leaf lettuce and highest on romaine types. The sting results from the field were highly correlated with the results from insect cages (r = 0.770 and 0.756 for 2001 and 2002 tests, respectively), suggesting that a cage test can be used to screen for resistance in the field. Broad-sense heritability estimates for stings per unit leaf area in the field were 81.6% and 67.4% for 2001 and 2002 tests, respectively. The number of pupae produced per plant or per leaf was moderately correlated with sting density but was not correlated with leaf weight. Results suggest that both antixenosis and antibiosis exist in lettuce germplasm and resistant genotypes from choice tests remain resistant under no-choice conditions. These findings suggest that genetic improvement of cultivated lettuce for leafminer resistance is feasible.

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Leafminer (Liriomyza langei Frick) is a major insect pest of many important agricultural crops including lettuce (Lactuca sativa L.). The goals of this study were to evaluate lettuce genotypes for resistance to leafminer and to estimate heritabilities of three leafminer-resistant traits. Forty-six lettuce genotypes were evaluated in two tests in insect cages. Wild species (Lactuca serriola L., Lactuca saligna L., and Lactuca virosa L.) had significantly fewer leafminer stings than cultivated lettuce (L. sativa) in both tests. PI 509525 (L. saligna) had few leafminer stings and no flies emerged. Leaf (leaf and romaine) lettuce also showed significantly less stings than head (crisphead and butterhead) types, while differences between leaf and romaine lettuces, and between crisphead and butterhead types were nonsignificant. Broad-sense heritability for number of stings per unit leaf area was relatively high, averaging 65% over the two tests. Heritabilities for egg-hatching period and flies per plant were 10% and 15%, respectively. Stings per unit leaf area from the two tests were highly correlated (r = 0.828), suggesting that resistance was stable over different plant ages and against different pressures of leafminer. These results suggest that genetic improvement of cultivated lettuce for leafminer resistance is feasible.

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Packaged salad-cut lettuce (Lactuca sativa L.) for food service and salad mixes is an increasingly important component of the lettuce industry. The product is highly perishable; cold storage and modified-atmosphere (MA) packaging are used to extend its shelf life. Given the importance of this market, lettuce cultivars, breeding lines, and populations should be selected for increased shelf life in MA environments. The objectives of this research were to determine the genetic variation in lettuce for shelf life in low-O2 MA environments and to develop rapid evaluation methods suitable for a lettuce breeding program. Lettuce was processed from field-grown plants of 33 romaine and three crisphead cultivars over 2 years. Shelf life was evaluated after storage in MA bags and in CO2-free controlled-atmosphere (CA) chambers with gas ratios of 0.2% O2:99.8% N2, 1.0% O2:99.0% N2, or 5.0% O2:95.0% N2. Deteriorated leaf blade tissue was water soaked and wilted with a dull to dark or black color, and midrib tissue and heart leaves were water soaked with a translucent to dark brown color. Genetic variation for shelf life was detected using MA bags or CA chambers, and the results from both years and testing methods were significantly correlated. Oxygen concentration did not affect shelf life in the CA chamber experiment, which indicated that the observed symptoms in the majority of cultivars were probably not from low O2 damage or CO2 injury, although multiple mechanisms of deterioration may be involved. Selection for lettuce cultivars, breeding lines, and populations with extended shelf life is possible using MA bags or CA chamber testing methods and could facilitate a consistent release of germplasm with stable shelf life in MA environments.

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Leafminer (Liriomyza spp.) is a major insect pest of many important agricultural crops including lettuce (Lactuca sativa L.). The goals of this study were to evaluate lettuce genotypes for resistance to leafminers and to estimate the heritabilities of leafminer-resistant traits in the field, to examine the association among different resistant traits, and to study the mechanism of leafminer resistance in lettuce. Seventy-eight lettuce accessions and 232 F2 plants of crosses were evaluated for leafminer stings and the production of pupae and flies in the field in 2001 and 2002, and resistant genotypes were subjected to no-choice test. Wild species (Lactuca serriola L., L. saligna L., and L. virosa L.) had significantly fewer stings than cultivated lettuces. Among cultivated lettuces, sting densities were lowest on leaf lettuce and highest on romaine types. The sting results from the field were highly correlated with the results from insect cages (r = 0.770 and 0.756 for 2001 and 2002 tests, respectively), suggesting that a cage test can be used to screen for resistance in the field. Broad-sense heritability estimates for stings per unit leaf area in the field were 81.6% and 67.4% for 2001 and 2002 tests, respectively. The number of pupae produced per plant or per leaf was moderately correlated with sting density but was not correlated with leaf weight. Results suggest that both antixenosis and antibiosis exist in lettuce germplasm and resistant genotypes from choice tests remain resistant under no-choice conditions. These findings suggest that genetic improvement of cultivated lettuce for leafminer resistance is feasible.

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