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Abstract

Lactuca saligna L., Lactuca serriola L., and Lactuca virosa L., which are cross-fertile with cultivated lettuce (L. sativa L.) were tested for resistance to lettuce infectious yellows virus in greenhouse and field tests. Fifteen of 25 L. saligna accessions were resistant, whereas 50 accessions of L. serriola and seven accessions of L. virosa were susceptible.

Open Access

Lettuce infectious yellows virus (LIYV), transmitted by the sweetpotato whitefly, (Bemisia tabaci Genn.), seriously affected melon (Cucumis melo L.) production in the lower desert areas of the southwest United States from 1981 through 1990. Melon plant introduction (PI) 313970 was previously found resistant to LIYV in naturally infected field tests and controlled-inoculation greenhouse tests. Data from F1 and segregating generations from crosses of PI 313970 with LIYV-susceptible lines indicated that resistance in this accession is conditioned by a dominant allele at a single locus designated Lettuce infectious yellows (Liy).

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Lettuce aphid (Nasonovia ribisnigri Mosley) is a recent insect pest to lettuce (Lactuca sativa L.) production in the United States. The single dominant gene, Nr, conditions resistance to the lettuce aphid in Lactuca virosa accession IVT280 from The Netherlands and is available in a limited number of commercial lettuce cultivars. New and genetically unique sources of resistance are sought to broaden the genetic base for resistance to the lettuce aphid. About 1200 lettuce PI lines were evaluated for resistance to lettuce aphid in greenhouse tests using a strain of lettuce aphid obtained from commercial lettuce in Salinas Valley, Calif. In 2002, plants were individually infested with five 24-hour nymphs per plant (controlled protocol), and the numbers of aphids per plant were counted 10–14 days post-infestation (dpi). Beginning in 2003, plants were mass-infested (mass protocol) with nymphs and alates of various ages and numbers. Using the mass protocol, the number of aphids per plant 10–14 dpi were estimated and categorized using a 1–5 scale where 1 = 0 aphids per plant, 2 = 1–10 aphids per plant, 3 = 11–20 aphids per plant, 4 = 21–30 aphids per plant, and 5 = >30 aphids per plant. `Salinas' and `Barcelona' were included as susceptible and resistant controls, respectively. Most of the accessions were susceptible. A few accessions had a few plants with very low numbers of aphids after repeated infestation, but their progeny were susceptible. Two accessions were highly resistant: PI 491093, a Lactuca serriola accession from Turkey, and PI 274378, a L. virosa accession from France. Inheritance of resistance in these two accessions and their allelism to Nr remains to be determined.

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Lettuce aphid, Nasanovia ribisnigri (Mosely) (Homoptera: Aphididae), is an economically important pest of lettuce (Lactuca sativa L.). High-level resistance found in a wild relative, Lactuca virosa L. accession PIVT 280, is conditioned by the Nr gene, which has been transferred to European cultivars and is being commercially transferred to U.S.-adapted cultivars. New sources of resistance to guard against possible resistance-breaking strains of lettuce aphid were sought in a greenhouse screening of 1203 accessions of lettuce, which included 1047 accessions of L. sativa L., seven accessions of Lactuca perennis L., 18 accessions of Lactuca saligna L., 125 accessions of L. serriola L., and six accessions of L. virosa L. Two new and potentially unique sources of resistance to lettuce aphid were found in L. serriola accession PI 491093 and L. virosa accession PI 274378. The genetic relationship of resistance in PI 491093 and PI 274378 with that in PIVT 280 remains to be determined.

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The use of RAPD markers in disease resistance breeding has been successfully demonstrated in horticultural crops. The identification of RAPD markers and their potential role in disease resistance breeding was first demonstrated in tomato; the procedure of bulk segregant analysis for detecting linkages between RAPD markers and genes conditioning resistance to downy mildew was described in lettuce; the use of linked RAPD markers to facilitate the efficient pyramiding of epistatic rust resistance genes to prolong their durability has been illustrated in common bean; the development of allele specific or sequence characterized primers linked to resistance genes has been achieved in pea, bean, tomato, and lettuce Indirect selection for disease resistance based on molecular markers is not a replacement for classical breeding procedures. Used as tools, markers offer breeders unique opportunities to combine resistance to an array of different pathogens and efficiently pyramid epistatic resistance genes for highly variable fungal pathogens. This would not always be possible in the absence of linked markers.

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PI 414723 has received much attention from melon (Cucumis melo L.) breeders, pathologists, and entomologists for resistances to zucchini yellow mosaic and watermelon mosaic viruses, including resistances to virus multiplication and subsequent transmission by the melon aphid, powdery mildew resistance, and melon aphid (Aphis gossypii Glover). PI 414723 was derived from PI 371795, which was a contaminant in cucumber (Cucumis sativus L.) PI 175111 collected in 1948 by Walter N. Koelz in Mussoorie, Uttar Pradesh, India (altitude 1829 m). Its fruit, which have soft flesh and rind that split at maturity, are used in soups and stews, and the seeds are roasted and eaten. PI 414723, PI 371795, and the related Ames 20219 and progeny 92528a were resistant to California and Florida isolates of papaya ringspot virus watermelon strain (PRSV-W). Plants were either symptomless, or they exhibited local lesions, systemic necrosis, or systemic spots. Resistance to PRSV-W is conditioned by a single dominant gene. Allelism with Prv1 (PI 180280, Rajkot, Gujarat, India), Prv2 (PI 180283, Bhavnagar, Gujarat, India), Nm (`Vedrantais, Fance), and a recently described gene for PRSV-W resistance in PI 124112 (Calcutta, India) is yet to be determined.

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Two cold storage treatments were evaluated against eggs, early instars, and late instars of the oriental fruit moth [Cydia molesta (Lepidoptera: Tortricidae)] a quarantine pest for Mexico of stone and pome fruit from the United States. In the first, `Delicious' apples (Malus domestica) were infested with these life stages and treated for 13 weeks in cold storage at 38 °F (3.3 °C) in replicated studies. In the second, the same life stages infesting `Delicious' apples were exposed to air temperatures slightly above freezing, 33.3 ± 0.7 °F (0.7 ± 0.4 °C), up to 7 weeks to simulate near commercial storage conditions. Weekly subsamples of the life stages were examined for survival. At 38 °F, complete mortality was obtained for eggs and early instars by the eighth week, and for late instars by the tenth week. At near freezing temperatures, eggs and early instars died by the fourth week, and late instars eliminated by the sixth week. This study demonstrated that the treatments were effective against the infesting life stages.

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Powdery mildew is a major problem in melon (Cucumis melo L.) production worldwide. Three genes for resistance to Sphaerotheca fuliginea (Schlecht. ex Fr.) Poll. race 1 and race 2U.S. were identified in growth chamber and greenhouse tests in the cross of PI 313970 × `Top Mark'. A recessive gene conditioned resistance of true leaves to race 1. A recessive gene appeared to condition resistance of cotyledons to race 2U.S., although a second recessive gene may be involved. A semi-dominant gene conditioned resistance of true leaves to race 2U.S. Limited data suggested linkage of the recessive gene for resistance to race 1 and the semi-dominant gene for resistance to race 2U.S. The resistance reaction of PI 313970 to infection of true leaves by race 2U.S. included water-soaked spots and resistant blisters, but segregation data for the resistant blister reaction were inconclusive. Allelic relationships of these genes with previously reported melon powdery mildew resistance genes remain to be determined.

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