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Edward J. Ryder

Segregation data from crosses between necrotic and mottled lettuce (Lactuca sativa L.) parents showed that a single gene controls the difference in type of reaction to lettuce mosaic virus: necrosis is dominant to mottled. Segregation data from crosses between resistant and necrotic parents differed, depending on the necrotic parent. In crosses with the necrotic cultivar Prizehead, there were two independent genes, one controlling necrotic vs. mottled and the other resistant vs. susceptible. In a cross with the necrotic cultivar Maikonig, resistance was epistatic to necrotic, suggesting a second necrotic allele. Crosses among necrotic cultivars indicated a single gene for the necrotic reaction, with the possibility of more than one necrotic allele. Necrotic alleles identified are named Necrotic-1m and Necrotic-1p.

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Beiquan Mou and Edward J. Ryder

insecticides ( Keil and Parrella, 1990 ; Mason et al., 1987 ; Parrella and Trumble, 1989 ). Therefore, it is essential to develop alternative strategies for leafminer management, including the deployment of resistant varieties. Lettuce mosaic virus (LMV

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Beiquan Mou, Ryan J. Hayes and Edward J. Ryder

deployed in most resistant lettuce cultivars. Lettuce mosaic virus (LMV) causes serious disease problems in the production of lettuce worldwide. Plants susceptible to LMV exhibit systemic symptoms of vein clearing, mottling, leaf recurving, leaf margin

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Edward J. Ryder

In crosses of lettuce (Lactuca sativa L.) between parents producing a mild or susceptible reaction to lettuce mosaic virus, a single gene segregated. The heterozygote reacted in an intermediate manner. In crosses between mild-reacting and resistant parents, the mild reaction gene and the resistant gene segregated independently. The resistant and mild alleles together produced a new phenotype that is usually symptomless. The gene symbol proposed is Mi'Mi, where Mi' gives the mild phenotype. Breeding is in progress to combine the mild and resistant traits in new lettuce cultivars.

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Edward J. Ryder and Bert J. Robinson

Screening for lettuce (Lactuca sativa L.) big-vein resistance in the F2 generation is highly inefficient. Efficiency improves in the F3 and following generations with continued inbreeding. Traits useful in ascertaining resistance are 0% of plants showing symptoms and percentage of plants showing symptoms at a given date. Breeding lines identified as resistant in greenhouse screening have proved resistant under field conditions. Forty-nine cultivars have been identified in preliminary testing as potentially resistant. Of these, 11 have been confirmed as resistant in greenhouse and field tests.

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R.T. Nagata, V.L. Guzman, L.E. Datnoff and R.N. Raid

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V.L. Guzman, R.T. Nagata, L.E. Datnoff and R.N. Raid

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Edward J Ryder

Lettuce mosaic has been a serious virus disease for lettuce in all locations worldwide where lettuce has been grown. Consequently, the disease and its virus have been well studied. Lettuce plants react to lettuce mosaic virus in a variety of ways. The most common susceptible reaction is an overall vein clearing and mottling, followed by leaf recurving, leaf distortion, and stunting. However, some susceptible types manifest a mild mottling with little additional distortion. Others develop a necrotic reaction, which may be severe, mild, or seasonal. Finally, there are at least three resistant reactions, most frequently appearing as a systemic infection manifested with restricted yellowish lesions. Research is ongoing to sort out the various reactions and their genetic bases. This report describes the inheritance of the severe necrotic reaction and its relationship to the resistant reaction conferred by the allele mo-1. Several previous crosses among necrotic types indicate that the same necrotic allele is operating except that found in `Bibb'. Several crosses were studied. The cross `Salinas' (mot.) × `Crisp As Ice' (nec.) showed that necrotic is due to a single dominant allele. The cross `Salinas 88' (res.) × `Maikonig' (nec.) produced three phenotypes in F2, indicating the action of two loci. The crosses PI 251245 (res.) × `Prizehead' (nec.) and `Vanguard 75' (res.) × `Prizehead' disclosed two recombinant phenotypes, mottled and resistant-necrotic. Necrotic is dominant to nonnecrotic in both susceptible and resistant phenotypes. The genes are inherited independently.

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Ivan Simko, Ryan J. Hayes, Krishna V. Subbarao and Rebecca Grube Sideman

marker analysis revealed that both breeding lines and parental material carry the Mo1 allele (Simko, unpublished results) that confers susceptibility to Lettuce Mosaic Virus (LMV) ( Nicaise et al., 2003 ). This confirms earlier greenhouse testing, which

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Sylvie Jenni, David de Koeyer and George Emery

et al., 1999 ), head development ( Bassett, 1975 ), flowering time ( Ryder, 1996 ), and dwarfism ( Waycott et al., 1995 ); 2) resistance to diseases such as downy mildew [ Bremia lactucae Regel ( Norwood et al., 1985 )], lettuce mosaic virus ( Ryder