Lettuce (Lactuca sativa L.) is a major leafy vegetable that is grown and harvested year-round in the United States (Davis et al., 1997). California and Arizona produce 68% and 22%, respectively, of the lettuce grown in the United States (Mou, 2009) with the balance produced in 17+ other states (USDA, 2011a). Head and leaf lettuce crops were harvested from 56,660 ha in Monterey County, CA, and valued at $1.24 billion in 2010 (Lauritzen, 2011). Fresh lettuce leaves are commonly consumed in salads or sandwiches in the United States, Europe, and Australia, but in some countries, elongated stems are eaten; raw in Egypt; and cooked in China (Ryder, 2002).
Lettuce aphid, Nasonovia ribisnigri Mosley (Hemiptera: Aphididae), is an economically important pest of lettuce in Europe (Arend et al., 1999), Canada (Forbes and MacKenzie, 1982), the United States (Chaney, 1999; Palumbo, 2000), New Zealand (Stufkens and Teulon, 2003), Tasmania (Stufkens et al., 2002, 2004), and Australia (Anonymous, 2004). Lettuce aphid may colonize lettuce at any time from the seedling stage of growth onward. High densities of lettuce aphid can deform lettuce heads and change leaf color. Lettuce aphid is also listed as a vector of Cucumber mosaic virus and Lettuce mosaic virus (Blua, 1997), although no incidents of virus transmission have been reported from any lettuce production areas. Lettuce aphid not only reduces marketability of infested lettuce in domestic markets but poses as a phytosanitary barrier for lettuce exports to some overseas markets such as Japan and Taiwan where lettuce aphid is quarantined.
Lettuce aphid is difficult to manage. Its propensity for colonizing the young (core) leaves makes it virtually impossible to control with contact insecticides during head maturation (Liu, 2004). In addition, it can develop resistance to insecticides (Barber et al., 1999; Stufkens and Wallace, 2004). Postharvest control of lettuce aphid on exported lettuce is also challenging. Methyl bromide fumigation causes injuries to lettuce and there are no safe alternative fumigants. Although ultralow oxygen treatment was reported to be safe and effective to control lettuce aphid on harvested head lettuce, the treatment was not effective against leafminer, Liriomyza langei Frick, which is also quarantined in Japan and often intercepted on exported lettuce (Liu, 2005). Genetically based host plant resistance is an economical and environmentally desirable means to control lettuce aphid infestation of lettuce (Painter, 1980).
Two biotypes of lettuce aphid have been known in Europe since 2007 and were designated by The Netherlands Inspection Service for Horticulture (Naktuinbow) as Nr:0 and Nr:1 (Thabuis et al., 2011). Biotype Nr:0 is known worldwide, and biotype Nr:1 is, thus far, known only in Europe. Complete and partial types of resistance to Nr:0 were described in Lactuca virosa L., a wild, distant relative of cultivated lettuce (Eenink and Dieleman, 1983). Complete resistance to Nr:0 was the result of a single gene (Nr) that was partially dominant in segregating (F2) families from resistant L. virosa × susceptible L. virosa crosses but completely dominant after introgression to cultivated lettuce, L. sativa (Eenink and Dieleman, 1983; Eenink et al., 1982b). Partial resistance to Nr:0 in L. virosa was the result of a recessive allele, nr, at the same locus (Eenink and Dieleman, 1983). Complete resistance to Nr:0 in L. virosa accession IVT 280 was transferred to cultivated lettuce (Arend et al., 1999) and is highly effective against California strains of Nr:0 (Liu and McCreight, 2006).
Sixty-four L. serriola and L. virosa accessions in the Center for Genetic Resources, The Netherlands (CGN) collection were reported resistant to Nr:1 (Anonymous, 2008). Dominant resistance to Nr:0 and Nr:1 was claimed in L. serriola accession 10G.913571 (Thabuis et al., 2011).
The potential for the breakdown or failure of resistance conferred by the Nr gene was recognized shortly after release of Nr:0-resistant cultivars (Arend, 2003) and subsequently realized in Europe where the Nr gene was widely deployed (Thabuis et al., 2011). Although the gene has not been widely deployed in the United States as a result of different market requirements and numerous production niches (Davis et al., 1997), alternative sources of resistance to Nr:0 conditioned by other genes or mechanisms are desirable to prolong the effective life of Nr:0-resistant lettuce cultivars. Two potentially unique sources of resistance to Nr:0 were found in a survey of ≈1200 cultivated and wild lettuce accessions (McCreight, 2008). Our objectives were to characterize and determine the inheritance of resistance to Nr:0 in Lactuca serriola accession PI 491093 and L. virosa accession PI 274378.
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