Lettuce (Lactuca sativa L.) is a globally important horticultural crop, and many pre-existing and emerging diseases constrain production throughout the world. Host resistance is often the most sustainable and economically viable control method, if available (Lebeda et al., 2009). To achieve this, continual screening for new/additional resistances is necessary because new diseases emerge or change. Verticillium wilt caused by Verticillium dahliae Kleb. is a serious disease of lettuce that was first identified in the central coast of California in 1995 (Subbarao et al., 1997). The disease has subsequently spread within coastal California (Atallah et al., 2010) and has been identified in the Mediterranean basin (Garibaldi et al., 2007; Ligoxigakis et al., 2002). The pathogen is seed-transmitted in lettuce and other crops commonly grown in rotation with lettuce, raising concerns regarding its spread to other lettuce production areas (Atallah et al., 2010; Vallad et al., 2005). The disease is particularly destructive on lettuce, and all market types are susceptible. Plants often remain symptomless until they near harvest maturity, at which time the symptoms develop quickly.
Host resistance is the best long-term control method in lettuce, because current cultural control methods are cost-prohibitive, potentially damaging to the environment, or of limited feasibility (Subbarao et al., 1997). Verticillium dahliae of lettuce exists as two pathogenic races (Race 1 and Race 2). The Batavian cultivar La Brillante and several other heirloom cultivars are resistant to Race 1 isolates, whereas no source of resistance against Race 2 isolates is known (Hayes et al., 2007; Vallad et al., 2006). The system is similar to that described in tomato (Alexander, 1962; Vallad et al., 2006), and the pathogenicity of isolates (Race 1 or Race 2) from lettuce and tomato is strongly correlated (Maruthachalam et al., 2010). Race 1 resistance in ‘La Brillante’ is complete (no symptom development) (Vallad and Subbarao, 2008) and continues to be effective in grower fields for the time being (Hayes et al., 2007). In tomato, widespread use of cultivars carrying the Race 1 resistance gene Ve+ led to the discovery of resistance-breaking Race 2 isolates that now predominate in many tomato production regions (Alexander, 1962; Pegg and Brady, 2002). Although the existence of Race 2 lettuce isolates in California production districts has been established (Maruthachalam et al., 2010), no fields that are predominantly or exclusively infested with Race 2 isolates have been reported. Regardless, it is highly likely that production of Race 1-resistant lettuce cultivars will increase the frequency of Race 2. Therefore, there is a need to rapidly identify sources of resistance to Race 2 isolates of V. dahliae in lettuce.
Substantial diversity exists within cultivated lettuce and wild relatives for resistance to biotic and abiotic stresses and for horticultural traits (Ryder, 1999). The primary gene pool of lettuce comprises germplasm that is fully interfertile with L. sativa, including the wild species L. serriola and several L. serriola-like species. Lactuca saligna and L. virosa represent the secondary and tertiary gene pools, respectively (Lebeda et al., 2009; Ryder, 1999). All are 2n = 2x = 18 and are autogamous, resulting in cultivars, landraces, accessions, and wild populations of highly homozygous plants. Within cultivated lettuce, numerous market types are known, including crisphead, romaine, butterhead, Latin, and red or green leaf and the less widely grown stem and oil seed types (Ryder, 1999). Cultivated lettuce may be derived from multiple gene pools (Kesseli et al., 1991), and each lettuce type may represent an important and unique resource for the identification of useful new genes.
Screening diverse Lactuca for disease resistance is often complicated by correlations with morphology, maturity, or rate of bolting. In the V. dahliae–lettuce pathosystem, disease severity and incidence may be related to market or harvest maturity as well as to reproductive maturity (Hayes et al., 2007; Vallad and Subbarao, 2008). Disease evaluations must be delayed past market maturity or until flowering to reduce the number of lines incorrectly categorized as resistant. Furthermore, foliar symptoms can be similar to other diseases or disorders. Examination of root and foliar symptoms in conjunction with plating plant samples on semiselective media may be required for accurate evaluations. Taking these factors into account, successful strategies were used to identify resistance to Race 1 isolates (Hayes et al., 2007) and are expected to successfully identify resistance to Race 2 isolates. The objective of this research was to identify accessions with resistance to Race 2 isolates of V. dahliae.
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