Iceberg Lettuce Breeding Lines with Resistance to Verticillium Wilt Caused by Race 1 Isolates of Verticillium dahliae

The Agricultural Research Service, U.S. Department of Agriculture and the University of California, Davis, announce the release of two breeding lines of lettuce (Lactuca sativa L.). Lines RH08-0472 and RH08-0475 are F9 iceberg-type lettuce breeding lines with resistance to verticillium wilt caused by V. dahliae. They were selected from the cross ‘Tiber’ · (‘La Brillante’ · ‘Pacific’). Resistance is derived from ‘La Brillante’, a Batavia-type lettuce cultivar with resistance to Race 1 isolates of V. dahliae. These breeding lines are suitable for commercial production in both V. dahliaeinfested and non-infested fields. They are being released for commercialization of verticillium wilt-resistant iceberg cultivars and for use as parents in further breeding of resistant cultivars. They have been deposited in the National Plant Germplasm System as W6 39944 (RH080472) and W6 39945 (RH08-0475). Verticillium wilt, caused by the soilborne fungus Verticillium dahliae Kleb. , is a serious soilborne disease affecting a broad range of economically important crops (Pegg and Brady, 2002). Lettuce (Lactuca sativa L.) was not considered a host for V. dahliae until the mid1990s, when the disease was discovered in coastal production districts of California (Subbarao et al., 1997). The disease has subsequently spread within coastal California (Atallah et al., 2010) and has also been detected on lettuce in the Mediterranean basin (Garibaldi et al., 2007; Ligoxigakis et al., 2002). Verticillium dahliae is seed-transmitted in lettuce and other vegetable crops grown in rotation with lettuce, raising concerns regarding its spread to other lettuce production areas (Atallah et al., 2010; Vallad et al., 2005). Although all types of lettuce are susceptible, verticillium wilt is most damaging to icebergtype cultivars. Plants often remain symptomless until they near harvest maturity, at which time the symptoms develop quickly. Basal or ‘‘wrapper leaves’’ that completely cover the outer part of the head wilt and then collapse as the disease progresses, leading to premature plant death and an unharvestable head. Other key foliar symptoms include stunting, defoliation, and other developmental abnormalities. Before the onset of foliar symptoms, the vascular discoloration of root and stem tissues, revealed by vertical sectioning of the plant, are the only other diagnostic features. Host resistance is the best long-term control method in lettuce, because current cultural and chemical control methods are cost-prohibitive, potentially damaging to the environment, or of limited feasibility (Subbarao et al., 1997). Two pathogenic races (Race 1 and Race 2) of V. dahliae on lettuce have been described. The Batavian cultivar La Brillante and several other heirloom cultivars are resistant to Race 1 isolates, whereas no known source of resistance to Race 2 isolates has been reported (Hayes et al., 2007; Vallad et al., 2006). Race 1 resistance in ‘La Brillante’ is complete (no symptom development) (Vallad and Subbarao, 2008) and continues to be effective in grower fields (Hayes et al., 2007). We are reporting on the development of iceberg breeding lines RH08-0472 and RH08-0475, which have resistance to Race 1 isolates of V. dahliae inherited from ‘La Brillante’.