2013 ASHS Annual Conference

Verticillium wilt in lettuce (Lactuca spp.) is a threatening disease caused by the soil borne fungus Verticillium dahliae. In lettuce, two races of the pathogen are known (race 1 and 2). Resistance to race 1 is controlled by a single dominant gene named Verticillium resistance 1 (Vr1) located on linkage group 9 in  'La Brillante'. Several other lettuce cultivars also express resistance to race 1; however, no genetic studies have been conducted to determine if Vr1 is responsible for the resistance in them. Approximately 300 F₂ plants from crosses between 'La Brillante' and 'Annapolis', 'Merlot', 'Little Gem', 'Pavane', and 'Eruption' were assessed for their resistance to race 1 V. dahliae isolate Ls16 in the greenhouse, and 238 F₂ plants of the cross 'La Brillante' x 'Merlot' in the field. The parents of each cross and the susceptible 'Salinas' were also included. Each test of Salinas or a parent cultivar consisted of four reps.; five plants per rep. Each plant was assessed for root symptoms (vascular discoloration) and foliar wilting. Crown sections of each symptomatic F₂ plant were plated on semi-selective NP10 media to confirm that the symptoms were due to V. dahliae. The fungus was isolated from the positive plates, single-spored, and the race determined by PCR using race-specific primers. A plant with any degree of root discoloration or foliar wilting and from which V. dahliae was recovered was considered symptomatic for Verticillium wilt. ‘Salinas’ exhibited > 50% incidence of symptomatic plants in greenhouse experiments and 80% in the field experiment, while no symptomatic plants were detected in the resistant parents in the greenhouse experiments or the field experiment. No symptomatic F₂ plants were detected in the greenhouse experiments from crosses of ‘La Brillante’ with ‘Annapolis’, ‘Merlot’, ‘Little Gem’, ‘Pavane’, or ‘Eruption’. An insignificant percentage (0.8%) of F₂ plants were symptomatic in the field test. The F₂ families in these experiments either did not segregate or segregated in an unacceptable fit to a two independent dominant gene (15 non-symptomatic : 1 symptomatic) model. Resistance to V. dahliae race 1 in lettuce appears therefore to be controlled by Vr1 or a closely linked gene in  'Annapolis', 'Merlot', 'Little Gem', 'Pavane', and 'Eruption'.