Delayed Wilt Symptoms Caused by Verticillium dahliae As a Resistance Characteristic in Iceberg Lettuce (Lactuca sativa)

Verticillium wilt of lettuce is caused by two races of the fungus Verticillium dahliae. Complete resistance to race 1 is conferred by the Vr1 gene, while only partial resistance to race 2 is known. Additional sources of race 2 or race non-specific resistance are needed. The disease causes discoloration of root vascular tissues followed by wilting of lower and cap leaves in iceberg lettuce resulting in plant death. Iceberg cultivars that delay symptoms past market maturity could reduce crop losses, but it is not known if this is a heritable trait. The objective was to evaluate the rate of wilting symptom development relative to market maturity in 34 iceberg cultivars adapted to diverse locations. The susceptible cultivar Salinas and the breeding line RH11-1798 carrying the Vr1 gene were used as controls. Cultivars were tested in three replicated experiments in a Salinas, CA field infested with a race 1 isolate. At three time points, each plot was evaluated for foliar symptom severity (FS, 0=no symptoms to 5=all visible leaves wilted) and head maturity (HM, 0=open heads, 3=harvest maturity to 5=over-mature heads). Ten heads per plot were evaluated for root discoloration, which was used to calculate disease incidence (DI, % symptomatic plants). Cultivars were compared using Relative Marginal Effects (RME) of ranked data in a repeated measures ANOVA. Cultivars differed significantly for HM, DI, and FS (P<0.001) at each time-point. Therefore, the time needed for each cultivar to reach market maturity and their rate of symptom development was different. FS and HM were significantly correlated at the last rating (r=0.39 P<0.001). The RME for FS on the last rating for Climax (1.7), Anuenue (2.8), Desert Storm (2.8), Reine des Glaces (3.1), and Bubba (3.2) were significantly lower than Salinas. All cultivars had significantly higher FS than RH11-1798 (0.6). The HM of these cultivars was past harvest maturity, ranging from 3.6 to 4.3 except for Bubba (2.8). Despite low FS, DI was high in Desert Storm (65%), Bubba (80%) and Climax (85%) indicating susceptibility. Iceberg cultivars with delayed foliar symptoms may be useful for reducing crop losses from Verticillium wilt. Additional research is needed to determine if these cultivars exhibit delayed foliar symptoms against both races. Crosses have been made with cultivars with delayed foliar symptoms and those that quickly develop wilting to generate populations to study the suitability of this trait for breeding resistance to V. dahliae.