Harvest of Lettuce from Verticillium-Infested Fields Has Little Impact on Postharvest Quality

Verticillium wilt of lettuce, caused by the soilborne pathogen Verticillium dahliae, poses a serious threat to the California lettuce industry. Knowledge of disease development and its impact on postharvest marketability would facilitate better management of the affected fields. This study investigated postharvest marketability of 22 lettuce varieties harvested from two Verticillium-infested commercial lettuce fields in Salinas and Watsonville, CA, in 2005 using a randomized complete block design. Periodic sampling to monitor disease in several crisphead varieties in the field demonstrated that root symptoms developed quickly at later stages of heading, followed by the onset of foliar symptoms as the crop reached harvest maturity. Harvested marketable heads were vacuum cooled soon after harvest to about 4°C and maintained at this temperature in commercial coolers. The impact of V. dahliae on postharvest marketability was assessed based on the percentage of heads per case deemed marketable following 1, 2, and 3 weeks of refrigerated storage. Across both field experiments, the average disease incidence and postharvest marketability ranged from 4.2 to 87.5% and from 69.4 to 100.0%, respectively, among lettuce types and varieties. The Pearson correlation analysis detected no significant relationship between disease incidence and postharvest marketability across all varieties tested (r = 0.041, P = 0.727), or within lettuce types, even though V. dahliae was recovered from 34% of the plants harvested, and recovery ranged from 0 to 73.3% for V. dahliae and from 10 to 91.7% for non-V. dahliae (V. isaacii or V. klebahnii) species. These findings demonstrate that growers can harvest lettuce from an infested field before foliar symptoms develop with negligible impact by Verticillium spp. on postharvest marketability or quality.