Role of Crop Debris and Weeds in the Epidemiology of Bacterial Leaf Spot of Lettuce in California

Abstract

Bacterial leaf spot of lettuce (BLS), caused by Xanthomonas campestris pv. vitians, has increased in importance in California over the past 5 years. The pathogen can be seedborne, but it was not recovered from selected commercial lettuce seed lots planted during this time. Survival of X. campestris pv. vitians in association with plant debris was investigated in a 3-year field experiment in Salinas, CA. The initial lettuce spring crop was spray inoculated with X. campestris pv. vitians, which resulted in 100% disease incidence. Spring crops were followed by a 1-month summer fallow period, whereas fall crops were followed by a 5-month winter fallow period. High populations of X. campestris pv. vitians (up to 106 CFU/g) were recovered from lettuce plant debris after the 1-month summer fallow and BLS developed on all subsequent fall lettuce crops. During the winter fallow period, X. campestris pv. vitians populations associated with plant debris declined and, by 2 months after harvest, only small populations were detected. Spring crops also developed BLS, but at reduced levels. X. campestris pv. vitians was recovered from leaves of several symptomless weed species collected around commercial infested fields, but not from weeds collected around previously infested fields during fallow periods. During the course of this study, an X. campestris pv. vitians-specific polymerase chain reaction primer pair was developed.