Role of crop debris and weeds in the epidemiology of bacterial leaf spot of lettuce in California

Bacterial leaf spot of lettuce (BLS), caused by Xanthomonas campestris pv, vitians, has increased in importance in California over the past 5 years. T he pathogen can be seedborne, but it was not recovered from selected commer cial lettuce seed lots planted during this time. Survival of X. campestris pv. vitians in association with plant debris was investigated in a 3-year f ield experiment in Salinas, CA. The initial lettuce spring crop was spray i noculated with X. campestris pv. vitians, which resulted in 100% disease in cidence. Spring crops were followed by a 1-month summer fallow period, wher eas fall crops were followed by a 5-month winter fallow period. High popula tions of X. campestris pv. vitians (up to 10(6) CFU/g) were recovered from lettuce plant debris after the 1-month summer fallow and BLS developed on a ll subsequent fall lettuce crops. During the winter fallow period, X. campe stris 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 r ecovered from leaves of several symptomless weed species collected around c ommercial infested fields, but not from weeds collected around previously i nfested fields during fallow periods. During the course of this study, an X . campestris pv. vitians-specific polymerase chain reaction primer pair was developed.