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ARS Home » Pacific West Area » Albany, California » Western Regional Research Center » Produce Safety and Microbiology Research » Research » Publications at this Location » Publication #136784

Title: MOVEMENT OF XANTHOMANAS CAMPESTRIS PV. VITIANS IN THE STEMS OF LETTUCE AND SEED CONTAMINATION.

Author
item Barak Cunningham, Jeri
item KOIKE, STEVEN
item GILBERTSON, ROBERT

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2002
Publication Date: 5/5/2002
Citation: Barak Cunningham, J.D., Koike, S.T., Gilbertson, R.L. 2002. Movement of xanthomanas campestris pv. vitians in the stems of lettuce and seed contamination. Plant Pathology. 51:506-512.

Interpretive Summary: The causal agent of bacterial leaf spot of lettuce (BLS), can be seedborne, but the mechanism by which the bacteria contaminates and/or infects lettuce seed is not known. In this study, the capacity of this bacterium to enter and translocate within the vascular system of lettuce plants was examined. The stems of lettuce plants were stab-inoculated and the bacteria recovered from 2 to 10 cm above (depending on stem length) and 2 cm below the inoculation site. The bacteria also was recovered from surface-disinfested stem sections of spray-inoculated plants. Together, these results are consistent with the bacteria invading and moving throughout the vascular system of lettuce plants. To investigate the mechanism of seed contamination, lettuce plants were spray-inoculated with the bacteria and allowed to develop BLS. Seed collected from these plants had a 2% incidence of bacterial external colonization, but no bacteria were recovered from within the seed. Although lettuce seed failed to become infected, symptomless systemic infections of lettuce stems could favor survival of the bacteria in association with lettuce debris, as stem tissue is likely to persist on or in soil longer than leaves. Symptomless systemic infections of lettuce plants could also result in external contamination of seed in the absence of disease symptoms. In such cases field inspections of seed crops would fail to reveal this source of seed contamination. Therefore, it seems advisable to perform seed assays to confirm that seed lots are not contaminated with the causal agent of BLS, particularly if such lots are to be planted in areas where conditions are favorable for disease development.

Technical Abstract: Xanthomonas campestris pv. vitians, the causal agent of bacterial leaf spot of lettuce (BLS), can be seedborne, but the mechanism by which the bacteria contaminates and/or infects lettuce seed is not known. In this study, the capacity of X. campestris pv. vitians to enter and translocate within the vascular system of lettuce plants was examined. The stems of 8- to 11-week-old lettuce plants were stab-inoculated and movement of X. campestris pv. vitians was monitored at various time points. Four, 8, 12, and 16 hours post inoculation (hpi), X. campestris pv. vitians was recovered from 2 to 10 cm above (depending on stem length) and 2 cm below the inoculation site. X. campestris pv. vitians also was recovered from surface-disinfested stem sections of spray-inoculated plants. Together, these results are consistent with X. campestris pv. vitians invading and moving systemically within the vascular system of lettuce plants. To investigate the mechanism of seed contamination, lettuce plants at the vegetative stage of growth were spray-inoculated with X. campestris pv. vitians and allowed to develop BLS. Seed collected from these plants had a 2% incidence of X. campestris pv. vitians external colonization, but no bacteria were recovered from within the seed.