Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2009
Publication Date: 5/1/2009
Citation: Gottwald, T.R., Graham, J., Bock, C., Bonn, G., Civerolo, E.L., Irey, M., Leite, R., Lopez, M., Mccollum, T.G., Parker, P., Ramallo, J., Riley, T., Schubert, T., Stein, B., Taylor, E.L. 2009. The epidemiological significance of post-packinghouse survival of Xanthomonas citri ssp. citri for dissemination of Asiatic citrus canker via infected fruit. Crop Protection. 29:508-524. Interpretive Summary: The risk of introduction of citrus canker bacteria to new, unaffected citrus producing areas is a major concern for those citrus industries attempting to remain free of citrus canker. If introduced, quarantines and restrictions could ensue that would severely limit or deny access to domestic and international markets. This study examines the citrus fruit, as a potential pathway for citrus canker to enter and become established in these areas. At present canker is assumed to be a risk. We examined survival of canker bacteria in disease lesions on fruit and on fruit surface after disinfectant treatment as used commercial packing house. We also examined the survival of canker bacteria in culled fruit that were discarded in cull piles and conducted studies on possible dissemination of the bacteria and disease establishment in simulated rainstorms. Taken as a group, this series of studies on canker bacteria viability in and on fruit surfaces, and bacteria dissemination, demonstrate that harvested and packinghouse-disinfested citrus fruit are extremely unlikely to be a pathway for canker to reach and become established in canker-free areas.
Technical Abstract: The risk of introduction of Xanthomonas citri spp. citri (Xcc) to new, unaffected citrus producing areas is a major concern for those citrus industries attempting to remain free of citrus canker. Citrus fruit, as a potential pathway for Xcc to enter and become established in these areas, is assumed to be a risk. However, there is little information relative to the potential of harvested fruit to act as an inoculum source. A multi-national research team was established to investigate the potential of bacterial survival in infected citrus fruit lesions and as surface contaminants on disease-free fruit, and to examine fruit as a viable inoculum source. Experiments were conducted in various locations in Florida and Argentina. Bacterial recovery and culture plating were problematic due to the presence of nonpathogenic bacteria with cultural characteristics that were difficult to distinguish from Xcc in culture. Therefore, in all experiments, although culturing on semi-selective agar media was used as an indication of overall bacterial populations, bioassays were conducted via needleless injection and infiltration of suspect bacterial suspensions into susceptible ‘Duncan’ grapefruit leaves. Inoculation sites were subsequently assessed for symptoms of citrus canker and if lesions developed, they were individually enumerated to confirm the presence of Xcc. In commercial packing lines in Florida and northwest Argentina, prewashing the fruit removed dirt and debris that could tie up chlorine and antimicrobial compounds and thus reduced surface bacterial populations. Bioassay demonstrated that the highest incidence of Xcc from fruit after packing line antimicrobial treatment occurred with symptomatic fruit (2.5-50.6 lesions per leaf), and zero to very low levels with fruit from apparently healthy trees (0-1.74 lesions per leaf). As anticipated, recovery of Xcc from fruit surfaces increased when active citrus canker lesions were present but total bacterial recovery decreased after processing, and bioassays demonstrated that the quantity of viable Xcc declined as fruit remained in cold storage, or as they aged on the trees. Furthermore, the proportion of injection-infiltration bioassay sites that developed lesions consistently fell in each of the three packinghouse studies, also showing that as fruit senesce and lesions age the ability of fruit to generate Xcc bacteria is increasingly compromised. Xcc survived in wounds on mature, fruit attached to the tree, but Xcc populations declined in wounds of processed or non-processed harvested fruit. Discarded canker-infected fruit in cull piles was an ineffective source of inoculum for dispersal. Transmission from cull piles to surrounding trap plants, even less than 1 m away, did not occur under natural conditions. Both cull piles and infected fruit suspended on wires in a frame 1 m above ground level, were subjected to simulated wind and rain, and trap plants placed downwind from fruit did not become infected in several repeated experiments. However, a single lesion was formed on one trap plant 0 m distant from Xcc-infected fruit in a single experimental replicate, in simulated wind-blown splash at wind speeds greater than 25 m/s. Taken as a group, this series of studies on Xcc viability in and on fruit surfaces, and Xcc inoculum viability and dissemination, demonstrate that harvested and packinghouse-disinfested citrus fruit are extremely unlikely to be a pathway for Xcc to reach and become established in canker-free areas.