The epidemiological significance of post-packinghouse survival of Xanthomonas citri subsp. citri for dissemination of Asiatic citrus canker via infected fruit

The risk of introduction of Xanthomonas citri subsp. 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, are 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 symptom-free fruit, and to examine the potential of infected 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 non-pathogenic bacteria with cultural characteristics that were difficult to distinguish from Xcc. 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 cv. Duncan grapefruit leaves. Inoculation sites were subsequently assessed for symptoms of citrus canker and lesions were individually enumerated to confirm the presence of Xcc. In commercial packing lines in Florida and northwest Argentina, prewashing the fruit to remove dirt and debris reduced surface bacterial populations. 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. Bioassays demonstrated that the highest incidence of Xcc from fruit after the packing line antimicrobial treatment occurred with symptomatic fruit (2.5–50.6 lesions leaf−1), and zero to very low levels with fruit from apparently healthy trees (0–1.74 lesions leaf−1). Furthermore, the proportion of injection–infiltration bioassay sites that developed lesions consistently decreased with time after processing in each of the three packinghouse studies, also showing that as fruit senesce and lesions age the ability of fruit to generate or sustain Xcc bacteria was increasingly compromised. The packing line process reduced canker lesion activity by as much as 50% compared to unprocessed fruit. 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 ineffective as a source of inoculum for dispersal. Transmission from cull piles of packing line-processed fruit to surrounding trap plants, even less than 1 m away, did not occur under natural conditions. However, with severely infected piles of culled fruit subject to extreme simulated wind (25 m s−1) and rain conditions, only a single lesion, associated with leaf injury, developed on a trap plant immediately downwind of the cull pile, suggesting an exceedingly low risk of spread. Taken as a group, this series of experiments demonstrate that harvested and packinghouse-disinfested citrus fruit are extremely unlikely to be a pathway for Xcc to reach and infect susceptible citrus and become established in canker-free areas.