Infection and decontamination of citrus canker-inoculated leaf surfaces

Citrus canker (Xanthomonas citri subsp. citri, Xcc) is now considered endemic in Florida and continues to spread. Various surfaces, including plant material, personnel and equipment can become contaminated. Decontamination is practiced in both disease-endemic and disease-free areas to reduce the risk of bacterial spread by man or machinery. We used grapefruit leaf surfaces to explore the efficacy of a commonly used personnel sprayer system applying a quaternary amine decontaminant. In three experiments, plants in flush (leaves ¾ expanded) were inoculated (∼10⁵ CFU ml⁻¹). Immediately after inoculation, plants were passed through the sprayers 0, 1, 2, 3, or 6 times. Leaves were sampled at 0.5, 10 and 20 min after decontamination and tested for viable Xcc by dilution plating. There was a large and rapid decline in the quantity of live bacteria with one pass through the decontamination sprayer (>80% decrease in CFU ml⁻¹), and multiple sprays (2-6) resulted in up to 100% mortality of surface Xcc. Presumably more thorough coverage with multiple sprays killed remnant bacteria, although the first spray invariably caused the highest proportion of population mortality. The effect of the decontaminant spray was immediate (within 0.5 min only 3-11% of surface bacteria survived, and by 20 min <1-3% survived). Based on these results, use of a personnel sprayer with a quaternary amine compound is highly effective for reducing surface inoculum. A single spray kills a high proportion of the population, but multiple sprays increase mortality of Xcc. All the Xcc-inoculated plants subsequently developed symptoms of citrus canker. No significant difference in incidence or severity of grapefruit leaf infection was detected among decontamination treatments or compared to the untreated control. This finding indicates that infection occurred at, or very soon after, inoculation, and that Xcc was in protected sites inside the leaf before exposure to the decontaminant spray.