Diagnosis of Xanthomonas axonopodis pv. citri, causal agent of citrus canker, in commercial fruits by isolation and PCR-based methods

AIMS

To show the results of the detection of an EU quarantine organism, Xanthomonas axonopodis pv. citri (Xac), in citrus fruits imported from countries where this bacterium is present, using an integrated approach that includes isolation, pathogenicity assays and molecular techniques.

METHODS AND RESULTS

Citrus fruits with canker-like symptoms, exported to Spain from South American countries were analysed by several methods. Bacterial isolation, three conventional polymerase chain reaction (PCR) protocols, and real-time PCR with SYBR Green or a TaqMan probe, were compared. Canker-like lesions were disrupted in PBS buffer, and the extract used for bacterial isolation and DNA extraction followed by PCR amplification. Canker lesions, identified by PCR, showed viable bacteria in eleven of fifteen fruit samples. In 16 out of 130 lesions analysed from these samples, Xac was isolated, and pathogenicity on grapefruit leaves confirmed. By real-time PCR, using SYBR green or a Taqman probe, Xac was detected in 58 and 80 lesions respectively. By conventional PCR the bacterium was detected in 39-52 lesions depending on the protocol employed.

CONCLUSIONS

An integrated approach for reliable detection of Xac in lesions of fruit samples, employing several techniques and with real-time PCR using a TaqMan probe as the fastest and most sensitive screening method, has been established and validated and is proposed as a useful tool for the analysis of Xac on fresh fruits.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work faces up to the real threat of the importation of citrus fruits that can harbour quarantine bacteria and will be useful in diagnostic laboratories for the analysis of commercial fresh fruits from countries where citrus canker is present.

Development of a highly sensitive nested-PCR procedure using a single closed tube for detection of Erwinia amylovora in asymptomatic plant material

A novel method, which involves a nested PCR in a single closed tube, was developed for the sensitive detection of Erwinia amylovora in plant material. The external and internal primer pairs used had different annealing temperatures and directed the amplification of a specific DNA fragment from plasmid pEA29. The procedure involved two consecutive PCRs, the first of which was performed at a higher annealing temperature that allowed amplification only by the external primer pair. Using pure cultures of E. amylovora, the sensitivity of the nested PCR in one tube was similar to that of a standard nested PCR in two tubes. The specificity and sensitivity were greater than those of standard PCR procedures that used a single primer pair. The presence of inhibitors in plant material, very common in E. amylovora hosts, is overcome with this system in combination with a simple DNA extraction protocol because it eliminates many of the inhibitory compounds. In addition, it needs a very small sample volume (1 microl of DNA extracted). With 83 samples of naturally infected material, this method achieved better results than any other PCR technique: standard PCR detected 55% of positive samples, two-tube nested PCR detected 71% of positive samples, and nested PCR in a single closed tube detected 78% of positive samples. When analyzing asymptomatic plant material, the number of positive samples detected by the developed nested PCR was also the highest, compared with the PCR protocols indicated previously (17, 20, and 25% of 251 samples analyzed, respectively). This method is proposed for the detection of endophytic and epiphytic populations of E. amylovora in epidemiological studies and for routine use in quarantine surveys, due to its high sensitivity, specificity, speed, and simplicity.

A simple extraction procedure for efficient routine detection of pathogenic bacteria in plant material by polymerase chain reaction

A simple and rapid method for extracting DNA from plants based on the use of an extraction buffer and precipitation with isopropanol was assayed to see its usefulness in detecting pathogenic bacteria in plant material. The method was compared with a phenol-chloroform standard procedure obtaining higher sensitivity levels of detection. The protocol developed was efficient for detecting a Gram-positive bacterium, Clavibacter michiganensis subsp. sepedonicus and several Gram-negative pathogenic bacteria (Ralstonia solanacearum, Erwinia amylovora, Xanthomonas axonopodis pv. citri) with a sensitivity of 10(2)-10(3) cfu/ml in spiked samples. It was also efficient to specifically identify such bacteria in naturally infected plant material. This procedure is proposed as a routine tool for detection of plant pathogenic bacteria, as well as in environmental microbiology and biotechnology studies.

A simple and efficient PCR method for the detection of Agrobacterium tumefaciens in plant tumours

A simple PCR protocol was developed for identifying Agrobacterium as the causal agent of the tumours produced by this bacterium in plant material. The sensitivity of this method was compared with that of bacterial isolation using common and selective media with a previous enrichment step. More than 200 samples from tumours of naturally infected and inoculated plants from several hosts including almond, peach x almond hybrids, apricot, rose, tobacco, tomato, raspberry, grapevine and chrysanthemum, were analysed by both methods. PCR was the most efficient method for detecting the bacterial aetiology of the plant tumours. Agrobacterium tumefaciens was better detected in crown and root tumours than in aerial tumours with all the methods assayed in inoculated plants. A comparison between the efficiency of the diagnosis by analysing pieces from the external and internal part of the tumour showed no differences between them.