Identification of Xanthomonas fragariae, the cause of an outbreak of angular leaf spot on strawberry in South Australia, and comparison with the cause of previous outbreaks in New South Wales and New Zealand

Abstract

In February 1994, symptoms consistent with angular leaf spot were found on strawberry plants on three properties in the Kenton Valley region of South Australia. To confirm the identity of the causal organism, bacteria were isolated from affected tissue and processed for analysis using whole genome DNA fmgerprinting and repetitive element polymerase chain reaction (PCR). Whole genome and PCR fingerprints from the isolated bacteria were compared to those generated from known isolates of Xanthomonas fragariae originally collected from New South Wales and New Zealand in 1975 and 1971–2, respectively The 1994 South Australian isolates exhibited a high degree of similarity to the New South Wales and New Zealand isolates, confirming their identity as X. fragariae. The New South Wales and New Zealand isolates had identical genomic and PCR fingerprints, suggesting that they were derived from the same source, in all probability a multiplication scheme from which both areas obtained runners in the early 1970s. The differences between the 1994 outbreak and those in New South Wales and New Zealand show that a separate introduction of X. fragariae into South Australia has occurred in recent times. These three outbreaks highlight the problems of relying on passive detection of this bacterium, which requires specific environmental conditions for symptom expression.