Asiatic citrus canker detected in a pomelo orchard in northern Australia

Raised, roughly circular, corky scabs 4-5 mm in diameter and typical of citrus canker were observed on spring flush leaves, twigs, and fruits of pummelos (Citrus grandis) in a young orchard near Darwin, Australia, during a survey conducted in 1991 as part of the Northern Australia Quarantine Strategy. The causal agent was identified as Xanthomonas campestris pv. citri (Asiatic group, or group A), using pathogenicity in a series of hosts, fatty acid profiles, and DNA fingerprints. The 10 strains from Darwin were compared with two previously identified strains of X. c. citri (group A) from a canker outbreak (which has since been eradicated) on Thursday Island in the Torres Strait. Symptoms on inoculated leaves of sweet orange, West Indian lime, sour orange, and Duncan grapefruit included lesions of eruptive, calluslike white tissue and were produced by all strains. Lesions were larger and more erumpent on seedlings or detached leaves of West Indian lime, sweet orange, and Duncan grapefruit. On citrange C-35 (Poncirus trifoliata x C. sinensis) leaves, calluslike lesions were produced by Thursday Island strains, and small, light tan, necrotic areas were produced by the Darwin strains. The fatty acid profiles of the Darwin strains (with similarity indices of 0.610 and 0.810). All Darwin strains had identical DNA restriction patterns, which were similar (with a similarity coefficient of 94%) but not identical to those produced from the reference Thursday Island strains. These results confirm that the canker outbreak near Darwin was caused by X. c. citri (group A). All citrus trees within the diseased orchard have been destroyed. No further outbreaks have been detected.