Identification and pathogenicity of Colletotrichum species associated with twig dieback of citrus in Western Australia

Up to 32 Colletotrichum species have been reported to be associated with pre- or postharvest diseases of citrus globally, while in Australia, six species have been reported to cause citrus leaf and fruit disease. Twig or shoot dieback has recently been observed as an emerging disease in citrus orchards in Western Australia. Colletotrichum species were isolated from diseased twigs showing dieback (withertip) or lesions, with or without gummosis, collected from 12 varieties of orange, mandarin and lemon. Colletotrichum gloeosporioides sensu stricto, Colletotrichum karstii and Colletotrichum novae-zelandiae were identified using a polyphasic approach that included multigene phylogenetic analysis using sequences of internal transcribed spacer and intervening 5.8S nrDNA (ITS), glyceraldehyde-3-phosphate dehydrogenase (gapdh), β-tubulin (tub2), actin (act) and histone (his3) for isolates in the boninense species complex, and Apn2–Mat1–2 intergenic spacer and partial mating type (Mat1–2) (ApMat) and glutamine synthetase (gs) for isolates in the gloeosporioides species complex, as well as morphological characteristics. C. gloeosporioides was the most prevalent species associated with twig dieback in Western Australia, while C. novae-zelandiae was reported for the first time in Australia. Pathogenicity tests on shoot twigs from lemon and orange trees confirmed C. gloeosporioides, C. karstii and C. novae-zelandiae as the cause of twig dieback, with C. gloeosporioides being the most aggressive species. Knowledge of the species causing twig dieback and their lifestyle will assist the development of integrated control methods.