Researchers at the University of Melbourne have identified an Australian strain of fungus that causes citrus anthracnose.
“Our research group at the University of Melbourne analyzed Colletotrichum collected from samples of anthracnose lesions on citrus leaves, twigs and fruit,” researchers Weixia Wang and Paul Taylor wrote. “The study identified six Colletotrichum species infecting Australian citrus. One of these is a new species — Colletotrichum australianum — named after the country where the pathogen was first identified.”
According to Wang and Taylor, the results of the study revise current thought about the diversity of Colletotrichum species that cause anthracnose of citrus in Australia. Among the four species that have been previously reported as pathogens associated with citrus anthracnose in Australia, only C. fructicola and C. gloeosporioides were found in this study.
The other three Colletotrichum species, C. theobromicola, C. karstii and C. siamense, have been recorded as pathogens of a broad range of plants in Australia (including coffee, jackfruit and fig) but never previously associated with citrus anthracnose. This is also the first time that C. theobromicola has been reported anywhere in the world as a pathogen of citrus.
“We were able to confirm not only the presence of the pathogen, but to understand which species were actually associated with causing disease,” Wang and Taylor wrote. They added that relationships between the Colletotrichum species were determined by analyzing the DNA sequences of the fungal pathogens to produce tree-like diagrams called phylogenetic trees.
The researchers also measured morphological characteristics including spore length, spore width, spore shape and colony growth rate of Colletotrichum samples. To understand the ability of each species to cause disease — it’s so-called pathogenicity — the researchers tested them on orange fruit leaves and petals and on lemon leaves.
By comparing selected gene sequences of all the Colletotrichum samples, in combination with their morphological characteristics and pathogenicity, the researchers confirmed that Collettorichum gloeosporioides was the pathogen most frequently found on diseased citrus branches, leaves and fruit.
Wang and Taylor reported that Australian mandarins have already shown up in supermarkets in Bangkok with symptoms of anthracnose. “These fruits may have been cross-infected in Thailand; however, it’s more likely these were infected prior to export, and only showed symptoms of disease as the fruit matured,” the researchers reported.
Future research is being undertaken in collaboration with colleagues in Indonesian, Thai and Philippine universities to identify Colletotrichum species that are pathogens of citrus and that may pose a biosecurity risk to Australian citrus production should exotic pathogens be allowed into Australia.