First Report of Colletotrichum johnstonii Causing Fruit Rot of Citrus australasica in Australia

Abstract

In February 2018, fruit rot was observed on finger lime (Citrus australasica) in Rosebank, New South Wales, Australia. The disease was characterized by initial brown lesions on the fruit rind, which expanded to show white to pale grey mycelia. The causal organism was isolated from tissue samples and identified as Colletotrichum johnstonii based on morphological characteristics such as flat colonies with entire margins, white to pale grey aerial mycelium and orange acervuli. Conidia were hyaline, smooth-walled and cylindrical, while appressoria were pale to medium brown and elliptical. The pathogen was confirmed through sequence analysis of ITS, gapdh, tub2, act and his3 genes, showing 99.60 to 100.00% similarity to known C. johnstonii sequences. Pathogenicity was demonstrated by inoculating finger lime fruits with a conidial suspension, and C. johnstonii was re-isolated from inoculated fruits, satisfying Koch's postulates. This is thought to be the first report of C. johnstonii causing citrus fruit rot in Australia.