Brown spot, a newly recorded disease of Murcott tangor in Queensland, was demonstrated to be caused by Alternaria alternata (previously Alternaria citri), the fungal pathogen causing brown spot of Emperor mandarin. The strain of A. alternata isolated from Murcott tangor had an identical host range to that of the Emperor mandarin strain; a host range test with an isolate from Emperor mandarin confirmed previous work. The genetic basis of susceptibility to the mandarin pathotype of A. alternata is discussed in relation to progeny of Murcott tangor crosses and progeny of the susceptible mandarin cultivar Dancy.