Recent surveys of canola (Brassica napus) crops across southern Australia highlighted that Alternaria leaf spot on canola is not solely caused by Alternaria brassicae but that other Alternaria spp. are also involved, including A. japonica. Studies were undertaken into the effects of different temperatures (14 and 10°C [day and night] or 22 and 17°C [day and night]) on development of Alternaria leaf spot caused by A. japonica as compared with A. brassicae in cotyledons (embryonic leaves) and true leaves (first leaves) of canola (B. napus 'Thunder TT') and mustard rape (B. juncea 'Dune'). Both pathogens expressed less disease at lower temperatures of 14 and 10°C with percent disease index (%DI) of 19.1 for A. japonica and 41.8 for A. brassicae, but expressed significantly more disease at higher temperatures of 22 and 17°C with %DI of 80.8 and 88.2 for the same pathogens, respectively. At 14 and 10°C, mustard rape cotyledons showed less disease (percent cotyledons disease index [%CDI] = 18.1) from A. japonica but showed more disease (%CDI = 75.0) from A. brassicae. However, at 22 and 17°C, cotyledons and true leaves of both canola and mustard rape showed significantly more disease and varied in expressing the disease severity to the two pathogens; true leaves of mustard rape showed less disease (percent true leaf disease index [%TDI] = 48.4) from A. japonica but showed more disease (%TDI = 92.0) from A. brassicae. At 22 and 17°C, cotyledons of canola expressed more disease from A. japonica (%CDI = 99.1) than from A. brassicae (%CDI = 70.7). At the lower temperature, both host species showed the least disease, with mean %DI of 27.3 and 33.5 for canola and mustard rape, respectively, as compared with the higher temperatures, where there was a greater DI, with %DI values of 87.9 and 81.2 for these same host species, respectively. We believe that these are the first studies to highlight the critical role played by temperature for A. japonica as compared with A. brassicae in Alternaria leaf spot disease development and severity. These findings explain how temperature affects Alternaria leaf spot severity caused by A. japonica as compared with A. brassicae on different foliage components of canola and mustard rape.