In vitro effects of temperature and wet periods on infection of oilseed rape by Alternaria brassicae

The influence of temperature and duration of wet periods on infection of oilseed rape by Alternaria brassicae was studied on detached leaves and pods, leaf disks and intact seedlings. Infections increased with age of leaf and the interaction between temperature and leaf age was highly significant. On older leaves infection was optimal at 25°C. There were many infections also at 15, 20 and 29 C but relatively few infections at 10°C. On pods most infections were observed at 20 C, the highest temperature studied. Infection at each temperature increased progressively with duration of surface wetness. The minimum wet periods for infection of leaves were 3 h at 20–25°C, 4 h at 15°C, 6–9 h at 10 C and 12–24 h at 5 C and for infection of pods, between 6 h and 9 h at 10°C and 6 h (or less) at 15°C and 20 C. On leaves, dry periods interrupting wet periods limited lesion development to that obtained with the initial wet period only; on pods some further infections developed when pods were re‐wetted. Dry periods of 3 h and 6 h following the inoculation of pods reduced subsequent infection but there was no further reduction by longer periods of drying to 48 h.