Effects of temperature and humidity on diseases caused by Phoma medicaginis and Leptosphaerulina trifolixs in lucerne (Medicago sativa)

The post-inoculation temperature and humidity conditions most favourable for development of Phoma black stem and leaf spot (Phoma medicaginis) and Leptosphaerulina leaf spot (Leptosphaerulina trifolii) diseases on lucerne (Medicago sativa) were determined using controlled-environment cabinets. P. medicaginis caused more severe leaf disease at day/night temperatures of 21/16 and 18/13°C compared with 15/10°C; petiole disease was most severe at 21/16°C compared with 18/13°C, and least severe at 15/ 10°C. L. trifolii generally caused more severe leaf disease at 18/13°C compared with 21/16°C or 15/10°C, but the most severe petiole disease occurred at 21/16°C with least at 15/10°C Leaf disease caused by either P. medicaginis or L. trifolii was always more severe with the longest period (168 h) of high-humidity incubation and, generally, increased with increasing periods of high humidity. The severity of petiole disease was much less affected than that of leaf disease by an increase in the period of high-humidity incubation.