Lowbush blueberry, a valuable horticultural crop managed as a wild plant, is prone to leaf spot and stem canker [Sphaerulina amelanchier] which causes premature defoliation in fruiting fields and affects yields and fruit quality. In this study, we investigated the impact of relative humidity (RH), temperature, photoperiod and plant age on disease development and progress under controlled conditions. With an increase in the accumulated periods of high RH (>95%), leaf infections increased on the inoculated plants with a 4-day exposure showing high severity. Exposure to high RH (>95%) was more important than wet periods for leaf infections as inoculated plants kept in a high RH chamber for 3 days showed a high leaf spot severity after 3 weeks. Inoculated blueberry leaves kept in a Petri dish chamber under free water or high RH (?97.5%), or inoculated plants kept in a high RH humid chamber, showed 70.5?90.5% spore germination after 3 days. Leaf spot severity on the inoculated plants increased over time reaching maximum after 30 days. Inoculated plants kept for 3 days under a 14-h-photoperiod showed high leaf infections, increased spore germination (51.56%) and increased mycelium penetration (19.56%) after 4 weeks of inoculation. Inoculated plants showed very high leaf infections at 20 and 25°C and leaf spot severity increased with an increase in exposure time at high RH (>95%). Incubation temperatures of 20 and 25°C also showed high leaf spot severity under continuous high RH (>95%). Leaf spot severity was 2.69-fold higher on 4-week-old plants compared with 8-week-old plants.