Temporal sporulation of Thekopsora areolata and Chrysomyxa spp. in Finnish Norway spruce seed orchards

•Thekopsora areolata incidence varied greatly among years in Finnish Norway spruce seed orchards.•Amount of rainfall in May-June explained the annual T. areolata incidences on Prunus padus.•Rainfall, temperature, time and their interactions were significant variables in a disease model.•To control T. areolata the number of overwintering diseased Prunus leaves should be minimized.•Control of C. pirolata should concentrate on Pyrola in early May and on P. abies in late May.