Abstract
Studies conducted during 2009-2013 at the Experimental Orchard located in the central region of Poland showed that the highest number of V. inaequalis pseudothecia occurred on ‘McIntosh’ and ‘Cortland’ leaves. A smaller number of pseudothecia was found on the leaves of the Alwa and Jonagold cultivars, but there were no statistical differences. The first discharges of ascospores were noted at a similar time on the leaves of all of the apple cultivars, usually at the end of March or in early April. For the forecasting of ascospore discharges and dates of critical periods, a simulation model by RIMpro was used. The number of critical periods and apple scab severity differed in each season. The most critical periods (above 20) were recorded in 2010 and 2013, which also corresponded with a high severity of apple scab. The number of infected leaves and fruits on the untreated trees of the very susceptible cultivar McIntosh reached nearly 100% during these seasons. In other years, the severity of the disease on the same apple trees was lower and amounted to 30-54% of infected leaves and 23-80% of infected fruits.