Venturia inaequalis pseudothecia development in relatively warm and cold winter regions
Abstract: Potential ascospore dose (PAD) is an estimate of an orchard’s level of risk for scab build up. Two density components that determine PAD, pseudothecial density per fertile lesion(PD) and ascal density per pseudothecium (AD), were compared in the warm winter region of Elgin (E) and the cold winter region of Koue Bokkeveld (KB) over two seasons (2012, 2013). From the two regions, apple leaves with scab lesions were collected during leaf-drop and overwintered in their region of origin or in the other region to investigate the effect of temperature and precipitation on PD and AD. In both seasons, the PD for leaves collected and overwintered in the cold winter region was significantly higher than leaves collected in the cold and overwintered in the warm region, as well as leaves collected and overwintered in the warm winter region. Significantly lower temperatures in the first weeks after leaf drop in the cold winter region are thought to have affected pseudothecial formation. However, in both seasons, the PD for leaves collected and overwintered in the warm winter region did not differ significantly from leaves from the warm region that overwintered in the cold region. Asci developed normally and produced mature ascospores in spring, and there were no significant differences for AD values in any treatments or seasons, despite significantly different temperature differences in the two regions. This is possibly due to different fitness of the EL pathogen population that has adapted to shorter, warmer winters causing fewer pseudothecia to be formed. Additional seasons for this experiment are needed to verify these results before PD and AD constants can be determined for application of PAD in South Africa.