Epidemiological studies on primary inoculum of Venturia inaequalis and V. asperata for apple scab management

Apple scab, caused by the ascomycete Venturia inaequalis, is the most important disease of apple, causing great losses worldwide. Its management is largely based on fungicide treatments, especially targeting ascosporic infections. To reduce the pesticide input, sanitation practices against overwintering inoculum, decision support systems for an optimal timing of treatments, and resistant cultivars can be used. However, reducing pesticide pressure increases the risk of emerging new and/or secondary pathogens. This is the case of V. asperata that was isolated on cultivars resistant to V. inaequalis and already reported in several regions of Italy and France. It causes atypical scab-spots on fruit and leaves. Limited information is available on the epidemiology of V. asperata. The specific objectives of the thesis were: i) to set up a method of reducing the overwintering inoculum of apple scab by irrigating overwintering leaves on the ground in dry days, ii) to understand the aerial dispersal of ascospores with the under-canopy irrigation, and iii) to identify the conditions of maturation of pseudothecia and ascospore discharge of V. asperata. The effect of irrigation was evaluated comparing irrigated and non-irrigated leaf litter. Irrigation in dry days induced the release of a noticeable amount of ascospores of V. inaequalis under conditions unsuitable for infection, thus reducing the inoculum during rainy days. In mean of all trials, the ascosporic inoculum was reduced by approx. 50% in irrigated plots. In addition, irrigation caused an earlier depletion of the ascospore supply. Field trials were carried out to evaluate the efficacy of irrigation in reducing scab in untreated and fungicide-treated plots. By the end of the primary season, the overall incidence of infected leaves and fruit in untreated and fungicide-treated plots had decreased by more than 50%. To understand the aerial dispersal of ascospores of V. inaequalis with under-canopy irrigation, rotating-arm spore traps were placed at heights, ranging from 0.3 m to 3.0 m above the ground. Irrigation was carried out above overwintered apple leaves infected with scab in two different situations, i.e., in a wind-protected enclosure and under real orchard conditions. Ascospores were found to disperse above the irrigated layer, with more spores detected above the sprinklers than below. Based on these results, since ascospores could settle on susceptible apple tissues, it is essential to ensure a rain-free period of at least 24 hours post-irrigation to prevent scab infections. The primary infection season of V. asperata was studied by comparing the development of pseudothecia and ascospore discharge to V. inaequalis in overwintered leaf litters. Venturia asperata showed a delay in pseudothecial maturation and emptying in relation to degree days accumulation, compared to V. inaequalis. The time of spore release for V. asperata was postponed compared to V. inaequalis. The delayed spore ejection and pseudothecial development of V. asperata, in comparison with V. inaequalis, could partly explain the late appearance of symptoms during the growing season in the orchards. In conclusion, targeted irrigation could represent a sustainable and easily applicable method to reduce the incidence of apple scab, and consequently to reduce the number of treatments or increasing their efficacy. Further research on epidemiology of V. asperata is necessary to find the optimal weather conditions for ascospore discharge and for primary and secondary infections, as well as its latency period. Future breeding programs for apple should also take into consideration resistance against V. asperata