Epidemiological studies and sustainable approaches for the control of strawberry powdery mildew

Strawberry powdery mildew, caused by Podosphaera aphanis, is one of the most destructive diseases of strawberries worldwide, particularly under protected cultivation. Difficulties in detecting and managing early infections result in rapid development and yield losses up to 70%. Control still relies on frequent applications of chemical synthetic fungicides, due to the broad range of environmental conditions favorable to pathogen development and lack of effective alternatives. Although extensive research is available, existing knowledge remains fragmented and offers limited support for effective disease management. The first objectives of the thesis were (i) to revise existing literature and (ii) assess growers’ challenges to identify current research gaps and practical constraints in the field. After analyzing the extensive literature, major gaps persist regarding key aspects of disease epidemiology and the development of effective management approaches. In particular, the main drivers of rapid disease spread under field conditions, the management of overwintering inoculum to limit early-season epidemic onset, and the effective integration of alternative natural fungicides into residue-minimizing strategies remain poorly investigated. To address these gaps, the experimental objectives were: (iii) to characterize the role of wind in the spatiotemporal dispersal of the disease in high tunnels; (iv) to quantify the role of overwintering inoculum and evaluate post-winter leaf removal for the control of early-season primary infection; and (v) to develop a management strategy by limiting synthetic fungicides to early-season applications and evaluating the performance of alternative products post-flowering. Disease spreading was evaluated in a two-year trial, where spatiotemporal colony development (along a 17-m high-tunnel, for 17 days) originated from a single source of inoculum, was compared under both natural and reduced wind conditions. Results showed that wind strongly influenced spatial spread and secondary hot-spot formation, yet P. aphanis dispersed efficiently even under reduced wind, advancing up to ~1.7 m day⁻¹. To quantify the role of overwintering inoculum in epidemic onset, the effect of post-winter leaf removal in an unsprayed plot was assessed by comparing non-sanitized plants with sanitized plants in which complete leaf removal was performed. In parallel, chasmothecium development in autumn and chasmothecium opening in spring were monitored according to temperature and leaf wetness and related to primary infection onset. Results showed that post-winter removal of infected/old leaves consistently reduced disease incidence by 50% and delayed onset. Chasmothecium development started after 8 and 16 cumulative hours in 2023 and 2024, at late-summer temperature <13 °C, while chasmothecium opening in spring started after 21 and 18 cumulative hours in 2023 and 2024 at temperatures >10° and at least 2 hours leaf wetness. Finally, to design a strategy to reduce the number and quantity of residues, we compared a standard chemical fungicide program with three integrated strategies each involving the substitution of synthetic fungicides after flowering with application of alternative products, namely an aqueous extract from germinated Lupinus albus L. seeds, potassium bicarbonate and sulfur. Results showed that limiting synthetic fungicides to the pre-flowering period and relying on natural products thereafter reduced residue numbers by up to 57% while maintaining disease control comparable to the standard program. Overall, this work showed that P. aphanis. spreads rapidly in high tunnels even under minimal wind, implying that few initial foci can trigger fast epidemics. Combining rigorous post-winter leaf removal and synthetic chemical fungicides early in the season is therefore pivotal for reducing primary infection risk and delaying disease onset. When followed by the use of effective alternatives, low-residue strawberry production becomes achievable, supporting more sustainable and reliable disease management.