Crop System and Fertilization Management Determine Onion Downy Mildew Severity

Traditional farming heavily relies on agrochemicals including mineral fertilizers and pesticides, which are known for their detrimental effects on ecosystems and soil health, especially when these chemical inputs are applied excessively. We conducted two field experiments to compare conventional, integrated and agroecological farming systems, as well as to evaluate the influence of fertilization management (mineral and organic) on the incidence and severity of onion downey mildew. In the conventional system, onion was monocultured with extensive use of agrochemicals. The integrated system involved polyculture, combining onion and white cabbage with a mix of organic and chemical inputs. Agroecological management involved growing onion in polyculture with white cabbage and black beans. A second experiment assessed different fertilization schemes: mineral phosphonitrate, vermicompost, and bokashi. An outbreak of onion downy mildew caused by Peronospora destructor coincided with the rainy season a month before harvest. The results revealed that disease severity was higher in integrated plots (level 4–5 according to the scale) compared to agroecological ones (around level 3), but not significantly different from conventional farming (level 3). Regarding fertilizers, mineral application led to the highest disease severity (level 4–5), while organic fertilizers reduced it (level 3). In addition monitoring of onion downey mildew also crop parameters including yield and the concentration of nitrogen N and phosphorous P in onion tissue were analyzed. The N and P concentration in onion tissue did not appear to associate with downey mildew severity, but there were differences based on the type of management, with approximately 18 mg N g⁻¹ N in conventional systems and 12 mg N g⁻¹ in agroecological systems. Conversely, the concentration of P was lower in conventional systems (2.6 mg g⁻¹) and higher in integrated and agroecological systems (4.0–3.6 mg g⁻¹). Onion fresh biomass production was lowest in agroecological system (0.5 kg m⁻²), whereas no significant differences were found between fertilization types with yields of 1.3 kg m² in integrated and 1.1 kg m⁻² in the agroecological systems. In summary, our study demonstrated that both farming system and fertilizer type influence onion downy mildew severity with the lowest severity observed in the agroecological system and under compost fertilization.