Severity of onion downy mildew in different production systems

ABSTRACT Downy mildew, caused by Peronospora destructor, is the most important disease of onion when mild temperatures and high humidity predominate. In the conventional systems, excessive use of curative fungicides is recurrent, and the evaluation of differentiated production systems is consistent with the indication of the best method for the sustainability of the crop. Thus, the present study aimed to assess conventional, rational and organic production systems in reducing the severity of onion downy mildew. This study was conducted at Epagri, Ituporanga Experimental Station, in 2013 and 2014. The used cultivar was Empasc 352 Bola Precoce, arranged in a randomized block design with four replicates. Treatments consisted of five production systems, named conventional, rational I (official recommended fertilization [CQFS-RS/SC], reduction in the number of applications and in the toxicity of phytosanitary products), rational II (increase in the amount of N and its fractionation, P and K, according to CQFS-RS/SC, and reduction in the number of applications and in the toxicity of phytosanitary products), organic I (fertilization according to CQFS-RS/SC and five applications of low-toxicity fungicide with products allowed for organic systems) and organic II (increase in the amount of N and its fractionation, P, according to CQFSRS/SC, and the same phytosanitary treatment as in organic I). Every week, after transplanting, the severity of downy mildew was evaluated in five random plants of each replicate based on the percentage of leaf area affected by the disease in each exposed leaf. The severity data were calculated and integrated by the area under the disease progress curve (AUDPC). The obtained average values of AUDPC, final severity and total commercial crop yield (Kg.ha-1) underwent analysis of variance according to F test and, when significant, were compared according to Tukey’s test at 5%. In 2013, the AUDPC was not significant among treatments and in 2014 the rational systems did not differ from the conventional system and were similar to the organic systems. Similarly, the final severity of the rational systems did not differ from the conventional system in both harvests but were different from the organic systems. The yield of the rational system II was equal to that of the conventional system, 37.59 Kg.ha-1 in 2013. The organic systems showed higher severity of downy mildew and lower yield, compared to the other systems. We concluded that the rational systems reduce the use of curative fungicides for the control of downy mildew, showing effectiveness similar to that of the conventional and the organic system. The isolated use of protective copper fungicide does not reduce the severity of downy mildew in the crop, in relation to its alternate use with curative fungicides.