Incidence of downy mildew in onion growing under mineral and organic fertilization

The incidence of downy mildew, Peronospora destructor Berk. Casp., in onion growing in chemical and organic fertilized plots was evaluated in Ituporanga, Santa Catarina State, Brazil. Two experiments were carried out between August to December 1998. The treatments in the first experiment with organic sources were: 75.0 kg/ha of N (swine manure; poultry manure; compost; turkey manure; humus); 37.5 kg/ha of N (swine manure half dose); control (NPK, 30-120-60 kg/ha of N-P2O5-K2O) and double rate of NPK (60-240-120 kg/ha of N-P2O5-K2O); and unfertilized control. In mineral fertilization experiment, the mineral levels were 1 and 3 times the recommended rates based on soil analysis: 30-120-60 kg/ha of N-P2O5-k2O; 90-360-180 kg/ha of N-P2O5-K2O; 75.0 kg/ha of N; 225.0 kg/ha of N; 80.0 kg/ha of P2O5; 240.0 kg/ha of P2O5; 60.0 kg/ha of K(2)0; 180.0 kg/ha of K(2)0. Control plots with organic fertilization received swine manure + phosphate rock in three treatments (75.0 kg/ha of N + 80.0 kg/ha of P2O5; 225.0 kg/ha of N + 240.0 kg/ha of P2O5; 37.5 kg/ha of N + 40.0 kg/ha of P2O5) and unfertilized check. Downy mildew incidence was similar between treatments with fertilizer and no fertilizer check. Mineral fertilization did not increase downy mildew incidence compared to the organic treatments. The relationship between nutrient and disease varied according to sampling date and differed for mineral and organic sources.

Allium cepa; Peronospora destructor; nutrient; organic agriculture; agroecology