Effect of potassium phosphite and manganese phosphite on downy mildew of the onion

This study aimed to evaluate the effects of potassium and manganese phosphites on the mildew severity as well as the yield of onion. For that, one experiment was carried out at Epagri/EEItu, in Ituporanga, SC, Brazil, from August to December 2006. In a randomized block design with four replications, 14 treatments were tested with different combinations of fungicides and potassium and manganese phosphites. All treatments containing fungicides reduced the downy mildew severity. The foliar spray of potassium phosphite reduced the development of downy mildew to an extent similar to chlorotalonil and tended to be more efficient than the manganese phosphite. The effect of potassium phosphite was similar to that of cholototalonil. None of the treatments affected the chlorophyll or
the carotenoid contents at the H-growth stage. Despite the disease reduction, none of the treatments significantly altered the yield or sugar content of onion bulbs, suggesting a compensation system that tolerates intermediate disease levels.