The Effect of Nitrogen on Insect and Disease Pests of Onions, Carrots, and Cabbage

Abstract

ABSTRACT Mineral nutrition may affect the ability of plants to resist harmful insects or diseases. The effects of nitrogen (N) nutrition on pests of onions (Allium cepa L.), carrots (Daucus carota L.), and cabbage (Brassica oleracea var. capitata L.) were evaluated on organic and mineral soils in Ontario, Canada in 2000 and 2001. Onions (cvs. Norstar and Winner) and carrots (cvs. Indiana, Idaho, and Annapolis) were grown on both soil types, and cabbage (cv. Atlantis) was grown on mineral soil. Nitrogen was applied at 0,50,100,150, and 200% (carrots and cabbage) and 0, 100, and 200% (onions) of the rate recommended by the Ontario Ministry of Agriculture, Food, and Rural Affairs using calcium ammonium nitrate preplant and potassium nitrate for sidedress applications. In cabbage, onion thrips (Thrips tabaci L.) damage was rated at harvest. Onion thrips (OT) populations were monitored in onions weekly. In carrots, the combined leaf blight symptoms caused by Cercospora carotae and Alternaria dauci was evaluated biweekly using a visual rating. The incidence and severity of cavity spot (Pythium spp.) was rated at harvest. The damage caused by OT was lower on cabbage that received 100–200% and 150% of the recommended N rate in 2000 and 2001, respectively, than the no N treatment in both years, and 200% of the recommended rate in 2001. This effect may have been due to delayed maturity of plants that received low and high N levels. Onion thrips populations in onions were not affected by N rate in either year or location. Leaf blight severity decreased with increasing N rate in most cases. There was no effect of N rate on cavity spot. Nitrogen management should be considered in the integrated pest/crop management program for cabbage, onions, and carrots.