Ozone and Botrytis spp. Interaction in Onion Leaf Dieback: Field Studies1

Abstract

Abstract A suspected interaction between ozone injury and Botrytis spp. infection on onions (Allium cepa L.) was investigated in the field by reducing one or both causal agents by chemical antioxidants and/or fungicides. ‘Autumn Spice’ and ‘Rocket’ onions received 4 sprays of the fungicide anilazine and/or one of the following chemicals with antioxidant properties: fenarimol, ancymidol, chlormequat, piperonyl butoxide, metiram, chlorthalonil, or DuPont DPX-4891. Ozone levels exceeded 4 hourly averages of 8 pphm on at least 7 occasions during July, ozone injury was routinely observed in the field, and Botrytis was frequently isolated from lesions on onion leaves. Single and combination treatments of all fungicides and antioxidants generally reduced leaf necrosis and the Botrytis incidence. Fungicide-antioxidant combination treatments were superior to either fungicide or antioxidants alone in reducing Botrytis infection. Yields of ‘Autumn Spice’ were increased by most treatments while ‘Rocket’ onion yields were generally decreased. DPX-4891, the only antioxidant which was nonfungitoxic to Botrytis in vitro, was frequently superior to fungicides in preventing Botrytis lesions and was also the most effective in reducing ozone injury. These results support the hypothesis that onions injured by ozone are more susceptible to Botrytis infection.