Cause During sunny weather after bloom, certain cultivars develop elongate, dark brown spots on leaves and flower stalks. No disease organism has been associated with these symptoms. It is believed to be the result of low temperatures during a certain growth stage of the plant.
Reference Fellows, J. and Hanks, G. 2007. NARCISSUS: The cause of 'physiological rust' disorder BOF 62. Hort Development Council, East Malling, Kent.