Molybdenum deficiency begins as a yellowing along poinsettia leaf margins, especially on leaves in the middle of the plant and upward. Leaf margins become brown and necrotic. Leaves may also pucker downward. 'Marblestar' commonly exhibits marginal chlorosis that may indicate molybdenum deficiency.
To prevent molybdenum deficiency, check media pH and use poinsettia fertilizer blends that contain sufficient molybdenum. Molybdenum deficiency is more likely to occur when the pH is low (pH of 5.3 or lower).