The deficiency may be caused by an excess of magnesium. Poinsettias have higher molybdenum requirements than other crops. Apply at least 0.1 ppm of this element throughout the whole poinsettia cycle using a soluble fertilizer that contains this micronutrient. Applications of this element can also be done through sprays. Fertilizer levels for dark leaf poinsettias.
Poinsettia cultivars with dark foliage require less fertilizer levels than light foliage cultivars.
Keep in mind that if the EC reaches levels below 1.2 mS cm -1, you should fertilize.