Yellow chlorosis on recently matured leaves in the middle of the plant is a common sign of micronutrient deficiency in poinsettia.
Poinsettias have a long crop time and higher micronutrient needs than many other floriculture crops. According to the e-GRO nutritional crop factsheet for poinsettia, the crop performs best when the substrate pH is between 5.5 and 6.5 and the PourThru EC is between 2.0 and 4.3 mS/cm. To maintain these levels, growers usually apply moderate to high fertility, around 200–300 ppm nitrogen, at each irrigation. Because poinsettias need more micronutrients than most crops, deficiencies are common.
A grower recently reported marginal chlorosis and interveinal chlorosis in several poinsettia cultivars, including Snowy White, Superba Marble, and Autumn Leaves. They were fertilizing with 16-3-16 Cal-Mag Special (Nutriculture from Plant Marvel) at 175–200 ppm and adding molybdenum at 0.1 ppm.
The crop pH was on the high side of the recommended range, between 6.3 and 6.4. Tissue samples were sent in to pinpoint which nutrients were behind the symptoms. Results showed deficiencies in iron (64.20 ppm; normal 100–250 ppm), manganese (36.51 ppm; normal 60–275 ppm), and molybdenum (0.41 ppm; normal 1–5 ppm). Boron and copper were also slightly low. All macronutrients, including nitrogen and phosphorus, were within normal ranges.
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