Molybdenum (Mo) deficient poinsettias exhibit halo-like marginal leaf chlorosis (yellowing) on recently matured leaves, leaf distortion or rolling and leaf edge burn. Poinsettias exhibiting interveinal chlorosis (yellowing) in older leaves, however, is an indicator of magnesium (Mg) deficiency.
These are the findings of MSU researching plant needs of poinsettia. Advising growers on how to grow strong plants, deficiency of these two elements is quite common.
Left: in poinsettia, molybdenum (Mo) deficiency symptoms develop as a thin, marginal chlorotic band or chlorotic halo that occurs on recently matured leaves from the leaf tip to base. Right: magnesium (Mg) deficiency symptoms develop in the lower or older leaves as interveinal chlorosis can work up the plant over time. Photos by W. Garrett Owen, MSU Extension.
To prevent Mo deficiency, consider providing supplemental applications of Mo. On a continuous basis, apply 0.1 ppm Mo provided by ammonium or sodium molybdate. These two sources can be applied as a corrective procedure at a rate of 2 ounces per 100 gallons of water. Once mixed, the solution can be applied as a substrate drench or foliar spray. If applied as a foliar spray, be cautious of phytotoxicity (Photo 3), especially if chelated Mo is used. Molybdenum is more readily available at higher pH levels, so avoid letting the substrate pH fall below 5.5 to help maximize uptake by the plant.
To prevent Mg deficiency, provide a calcium (Ca)-to-Mg ratio of 3:1 to 5:1. When this ratio is unbalanced, an antagonist effect can occur, i.e., excessive Ca supplied can cause Mg deficiency. In general, consider providing 30 to 40 ppm Mg either from a constant liquid feed program or via the irrigation water if Mg levels are sufficient.