Magnesium is sometimes a forgotten element. In many areas of North America, adequate Mg is available in the groundwater used for irrigation. Concentrations of 25 to 50 ppm Mg are sometimes available and provide adequate levels for plant growth. In addition, supplemental Mg is also supplied via the dolomitic limestone used to adjust the substrate pH. However not all growing locations are blessed with a supply of Mg.
Large pots of zinnias are a popular summer crop. With summer production, tip burn of the leaves that surround the flower bud and leaf distortion can occur (Fig. 1). Upon closer inspection, the tip burn occurs primarily on the set of leaves that form the whorl around the flower bud (Fig. 2). In extreme cases, the next sets of older leaves lower on the stem may also exhibit symptoms (Fig. 3).
A secondary Botrytis infection may opportunistically form on the dead tissue too (Fig. 4). This disorder has been observed each summer over the past few years. The symptoms are typical for a calcium deficiency. At NC State University, we conducted experiments on zinnias by withholding calcium and were able to induce the disorder, thus confirming the cause.
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