Lower leaf interveinal chlorosis on zinnias was observed. This was due to limited magnesium (Mg) in the water supply and the leaching of Mg from the lime charge over time due to frequent summer irrigations.
ummer production of plants can lead to unique symptomology that is not typical during other times of the year. Because magnesium (Mg) is a mobile element, and if Mg is limited in the plant, it will be translocated from the older leaves to the new tissue if required. Zinnia plants then can develop lower leaf interveinal chlorosis (yellowing) (Figs. 1-2). With advanced symptoms, necrotic (reddish-brown) spotting can also appear (Fig. 3). This can be confused with low pH induced iron (Fe)/manganese (Mn) toxicity (Fig. 4) which occurs when the substrate is below pH 5.5. So it is important to confirm your diagnosis with a substrate and/or tissue test. In general, the sufficiency range for Mg is between 0.15 to 0.40%, but can vary with species.
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.
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