Mexican sunflower or red sunflower (Tithonia rotundifolia) is a tall growing background plant that attracts bees, butterflies, and birds. During a grower visit, we were asked to look at a set of plants to help diagnose a case of lower leaf interveinal chlorosis (Figs. 1 & 2) and purplish-black coloration (Figs. 3 & 4). The symptomatic plants were the remainders from a landscape order that the grower fulfilled. As a result, they were mature in bloom, and fertilization was sporadic.
In North Carolina, our irrigation water is normally low in magnesium (Mg) because we do not have limestone bedrock to supply Mg. Therefore, when one observes lower leaf interveinal chlorosis, there is a very high probability that the cause is due to insufficient Mg. It is also common that late-season crops are more likely to develop a Mg deficiency after any Mg from the dolomitic limestone, which is added to adjust the substrate pH, has leached out.