The production cycle for Halloween or Dia de los Muertos marigolds is complete. What were some of the issues that growers faced this season? Part of Best Management Practices is being able to recognize the primary problems that can occur and prevent them.
The most common nutritional issue that marigold growers face is low substrate pH-induced iron (Fe) toxicity. The lower leaves develop a bronze coloration that can be mistaken for a magnesium (Mg) deficiency. The first step in diagnosing the situation is to conduct a PourThru pH and electrical conductivity (EC) test to determine values. In the case of a plant used in the research, the pH was 4.2, and the EC was 0.14 mS/cm. Albano and Miller (1998) reported that the bronzing was primarily an iron toxicity and not a manganese (Mn) problem (the other element that is associated with low substrate pH). Tissue values were excessively high in Fe (1630 and 1380 ppm reported in the two submitted samples). The other micronutrients were also high in the samples (Mn, zinc, copper, and boron). Maintaining the substrate pH between 5.8 and 6.2, utilizing a basic fertilizer to avoid pH drop, and balancing the fertility rate to avoid downward pH creep will all help in avoiding the problem.
A problem that can occur on the opposite end of the pH spectrum is an iron deficiency. In our iron deficiency trials at NC State University, we first observed Fe symptomology on the cultivar 'Zenith Red', an African-French hybrid. 'Discovery Orange' was the first African-type to exhibit mild symptoms, followed much later by 'Taishan Orange'. 'Antigua Orange' did not develop symptoms of Fe deficiency when Fe was withheld from the fertilizer mix. All three African-type cultivars appear not to be especially prone to developing Fe deficiency symptoms under low Fe fertilization conditions. Symptoms are more likely to occur when the plants are over-irrigated, which limits Fe uptake by the plant.
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