African marigolds (Tagetes erecta) can experience a physiological disorder of the leaves that appears as a speckled pattern of chlorosis (Photo 1) (Photo 2) or necrosis (Photo 3) and downward curling of the leaves (Photo 4) . Marigolds that show these symptoms usually also have high levels of iron in their tissues. While common with African marigolds, this problem also appears on New Guinea and Sultana impatiens, geraniums, vinca and some species of Brassica. This disorder is commonly named "iron toxicity". Joseph P. Albano and William B. Miller, from Clemson University, who have studied this phenomenon extensively, call it 'Bronze Speckle'.
The occurrence of the problem is always related to soilless media and iron chelates such as DTPA and EDTA. The symptoms may appear with a wide variety of iron levels in the tissue; a critical concentration has not been identified. Albano and Miller found that concentrations of iron-DTPA (e.g., 1 ppm) which are typically used in commercial production are sufficient to cause symptoms; increased concentrations are associated with increased severity and higher leaf iron concentrations. It is very difficult to control this problem by managing nutrient solution or medium pH because iron chelates remain fairly stable between pH 4-7.
Marigold plants respond to temporary iron deficiencies by modifying root physiology. These modifications include root induced acidification of the media to increase iron solubility in the root zone, and an enhanced ability to reduce, or chemically convert, ferric iron (Fe+3) to ferrous iron (Fe+2), the form of iron that plants take up. In other words, a marigold plant "detecting" less iron than needed "puts in motion" mechanisms that make it very efficient in scavenging iron. When iron levels go back to normal, some plants are unable to "turn off" these very effective mechanisms so they pick up iron to the point that it becomes toxic.
Since some cultivars are more susceptible than others, the best solution is prevention: selection of cultivars that do not tend to have this physiological disorder. If this is not possible/desirable, always apply the right iron level with your fertilizer program. Check your injector! Maintain medium pH at the right level (5.8 - 6.2). Store fertilizer solutions in opaque containers. Albano and Miller (1997) have shown that using an iron-chelate containing fertilizer solution that was subject to light can result in modifications in root physiology typically associated with iron deficiency stress (that results in iron toxicity).
For more detailed information about this topic you can read the scientific paper published by Joseph P. Albano and William B. Miller (1996).
