Iron Deficiency on Petunia and Calibrachoa

March 30, 2015

Iron deficiency symptoms have been observed on petunias. At this time in the growing season, Iron (Fe) deficiency symptoms are starting to appear on bedding plants and other spring greenhouse plants. The main symptom of Iron (Fe) deficiency is chlorosis or yellowing, usually starting at the shoot tip and newly developed leaves. The yellowing or chlorosis may appear only between the veins (interveinal chlorosis) or the entire leaf may be chlorotic. As the problem gets worse the color of the chlorosis may change from yellow to white. In severe cases the growing tips may die and are replaced by many growing tips that also turn white. Iron deficiency causes plants to be unsalable.

Iron (Fe) deficiency symptoms start to appear when pH of the growing medium decreases the solubility of micronutrients especially Iron (Fe) and Manganese (Mn) in the soil solution. Certain plant species, such as petunia, calibrachoa, vinca, nemesia, diascia, bacopa, dianthus, pansies, verbena, snapdragons and azalea are inefficient in taking up Iron (Fe) from the soil solution. The inefficient plants are sensitive to high pH because they lack the physiological mechanism to overcome Iron (Fe) deficiency by increasing uptake.

To prevent (Fe) deficiency on Iron (Fe) inefficient plants growers can: 
1. Monitor and maintain control of the growing medium pH for Iron inefficient crops at a low target pH range of 5.5-6.0. Use of acid forming fertilizers that have higher percentage of ammoniacal nitrogen will be enough to keep the pH of the growing medium in this range. However if the irrigation water is alkaline then acid injection into the irrigation water might be needed.
2. Fertilize Iron inefficient crops with iron. Fertilizing from time to time with Iron chelate (e.g Sprint 330) is a proactive way of preventing Iron (Fe) deficiency. The chelates are applied as soil drench and can be applied every 3-4 weeks if desired. It is important to rinse excess iron off the foliage and flowers because this can cause brown spots.

Geoffrey Njue, University of Massachusetts Extension

Resource:
How to Prevent Iron Deficiency in Spring Greenhouse Crops, (with photos) UMass Extension