Pest and disease management

Fertility Trends: Petunias Yellow? It May Not Be Iron Deficiency

The phenomenon of yellowing in the new growth of petunias, calibrachoa, and other related crops is a familiar concern among growers. Often observed as interveinal chlorosis, characterized by yellow leaves with green veins, or progressing to complete yellowing or whitening of the leaves, this issue is frequently attributed to iron deficiency. Iron deficiency can arise from inadequate iron application or high pH levels in the growing medium, rendering iron inaccessible for plant uptake.

Table 1. Crops prone to iron deficiency.
Crops Prone to Iron Deficiency
Bacopa (Sutera) Gerbera Snapdragon
Bracteantha Nemesia Torenia
Calibrachoa Petunia Verbena
Diascia Pansy Vinca
Dianthus Scaevola Viola

A simple way to correct iron deficiency is to apply an iron chelate drench to the growing medium. This will not only provide more iron, but a good chelating agent will allow iron to be available over a higher pH range as seen in Table 2.

Table 2. Common iron chelating agents used to correct iron deficiency in greenhouse crops.
Chelate Upper pH Limit Products Available % Iron Reliability
Iron - EDTA 7.0 Found in most water-soluble fertilizers, Sequestrene Fe 13% OK
Iron - DTPA 7.5 Sprint 330, Sequestrene 330 10% Better
Iron - EDDHA 9.0+ Sprint 138, Sequestrene 138 6% Best

In most cases an iron chelate will correct interveinal chlorosis in the new growth of the crops listed in Table 1, but not always. We are becoming conditioned that when we see interveinal chlorosis in the new growth of crops like petunias and calibrachoa, it is iron deficiency (Figure 1). In many cases this is true; however, manganese deficiency also has symptoms of interveinal chlorosis on the new leaves but can also display tan, sunken spots that appear in the chlorotic areas between the veins. (Figure 2).

Fe Def Calibrachoa plug Supertunia normal verbena PRO-MIX
Figure 1. The calibrachoa on the left and the petunia on the right are both exhibiting iron deficiency as interveinal chlorosis in the new leaves. As the problem progresses, the leaves eventually turn solid yellow and then eventually white. Source: Premier Tech

If an iron chelate is applied to a crop that only has manganese deficiency, the interveinal chlorosis, caused by manganese deficiency, may initially improve. However, after a few days, improvement stops and the interveinal chlorosis becomes much worse. A manganese fertilizer needs to be applied, but the rates may need to be quite high to compensate for the high levels of iron from the iron chelate. Iron should be found at 1.0-2.5 ppm in the growing medium for crops listed in Table 1; but, an iron chelate applied at 5 ounces per 100 gallons of water will provide 37.5-47 ppm iron, depending on the product. Respecting the ideal iron-to-manganese ratio of approximately 2:1, 10-20 ppm of manganese would have to be provided.

Fe Def in Calibrachoa plus and Supertunia normal verbena PRO-MIX
Figure 2. The petunia on the left and the soybean on the right are both exhibiting manganese deficiency as interveinal chlorosis in the new leaves. The symptoms are like iron deficiency and can be easily confused. Testing should be done to confirm what is causing the symptoms. Source for petunia: www.keywordsking.com/bG93IGlyb24gbGV2ZWxz/, Source for soybean: www.cargillag.ca/expert-network/expert-blog/micronutrient-watch-2016.

Next time interveinal chlorosis is found in petunias or calibrachoa, remember that it may not be caused by iron deficiency, but by manganese deficiency.

It is best to test the growing medium and plant tissue to verify which element or elements are deficient. Applying an iron chelate to a crop that is deficient in manganese will lead to more crop problems and negatively impact crop quality.