Tomatoes commonly develop symptoms of interveinal chlorosis (yellowing) on the lower leaves due to magnesium (Mg) deficiency. This e-GRO Alert highlights the symptomological development of Mg deficiency to help you identify the problem and discusses management procedures.

by Brian E. Whipker - bwhipker@ncsu.edu, Josh Henry, Paul Cockson and W. Garrett Owen - wgowen@msu.edu

Often times, lower leaf interveinal chlorosis appears on tomato plants (Fig. 1), which is a classical symptom of a magnesium (Mg) deficiency. Tomatoes commonly develop symptoms over time because they have a high demand for Mg and the high levels of calcium (Ca) supplied to plants to avoid blossom end rot can limit (antagonize) the plant’s ability to acquire adequate levels of Mg.

Figure 1. Typical symptoms of a magnesium deficiency include interveinal chlorosis (yellowing) of the lower leaves of tomatoes. Photo by: Brian Whipker.

Symptomology of Mg deficiency on tomatoes occurs on the lower, older leaves. That is because Mg is a mobile element, and if Mg is limited in the plant, it will be translocated from the mature leaves to the new tissue. Typical initial symptomology is lower leaf interveinal chlorosis (yellowing). Next, a few areas of the leaves develop a slight interveinal chlorosis (Fig. 2), which intensifies over time (Fig. 3). With advanced symptoms, necrotic spotting (Fig. 4) and dark purplish black spotting (Fig. 5) will develop. In general, the leaf tissue sufficiency range for Mg with tomatoes should be between 0.25 to 0.50%.

Figure 2. The initial symptom of a magnesium deficiency begins as faint interveinal chlorosis (yellowing) of the lower leaves. Photo by: Brian Whipker.

Magnesium deficiency can be confused with another problem common on the lower leaves of tomatoes. Tomatoes can also develop lower leaf necrosis as a result of excessively low substrate pH. Generally, when the substrate drops below pH 5.5, both iron (Fe) and manganese (Mn) can be taken up by the plant in toxic quantities (Fig. 6). Thus, it is important to confirm your diagnosis with a substrate and/or foliar tissue test.

Figure 3. Interveinal chlorosis (yellowing) expands between the veins of the older leaves and tan spotting develops as magnesium deficiency symptoms progress. Photo by: Brian Whipker.

Corrective procedures
The correction for a Mg deficiency is easy. Epsom salts (magnesium sulfate) can be applied at the rate of 2 pounds per 100 gallons of water (2.4 kg/1000L). Apply this as a 10% flow through leaching irrigation. This will stop the progression of symptoms but will not reverse any necrotic spotting. For areas which lack sufficient Mg in their irrigation water and Mg is not part of the regular fertilization program (e.g. 20-10-20 does NOT contain Mg), monthly applications of Epsom salts at a rate of 1 pound per 100 gallons of water (1.2 kg/1000L) is the common production practice to ‘green up’ plants and avoid deficiencies.

Figure 4. Necrotic (brown) spotting on the lower leaves is observed under advanced magnesium deficiency. Photo by: Brian Whipker.

Magnesium deficiencies commonly occur on tomatoes. Knowing how to identify the disorder will improve crop management.

Figure 5. On tomatoes, spotting can appear as a dark purplish-black coloration. This can be confused with low substrate pH induced iron and/or manganese toxicity symptoms. Photo by: Brian Whipker.

Figure 6. Low substrate pH induced iron and/or manganese toxicity mimics an advanced magnesium deficiency problem. Photo by: Brian Whipker.

Source: e-GRO Alert