From fighting free radicals to the splitting of water molecules Manganese is essential to several important plant functions. Nearly all forms of life exposed to oxygen contain enzymes that essentially neutralize harmful free radicals (superoxides) through super-oxide dismutase (SOD for short). A lack of SOD enzymes is known to cause premature aging, and a reduced resistance to radiation and disease. Manganese (Mn) is an essential building block to these enzymes. Mn is also necessary for oxygen evolution or, in plants, the splitting of a water molecule into oxygen and hydrogen. The oxygen is released into the atmosphere and the hydrogen is used by the plant to process carbohydrates – which are used as energy to support plant functions and microorganisms such as Mycorrhiza.  Like with most nutrients the availability of Manganese is influenced by pH – too high and manganese will become less and less available, but too low of a pH and plants can start to take up an exorbitant amount of the nutrient even to the point of toxicity.

Deficiency Symptoms

Although Mn deficiency can be easily confused with other more common deficiencies like iron, zinc, magnesium or nitrogen there are a few differences that make identification easier. Take a look at the areas near the leaf veins; in Mn deficiency the area along the veins that remains dark green will be slightly broader, less restricted to the vein itself, than in the case of an Iron deficiency. Zinc deficiency will start as lighter green blotches in between veins that will eventually grow and become almost white in color. Mn deficiency will be most notable in newer growth as it is an immobile nutrient and can not move from lower leaves to growing tips. Magnesium deficiency looks very similar, but starts with older leaves first. Mn deficiency is rare in plants that are properly watered and fed; almost all broad-spectrum fertilizers contain Mn. If you want to see a classic example of Mn deficiency look around for a Red Maple (A. rubrum) planted in a city-scape site; they are notorious for Mn deficiency and often appear wilted. You will see a pattern of dark green venial areas on a much lighter green leaf.

Causes

  • pH is above 7
  • High Iron content in soil – iron has a higher affinity for chelates than Mn. Adding Mn to high iron soils may exacerbate deficiencies.
  • Low Mn concentrations present in growing media

 Toxicity Symptoms

Manganese toxicity is more common than other micronutrients because, like nitrogen, plants will continue to take it up as long as it is available. Mn toxicity will affect lower leaves first often leaving them chlorotic with lesions and/or brown to red necrotic spots that become larger as the disease progresses, eventually causing large brown patches and leaf drop. These small brown spots can be confused with fungal leaf spots (such as Cercospora), which when sprayed with a manganese containing fungicide could aggravate the problem. Newer leaves often look puckered and stunted. Mn toxicity is almost always associated with acidic or waterlogged soil, and so may appear in conjunction with other deficiency symptoms.

Causes

  • Consistently waterlogged soil – unavailable forms of Manganese will oxidize into available forms and can be taken up in excess by plants
  • Over use of fungicides – some contain manganese which can build up in plant tissue
  • Soil pH is below 5.5 (can occur at levels lower than 6)

 Comparing & Contrasting

 Manganese deficiency example

Manganese deficiency (A. rubrum); Iron deficiency (Sweet Potato); Zinc deficiency (Citrus); Mn toxicity (Soybean)