Manganese toxicity

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Characteristics and occurrence


Manganese (Mn) is frequently an abundant constituent of soils, but its low solubility at neutral and alkaline pH prevents excessive uptake by plants. Therefore, manganese toxicity is nearly always associated with acid soils. Waterlogging may also induce or exacerbate manganese toxicity, as anaerobic conditions cause higher oxides of manganese to be reduced to plant-available Mn2+. Thus in some instances manganese toxicity may appear during wetter periods, with plants recovering as the soil dries out. Manganese is also a component of some fungicides, and may accumulate through repeated use of these fungicides, especially to crops grown on sandy soils.


Like boron toxicity, manganese toxicity causes necrotic spots to appear on the older leaves of the vine. Manganese toxicity also frequently causes chlorosis (pale or yellow colour), most severe on the younger leaves, due to an induced iron deficiency. Either one or both of these symptoms may be observed in crops affected by manganese toxicity.

Symptoms on older leaves begin with the appearance of small, irregularly shaped patches of pale tissue in interveinal zones. The pale tissue is slightly sunken, and small (0.5 - 2 mm), roughly circular spots of dark necrosis soon develop. The necrotic spots are usually more concentrated towards the tip and margins of the leaf. They are scattered within the interveinal tissue, and do not usually form a regular alignment between the main veins, as is seen in the case of boron toxicity or salinity. These lesions may be associated with the blackening of minor veins on the undersurface of the leaf. The necrotic lesions multiply and spread until they occupy most of the leaf area. Affected leaves eventually turn yellow and are shed.

The necrotic spots are caused by the accumulation of manganese in the tissue over a period of time. At relatively mild levels of manganese toxicity, necrotic spots on older leaves may be associated with little apparent reduction in vine growth. At greater severity, root function is impaired, and stunting in conjunction with iron deficiency symptoms may develop, often without necrotic spots. Symptoms indicative of induced iron deficiency are a pale yellow to white interveinal chlorosis of young leaves, and eventually necrosis of the young leaves and apex, resulting in arrested growth.

Possible confusion with other symptoms

The necrotic lesions caused by manganese toxicity may be confused with those of salinity or boron toxicity. In the case of manganese toxicity, necrotic spots are smaller and more evenly scattered across the interveinal tissue, rather than being predominantly aligned midway between the main veins. The spread of necrosis around the leaf margin is not usually observed until after extensive areas of interveinal tissue have been engulfed.

The necrotic lesions may also be mistaken for those caused by fungal pathogens, particularly Cercospora bataticola, which produces small, dark leaf spots. Fungal lesions may be recognised by their random distribution on the leaf blade. While they may be more common on older leaves, they do not show a consistent decrease in severity from the oldest to younger leaves.

Diagnostic soil and plant tissue tests

A critical concentration of 1600 mg Mn/kg in the 7th to 9th youngest leaf blades was determined in solution culture experiments using cultivar Wanmun. Concentrations as high as 8000 mg Mn/kg were measured in severely affected plants. However, a concentration of 1500 mg Mn/kg was measured in field-grown plants showing severe symptoms and considerable stunting. A study with tobacco found that tolerance to manganese toxicity increased with increasing temperature, despite greatly increased concentrations of manganese in the leaves of plants grown at the higher temperature. The combined effect of these two factors meant that the critical tissue concentration associated with the appearance of symptoms increased seven-fold, from 700 to 5000 mg Mn/kg, with an increase in the day/night temperature regime from 22/18oC to 30/26oC. It is likely that sweetpotato crops experience a similar interaction between manganese toxicity and temperature. Crops have been observed to recover from manganese toxicity as the season becomes warmer. This obviously makes interpretation of tissue manganese concentrations difficult.

In soils, levels of total (perchloric acid extractable) manganese of more than 2000 mg/kg are regarded as high. Measurement of “easily-extractable” manganese is more common, using chelating agents such as diethylenetriaminepentaacetic acid (DTPA) as extractant. Concentrations of DTPA-extractable manganese above 45 mg/kg are considered potentially harmful to root crops such as potato and carrot. In conjunction with low pH (<5.3 measured in water) or waterlogging, such concentrations may indicate a risk of manganese toxicity for sweetpotato.


Cultural control

As manganese toxicity often results from low soil pH, it can often be corrected by application of lime or dolomite to raise the pH above about 5.3 (measured in 1:5 soil:water). If the problem is associated with waterlogging, improved drainage may be effective. Raising the height of mounds or ridges may be enough to avoid problems associated with waterlogging in most situations.


CFL 1983. Soil Analysis Service Interpretation Charts. Consolidated Fertilizers Limited, Morningside, Queensland, Australia.

Clark, C.A. and Moyer, J.W. 1988. Compendium of Sweet Potato Diseases. APS Press, The American Phytopathological Society, St. Paul, USA.

Landon, R.J. 1991) (ed.) Booker tropical soil manual: a handbook for soil survey and agricultural land evaluation in the tropics and subtropics. Booker Tate Ltd; Longman, London.

O’Sullivan, J.N., Asher, C.J. and Blamey, F.P.C. 1997. Nutrient Disorders of Sweet Potato. ACIAR Monograph No. 48, Australian Centre for International Agricultural Research, Canberra, 136 p.

Rayment, G.E. and Higginson, F.R. 1992. Australian laboratory handbook of soil and water chemical methods. Inkata Press, Australia.

Rufty, T.W., Miner, G.S. and Raper, C.D.Jr. 1979. Temperature effects on growth and manganese tolerance in tobacco. Agronomy Journal 71, 638-644.



Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Brown spots and yellowing of leaves due to Mn toxicity (J. O'Sullivan).  

Pale green and sunken  patches between veins are early symptoms of Mg toxicity (J. O'Sullivan).

Small dark necrotic spots develop in the interveinal patches (J. O'Sullivan).

 The oldest leaves senesce with progressive yellowing followed by necrosis spreading and engulfing the previous spots (J. O'Sullivan).

On the lower surface of older leaves, the small veins become blackened (J. O'Sullivan).