Molybdenum deficiency

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Characteristics and occurrence

Molybdenum (Mo) is unusual among the micronutrients in becoming less available to plants at low soil pH. Therefore, molybdenum deficiency is usually associated with acid soils (pH <5.5), particularly those which are geologically old and highly leached. Molybdenum deficiency has not been reported in sweetpotato crops to date. As soils low in molybdenum are often also low in phosphorus and sulfur, molybdenum may not be apparent unless phosphorus and sulfur deficiencies have been corrected by fertilisation.

Molybdenum is required by plants in very small quantities. Its main (and possibly only) function in non-leguminous plants is as a component of the enzyme nitrate reductase, which is essential for the metabolism of nitrate, the main form of plant-available nitrogen in most soils. Therefore molybdenum deficient plants may appear as if they are deficient in nitrogen, having general chlorosis and stunted growth. Symptoms unlike those of nitrogen deficiency may occur as a result of nitrate accumulation to toxic levels in the tissue.

Symptoms

Although molybdenum deficiency has not been described in field-grown sweetpotato, symptoms resembling those of N deficiency are expected. Such symptoms would include a general pale green colour, stunted growth with small leaf size, and possible reddening of veins on the young leaves.

In plants grown in solution culture, the following symptoms were associated with a mild growth reduction from molybdenum deficiency. In cultivar Wanmun, older leaves developed silvery patches over interveinal tissue, as a result of the upper epidermis separating from the underlying tissue. Eventually these patches became necrotic, but necrosis did not spread readily from the isolated interveinal patches. In cultivar Beerwah Gold, the young to recently mature leaves displayed a mild interveinal chlorosis, with some interveinal areas becoming necrotic. In cultivar Hawaii, chlorosis was more uniform, and the recently mature leaves commonly developed a marginal scorch, especially at the tips of lobes.

Possible confusion with other symptoms

Molybdenum deficiency may induce symptoms similar to those of nitrogen deficiency, as molybdenum is required for the assimilation of nitrate taken up by the plant. Necrosis (dead tissue) on the margins and interveinal areas of older leaves may bear some resemblance to salinity damage or boron toxicity. However, these disorders generally cause much more extensive necrosis than has been seen in the case of molybdenum deficiency, and are not usually associated with acid soils.

Diagnostic soil and plant tissue tests

From solution culture studies, the critical concentration for deficiency of molybdenum was found to be approximately 0.2 mg Mo/kg in the 7th to 9th youngest leaf blades. Equivalent leaves from healthy plants contained 0.5 - 7 mg Mo/kg.

As molybdenum is required for metabolism of nitrate, deficiency of molybdenum can result in accumulation of nitrate in the plant tissues. Measurements of sap nitrate concentration have been used to distinguish molybdenum deficiency from nitrogen deficiency in a number of crops. A simple, semi-quantitative measurement of sap nitrate can be performed using nitrate test strips, such as those manufactured by Merk. Reagent-impregnated paper is mounted on the plastic strips, and changes from white to deep purple when wet with a solution containing nitrate, over the range 10-500 ppm. Sweetpotato sap can be tested by cutting through a stem or petiole, and applying the side edge of the paper to the droplet of sap which exudes from the cut, allowing the moisture to move in by capillary action. The sap should not be applied directly to the paper surface, as it will stain darkly. Sap from molybdenum-deficient plants will cause a strong colour change, while that from nitrogen-deficient plants produces little if any change. Sap from healthy plants will usually give a weak to moderate reaction. Note that sulfur deficiency may also increase sap nitrate levels.

Management

Cultural control

Molybdenum deficiency is relatively easily corrected, either by the application of small quantities of molybdenum to the soil, or by raising the soil pH. Application of sodium molybdate or ammonium molybdate at rates of 0.2-0.3 kg Mo/ha should be sufficient to correct the disorder in most situations, and may be effective for several years. Sodium molybdate may also be applied as a foliar spray. A solution of 50 g sodium molybdate/100 L water has proven successful with other crops such as sunflower. Some commercially-available fertiliser mixtures also contain molybdenum.

Liming to raise the soil pH above 5.5 is usually effective in alleviating molybdenum deficiency, and may improve conditions for crop growth in other ways also, such as through improved availability of phosphorus and alleviation of manganese or aluminium toxicity.

References

Blamey, F.P.C., Edwards, D.G. and Asher, C.J. 1987. Nutritional Disorders of Sunflower. Department of Agriculture, University of Queensland, St Lucia, Queensland, Australia. 72 pp.

Johnson, C.M. 1966. Molybdenum. In: Chapman, H.D. (ed.) Diagnostic criteria for plants and soils. Dept of Soils and Plant Nutrition, University of California Citrus Research Centre and Agricultural Experiment Station, Riverside, California.

McDonald, W.J. 1978. Molybdenum - if itís short yields will suffer. Sunflower 2, 22.

OíSullivan, 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.

Ulrich, A. 1993. Potato. In: Bennett, W.F. (ed), Nutrient deficiencies and toxicities in crop plants. APS Press, St. Paul, Minnesota, USA. pp 149-156.

 

Contributed by: Jane O'Sullivan

Characteristics and occurrence

Symptoms

Confusion with other symptoms

Diagnostic tests

Management

References


Silvery patches over tissues between veins (J. O'Sullivan).

 

Pale necrotic spots of irregular shape following silvery spots (J. O'Sullivan).

 


Interveinal chlorosis and necrotic spots on a mature leaf (J. O'Sullivan). 

General chlorosis and marginal necrosis, especially on lobe tips (J. O'Sullivan).