Due to the different roles played by each nutrient in the plant, each nutritional disorder tends to produce its own characteristic symptoms. Visible symptoms provide a useful diagnostic tool which is not dependent on costly laboratory equipment or time-consuming chemical analyses. However, some disorders produce rather similar symptoms or no symptoms at all, and the effects of insect pests and diseases may produce symptoms similar to those of nutritional disorders. Environmental conditions (eg. moisture supply, temperature, light) may affect the appearance and severity of nutrient disorders. Cultivars may also differ in their expression of symptoms. Sweetpotato, in particular, shows enormous diversity in the appearance of cultivars, and this is reflected in the range of symptoms they display. Nevertheless, there are distinct patterns, and a careful observer can usually reduce the number of possible causes to a few, if not to a single suspect. Tentative diagnoses can then be confirmed by applying fertiliser test strips, by soil tests, leaf painting or plant tissue analyses.

Visible symptoms often take the form of chlorosis, the reduction of green colour (chlorophyll pigment) in the leaves. Chlorotic tissue may be light green, yellow or whitish. Tissue furthermost from the veins is often the most severely affected, as it is last in the supply line. Hence, chlorosis patterns are frequently described as interveinal, if the tissue on and adjacent to the veins retains a darker colour than the remainder of the leaf. To what degree the minor veins retain their colour in addition to the major veins, and the distance over which the colour change is graduated, are additional features which aid diagnosis. If the chlorosis affects the whole leaf blade uniformly, it is referred to as ‘general’. ‘Vein clearing’, the case when the veins become paler than the rest of the blade, is more often a symptom of viral infection than of a nutritional disorder.

Necrosis is the death of tissue. It may occur following chlorosis as part of a progressive degradation, or it may arise in localised zones of the leaf due to a critical disfunction. The location, shape and size of necrotic lesions are useful discriminators, as are the colour and texture of the dead tissue.

Other types of symptoms include changes in the occurrence and intensity of secondary pigmentation (red or purple colours), changes in the shape or dimensions of plant parts such as thickening, cupping or curling of the leaf blade, size reduction of the blade, deformities causing irregularly shaped or incomplete leaf blades, or shortening of internodes on the stem.

In addition to the appearance of a particular symptom, the position or location of that symptom on the plant must be noted. Nutrients are absorbed by the root system, and distributed among the various plant parts. Some of these nutrients may be redistributed to younger parts of the plant during times of shortage either readily (eg. K, P), more slowly (eg. S) or hardly at all (eg. B, Ca). Thus, deficiencies of K and P are likely to be observed first on older leaves, that of S on both older and younger leaves, and those of B and Ca on the younger leaves. Elements taken up in excess of plant requirements continue to be accumulated during the life of a leaf. Thus, there will be a tendency for toxicity symptoms to appear first on the older leaves where accumulation has been occurring for the longest time.

In many locations, sweetpotato flowers sparingly or not at all. An unusually prolific or early production of flowers is usually a sign that the crop is under nutritional stress. However, as a number of nutrient deficiencies can cause increased flowering, this symptom is of little use in diagnosing the particular deficiency.

Source: 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.

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Diagnosing nutritional disorders

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Plant tissue analysis

Soil analysis

Related topics:

Soil management

Plant nutrients

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Causes of nutritional disorders

Correcting nutritional disorders