IPI International Potash Institute
IPI International Potash Institute

Potassium and biotic stress

Presented at the 1st FAUBA-FERTILIZAR-IPI Workshop on

Potassium in Argentina's Agricultural Systems

20-21 November 2001, Buenos Aires, Argentina

Potassium and biotic stress

by Dr. Adolf Krauss

Contents

Pests and diseases challenge food security
The importance of nutrition in the host/pathogen relationship
How to explain the nutritional effects on the host/pathogen relationship?
Plant metabolism as affected by N and K
Plant anatomy and morphology and nutrient supply
Coincidence in host and pathogen life cycles
Conclusion
References

Pests and diseases challenge food security

Global population increases annually by about 80 million and will hit the 8 billion mark within the next 20 years. Most of the population growth will occur in developing countries where, between now and the year 2020, the population will increase by additional 1.5 billion inhabitants to reach 6.3 billion in 2020 and even 8 billion in fifty years from now. The highest growth rate is expected to occur in Sub-Saharan Africa, followed by West Asia North Africa (WANA), South Asia and Latin America & Caribbean (LAC, figure 1).

Figure 1: Relative population increase, 1995-2020
Figure 1: Relative population increase, 1995-2020
after PINSTRUP-ANDERSEN et al., 1997

More people need more food. ROSEGRANT et al. (1995) calculated that the global demand for cereals will increase till 2020 by almost 1 billion t to 2.7 billion t, and for meat by 75% to 283 million tons. Taking losses at storage and conversion into account, the world agriculture should produce by 2020 about 3.4 billion t cereals. In addition to that, with increasing income, consumers demand more animal protein, fruits and vegetables. Quality becomes an important parameter in selecting the food. And last but not least, consumers look more than before whether food is healthy and safe, and thus, free of pathogens, but also free of residues of pesticides.

Most of the required higher crop production has to come from increased yield because land reserves for cropping become progressively scarce due to competition for urban settlements, industrialisation, civic needs, etc. As shown in figure 2, Latin America, in comparison to E. Asia, still has land reserves, which could contribute to increase cereal production, however, the bulk has to come from higher yields.

Figure 2: Sources of growth in cereal production, 1995-2020
Figure 2: Sources of growth in cereal production, 1995-2020
after PINSTRUP-ANDERSEN et al., 1999

However, growth in crop output is jeopardised by losses caused from pests and diseases. OERKE et al. (1995) estimated that, during 1988-90, from the total attainable production of 8 major crops (wheat, corn, rice, barley, soybean, cotton, potatoes and coffee), worth US$580 billion about 42% or US$240 billion are lost due to insects (15%), followed by pathogens (13%) and weeds (13%) (figure 3). To protect plants from pests and diseases, farmers have spent worldwide in 1998 a total of US$34 billion and will spend more with an annual growth rate in pesticide consumption of 4.4% (YUDELMAN et al., 1998).

Figure 3: Crop loss due to pests, diseases and weeds
Total value of attainable production $580b
Figure 3: Crop loss due to pests, diseases and weeds
after OERKE et al., 1995

In addition to losses of yield, higher production costs due to expenses for plant protection, the farmer also looses income from those crops, which are infested with pests and diseases, because they are not competitive at the market. To control pests and diseases, the farmer has several options, which can be combined in the integrated pest management approach:

  • Genetics: i.e. the cultivation of crops, which are less susceptible or even resistant to pests and diseases
  • Biological control: this refers to utilization of predators
  • Chemical control: through organic/inorganic fungicides and pesticides
  • Cultural practice: to create optimal growth conditions of the cultivated crops and/or to eradicate those conditions, which are favourable for multiplication of pests and diseases
  • Plant nutrition.

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The importance of nutrition in the host/pathogen relationship

GRAHAM and WEBB (1991) describe resistance in the host-pathogen relationship as the ability of plants to limit the penetration, development and/or reproduction of invading pathogens. Tolerance of host plants is measured in terms of the ability to maintain growth and yield production in spite of infection or invasion of pathogens. Although both factors are genetically controlled, the environment and thus nutrition of the host plant can modify to a certain extent its expression, especially in moderately susceptible or resistant genotypes/varieties.

Nutrition of plants has a substantial impact on the predisposition of plants to be attacked or effected by pests and diseases. By affecting the growth pattern, the anatomy and morphology and particularly the chemical composition, the nutrition of plants may contribute either to an increase or decrease of the resistance and/or tolerance to pests and diseases. Numerous reviews have discussed these topics (e.g. KRAUSS, 1969; GRAHAM, 1983; PERRENOUD, 1990; MARSCHNER, 1995). However, unlike in human nutrition where the effect of nutrition on “health” has gained considerable importance, the implementation of “healthy” nutrition to improve resistance and tolerance of plants lags behind its potential.

The ratio between nitrogen and potassium plays obviously a particular role in the host/pathogen relationship. PERRENOUD (1990) reviewed almost 2450 literature references on this subject and concluded that the use of potassium (K) decreased the incidence of fungal diseases in 70% of the cases. The corresponding decrease of other pests was bacteria 69%, insects and mites 63% and viruses 41%. Simultaneously, K increased the yield of plants infested with fungal diseases by 42%, with bacteria by 57%, with insects and mites by 36%, and with viruses by 78% (Figure 4).

Figure 4: Effect of potassium on yield increase and pest incidences
Figure 4: Effect of potassium on yield increase and pest incidences

The effect of K on crop specific host/pathogen relationships for rice in Asia was summarised recently by HAERDTER (1997). For example, stem rot, Helminthosporium sigmoideum, generally occurs at high nitrogen (N) supply in soils poor in K; with improved K supply, the incidence decreased and yields increased. A similar inverse relationship between disease incidence and plant nutrition with K was quoted for brown leaf spot in rice (Helminthosporium oryzae), rice blast (Piricularia oryzae) or sheath blight of rice (Thanatephorus cucumeris). A curative effect from applying K was also seen for bacterial diseases in rice like bacterial leaf blight, Xanthomonas oryzae, although highly susceptible varieties hardly responded to K in contrast to varieties with a moderate degree of resistance. Comparable varietal differences in the response to K also exist for the effects of pests. The number of whitebacked plant hopper, Sogatella furcifera, could be substantially reduced with K in the resistant rice variety IR 2035 but K had almost no effect with the susceptible variety TN 1.

In more recent publications, MONDAL et al. (2001) found a negative correlation between K content in soybean and sesame with disease incidence and a positive correlation with their respective yield. SWEENEY et al. (2000) reported that K fertilization reduced leaf rust (Puccinia triticina) severity and improved yield by increasing kernel weight, although part of the positive impact could also be attributed to the effect of chloride applied with KCl fertilizer.

The observation by LAST (1962), that N increased the level of infection of barley powdery mildew as well as the grain yield of the infested plant, is a good example of changes in tolerance. With N, the more vigorous growth of the plant supplied more assimilates which lowered the competitive effect of the pathogen (Table 1). In the same experiment, phosphorus (P) and K increased the resistance of barley because the level of infection was reduced and yields increased.

Table 1. Main effect of nitrogen, phosphorus and potassium on yield and incidence of barley powdery mildew

Treatment

Yield t ha-1

Infestation (%)

N +

3.4

19.5

N -

2.3

10.2

P +

3.3

11.7

P -

2.4

18.1

K +

3.0

13.3

K -

2.8

16.4

after LAST, 1962

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How to explain the nutritional effects on the host/pathogen relationship?

In principle, there are 3 major mechanisms involved in the host/pathogen relationship, namely the host's metabolism and the chemical composition, the host's anatomy and morphology and the host/pathogen coincidence in the life cycles of both host and pathogen.

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Plant metabolism as affected by N and K

Potassium is involved in numerous functions in the plant such as in enzyme activation, cation/anion balance, stomatal movement, phloem loading, assimilate translocation and turgor regulation to name only few. Stomatal resistance decreases and photosynthesis increases with increasing K content of leaves (Peoples and Koch, 1979). In tobacco plants well supplied with K, 32% of the total 15N taken up within 5 hrs was incorporated into protein, in K deficient plants only 11% (Koch and Mengel, 1974) (figure 5).

Figure 5: N utilization in tobacco plants as affected by K supply
Figure 5: N utilization in tobacco plants as affected  by K supply
Koch & Mengel, 1974

Potassium deficient leaf cells accumulate substantial quantities of low molecular weight organic compounds because they act as an osmoticum in the absence of sufficient potassium. Leaf sheaths of rice given only NP but no K contained nearly three times the amount of soluble N and more than twice the content of soluble sugar than plants fertilised with adequate K (figure 6).

Figure 6: Composition of leaf sheaths of rice as affected by K
Figure 6: Composition of leaf sheaths of rice as affected by K
after Noguchi & Sugawara, 1966

The concentration of soluble assimilates in a plant cell is an important factor for the development of invading pathogens especially for obligate parasites such as mildew or rust. This group of pathogens requires living plant cells to complete their life cycle. Thus the host cell must survive the invasion by the parasite if the latter is to survive. Ample N supplies such an environment, i.e. longevity of cells, high turnover of assimilates and high content of low molecular weight compounds. Facultative parasites, in contrast, require weak plants to be infested and killed to survive. Vigorous plant growth stimulated by ample N would suppress infestation by this group of pathogens. This may explain differences in the expression of plant diseases in relation to the nutrition of the host. Figure 7 summarises the effects of N and K on the severity of the infestation by both obligate and facultative parasites.

Figure 7: Effect of N and K on expression of diseases caused by obligate and facultative parasites
Figure 7: Effect of N and K on expression of diseases caused by obligate and facultative parasites
after MARSCHNER, 1995

Phenolic compounds play an important role in the host/pathogen relationship being the basis for many defence mechanisms. They act as phytoalexins or as precursors of lignin and suberin which act as mechanical barriers. As schematically shown in Figure 8, with unbalanced ample N supply, there is a high demand for carbon (C) from photosynthesis via the Krebs cycle for soluble organic N compounds, leaving little C for synthesis of secondary compounds such as phenols and quinones. Under N limited conditions, however, much more carbon from the Krebs cycle is available for the synthesis of phenolic compounds.

Figure 8: Synthesis of pathogen repellents in plants
as affected by the level of nitrogen supply
Figure 8: Synthesis of pathogen repellents in plants as affected by the level of nitrogen supply
schematic after GRAHAM, 1983

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Plant anatomy and morphology and nutrient supply

As a general observation, plants excessively supplied with N have soft tissue with little resistance to penetration by fungal hyphae or sucking and chewing insects. IPI sponsored on-farm trials in India with soybean showed considerable less incidences with gridle beetle, semilooper and aphids when supplied with adequate potash (figure 9).

Figure 9: Pest incidence in soybean as affected by potash supply
Figure 9: Pest incidence in soybean as affected by potash supply

Excessive growth due to an unbalanced N supply can also create microclimatic conditions favourable for fungal diseases. Lodging of cereals as commonly observed at over-supply with nitrogen and inadequate potash is a good example, humidity remains longer in lodged crops giving ideal conditions for germinating of fungi spores.

Insufficient K also causes a pale leaf colour that is particularly attractive to aphids, which not only compete for assimilate but transmit viruses at the same time. Wilting, commonly observed with K deficiency, is another attraction to insects.

Cracks, fissures and lesions that develop at K deficiency on the surface of leaves and fruits provide easy access, especially for facultative parasites.

Apart from K, deficiency of boron (B), calcium (Ca) can also cause damages on the plant's surface. A classical example is the relationship between B deficiency and the secondary infection of sugar beet with Peronospora schachtii, causing hollow heart rot of the root. Without the preconditions of Ca deficiency, this fungi could not infest a healthy beet root.

Calcium is indispensable for the integrity and stability of cell walls. With insufficient Ca, cell walls leak low molecular weight organic compounds used as feed by parasites. Calcium also strongly inhibits the activity of pectolytic enzymes released by fungi to dissolve the middle lamella of the cell wall. The activity of polygalacturonase and pectate transeliminase decrease substantially with increasing Ca content of beans, resulting in a higher resistance to soft rot disease, Erwinia carotovora (PLATERO and TEJERINA, 1976). The lower infestation of lettuce with grey mould, Botrytis cinerea, at higher Ca contents (KRAUSS, 1971) or the decreasing incidence of apple decay caused by Gloesporium perennans at increasing Ca contents can also be related to the control of pectolytic enzymes by Ca.

Trace nutrients also interact with pests and diseases. The fungicidal effect of the trace nutrients manganese (Mn), copper (Cu) and zinc (Zn) as constituents of fungicides is well established. But Mn in particular has also an important function in the synthesis of lignin and phenols and thus, in controlling pathogens. GRAHAM and WEBB (1991) describe the role of Mn in disease resistance, as due to:

  • Lignification: This is obviously the basis of resistance to both powdery mildew and take all disease. Both Mn and Cu are involved in activating the metabolic pathway to synthesise the precursors and lignin.
  • Soluble phenols: These are important phytotoxic substances and their synthesis is stimulated by Mn.
  • Aminopeptidase and pectin methylesterase: Both are important enzymes in the host/plant relationship. The first is activated by pathogens and degrades protein to supply amino acids to the parasite; and the second is a fungal coenzyme for degrading host cell walls. The activity of both enzymes is inhibited by Mn.
  • Photosynthesis: Acute deficiency of trace nutrients like Mn inhibits photosynthesis which weakens the competitiveness of the host plant. It is assumed that, after soil application of trace nutrients, the nutritional status of the plant is improved and the increased metabolic activity allows the plant to tolerate infestations of pests and diseases.

Silicon is thought to increase the resistance of rice to blast fungus Piricularia oryzae, and brown spot, Cochliobolus miyabeanus, by forming a kind of physical barrier to penetrating fungi hyphae. A similar mechanism might contribute to plant resistance to insects such as stem borers (UKWUNGWU and ODEBIYI, 1985). Factors, which lower the soil pH lead to higher Si uptake and content in plants. A positive effect of K on Si uptake of rice was reported by NOGUCHI and SUGAWARA (1966).

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Coincidence in host and pathogen life cycles

Insects actively select plants best suited as a food source by, amongst other factors, appearance, stage of development and composition of the plant. A precondition for successful infestation is the coincidence of certain developmental stages of both host and pathogen. The use of fertilizers can affect this coincidence by either accelerating or slowing down the development of the host plant relative to that of the pathogen. A good example is the effect of Cu on Adult Plant Resistance (APR). Each succeeding leaf is more resistant than the one before. Cu deficiency delayed the development of the wheat and the plants could not establish the appropriate level of APR so the disease index of powdery mildew infestation remained at a constant high level for several weeks. The control of stem cancer (Diaporthe phaseolorum) in soybeans by potash use is also related to an escape mechanism, because the fungus can attack soybean only at a particular phenological stage. Earliness due to balanced fertilization provides the possibility to escape (ITO et al., 2001).

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Conclusion

Infestation of cultivated crops with pests and diseases is a multiple burden to the grower: (i) it lowers the yield and thus, his income, (ii) it increases the production costs because of purchase of agrochemicals to protect the plants, and (iii) infested plants are of low value at the market. The economic damage is too high to be ignored. The grower should not ignore the fact that consumers of his produce ask progressively more for “healthy” and “safe” food without residues of agrochemicals. The consumer prefers to purchase food that is “naturally” produced.

Balanced fertilization complies with this demand. Plants, which are supplied with all necessary nutrients in a balanced manner, are, as shown, more resistant to pests and diseases. This lowers the need for particular pest and disease control measures and reduces the risk of unwanted residues of pesticides. With adopting balanced fertilization, the farmer can proof that he produced the food in a “natural” way and at the same time he is more competitive at the market.

Unfortunately, balanced fertilization is not yet common practice. Unbalanced nutrition is wide spread. Developing countries apply nitrogenous and potassic fertilizers at a ratio of 1:0.2, the situation in developed countries is slightly better with a NK ratio of 1:0.4. Argentina is no exception, N and K are used at a ratio of 1:0.06, or 16 times more N than K. This compares with the ratio at which crops absorb both nutrients, namely cereals 1:1, tuber crops and vegetables absorb even more potassium than nitrogen. It is high time to reconsider the fertilizing practice in Argentina towards balanced fertilization, which is to the benefit of the farmer and the consumer. With higher yields and better crops, the farmer has a better income, which contributes to the development of the rural area. And the consumer can trust the produce from Argentina as being healthy and safe.

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References

Graham, R.D. (1983): Effects of nutrient stress on susceptibility of plants to disease with particular reference to the trace elements. Adv. Botanical Research 10: 221-276.

Graham, R.D. and Webb, M.J. (1991): Micronutrients and disease resistance and tolerance in plants. In: Micronutrients in Agriculture, 2nd ed., SSSA Book Series, No. 4, pp. 329-370.

Härdter, R. (1997): Crop nutrition and plant health of rice based cropping systems in Asia. Agro-Chemicals News in Brief, Vol. 20, No. 4, pp. 29-39.

Ito, M.F., Mascayrenhas, H.A.A., Tanaka, R.T., Martins, A.L.M., Otsuk, I.P., Carmello, Q.A.C. and Muraoka, T. (2001): Control of stem canker in soybeans by liming and potassium fertilizer. Rev. de Agricultura, Piracicaba, V. 76, fasc. 2, pp. 307-316.

Koch, K. and Mengel, K. (1974): The influence of the level of potassium supply to young tobacco plants (Nicotiana tabacum L.) on short-term uptake and utilisation of nitrate nitrogen. J. Sci. Food Agric. 25: 465-471.

Krauss, A. (1969): Einfluss der Ernährung der Pflanzen mit Mineralstoffen auf den Befall mit parasitären Krankheiten und Schädlingen. Z. Pflanzenernähr., Bodenkd. 124: 129-147.

Krauss, A. (1971): Einfluss der Ernährung des Salats mit Massennährstoffen auf den Befall mit Botrytis cinerea Pers. Z. Pflanzenernähr., Bodenkd. 128: 12-23.

Last, F.T. (1962): quoted by Graham (1983).

Marschner, H. (1995): Mineral nutrition of higher plants. 2nd ed., Academic Press, pp. 436-460.

Mondal, S.S., Pramanik, C.K. and Das, J. (2001): Effect of nitrogen and potassium on oil yield, nutrient uptake and soil fertility in soybean (Glycine max) – sesame (Sesamum indicum) intercropping system. Indian J. Agric. Sci. 71: 44-46.

Noguchi, Y. and Sugawara, T. (1966): Potassium and japonica rice. International Potash Institute, Basel, Switzerland, 102 p.

Oerke, E.C., Dehne, H.W., Schohnbeck, F. and Weber, A. (1995): Crop production and crop protection: Estimated losses in major food and cash crops. Amsterdam, Elsevier (quoted in IFPRI Discussion paper 25, 1998).

Peoples, T.R. and Koch, D.W. (1979): Role of potassium in carbon dioxide assimilation in Medicago sativa L. Plant Physiol. 63: 878-881.

Perrenoud, S. (1990): Potassium and plant health. IPI Research Topics No. 3, 2nd rev. edition. Basel/Switzerland.

Pinstrup-Andersen, P., Pandya-Lorch, R. and Rosegrant, M.W. (1997): The world food situation: Recent developments, emerging issues, and long term prospects. International Food Policy Research Institute, Washington DC, USA.

Pinstrup-Andersen, P., Pandya-Lorch, R., and Rosegrant, M.W., 1999: World food prospects: critical issues for the early twenty-first century. International Food Policy Research Institute, Washington DC, USA.

Platero, M. and Tejerina, G. (1976): Calcium nutrition in Phaseolus vulgaris in relation to its resistance to Erwinia carotovora. Phytopath. Z. 85: 314-319.

Rosegrant, M.W., Agcaoili-Sombilla, M. and Perez, N.D. (1995): Global food projections to 2020: Implications for investment. Food, agriculture, and the environment, Discussion paper 5, International Food Policy Research Institute, Washington DC, USA.

Sweeney, D.W., Granade, G.V., Eversmeyer, M.G. and Whitney, D.A. (2000): Phosphorus, potassium, chloride, and fungicide effects on wheat yield and leaf rust severity. J. Plant Nutr. 23:9, 1267-1281.

Ukwungwu, M.N. and Odebiyi, J.A. (1985): Incidence of Chilo zacconius Bleszynski on some rice varieties in relation to plant characters. Insect Sci. Appl. 6: 653-656.

Yudelman, M., Ratta, A. and Nygaard, D. (1998): Pest management and food production. In: Food, agriculture and the environment. Discussion paper 25, International Food Policy Research Institute, Washington DC, USA.

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