Plant diseases continue to play a major limiting role in agricultural production. Nutrients can reduce disease to an acceptable level, or at least to a level at which further control by other cultural practices or conventional organic biocides are more successful and less expensive. Integrative plant nutrition is an essential component in sustainable agriculture, because in most cases it is more cost effective and environmentally friendly to control plant disease with the adequate amount of nutrients and with no pesticides.
Nutrients can affect disease resistance or tolerance of plants. They can affect the development of a disease by affecting plant physiology or by affecting pathogens, or both of them.
Nitrogen (N) is an essential component of amino acids, enzymes, hormones, phenolics and proteins, all which can have direct effects on disease development. There is a difference in the response of obligate parasites to N supply, when there is a high N level there is an increase in severity of the infection.
In contrast, in facultative parasites at high N supply there is a decrease in the severity of the infection. Regarding the obligate parasites, e.g. Puccinia graminis and Erysiphe graminis, when there is high N supply there is an increase in severity of the infection. However, when the disease is caused by facultative parasites, e.g. Alternaria, Fusarium and Xanthomonas spp., high N supply decreases the severity of the infection. In the case of obligate fungal parasites, the nutritional requirements of the parasites cause changes in the anatomy and physiology of the host plant in response to N.
At high rates, there is a higher growth rate during the vegetative stage and the proportion of the young to mature tissue shifts in favour of the young tissues, which are more susceptible.
Potassium (K) decreases the susceptibility of host plants up to the optimal level for growth and beyond this point, there is no further increase in resistance. The high susceptibility of the K-deficient plant to parasitic disease is due to the metabolic functions of K in plant physiology. Under K deficiency synthesis of high molecular-weight compounds (proteins, starch and cellulose) is impaired and there is accumulation of low-molecular-weight organic compounds. K promotes the development of thicker outer walls in epidermal cells, thus preventing disease attack. Application of K can decrease leaf blight severity and increase grain yields in wheat and seedling rot caused by Rhizoctonia solani.
Phosphorus (P) is part of many organic molecules of the cell Deoxyribonucleic acid (DNA), Ribonucleic Acid (RNA), Adenosine Triphosphate (ATP), and phospholipids and is also involved in many metabolic processes in the plant and also in the pathogen. P has been shown to be most beneficial when it is applied to control seedlings and fungal diseases where vigorous root development permits plants to escape disease. Studies have shown that P application can reduce bacterial leaf blight in rice, downy mildew, leaf curl virus disease in tobacco, pod and stem blight in soybean, yellow dwarf virus disease in barley. However, in other studies application of P may increase the severity of diseases such as flag smut in wheat. Foliar application of P can induce local and systemic protection against powdery mildew in cucumber, roses, wine grapes, mango, and nectarines.
Calcium (Ca) is important for the stability and function of plant membranes. Ca deficiency causes membrane leakage of low-molecular weight compounds, e.g. sugars and amino acids, from the cytoplasm to the apoplast, which stimulate the infection by the pathogens. Ca is an important component of the cell wall structure as calcium polygalacturonates are required in the middle lamella for cell wall stability.
When Ca concentration drops, there is an increased susceptibility to fungi, which preferentially invade the xylem and dissolve the cell walls of the conducting vessels. Ca treatment of fruits before storage is therefore an effective procedure for preventing losses both from physiological disorders and from fruit rotting. Application of Ca to the soil eliminates the occurrence of the disease as it confers resistance against Pythium, Botrytis and Fusarium spp.
The effect of micronutrients in reducing the severity of diseases can be attributed to the involvement in physiology and biochemistry of the plant, as many of the essential micronutrients are involved in many processes that can affect the response of plants to pathogens. Manganese (Mn) can control a number of diseases. the Mn has an important role in lignin biosynthesis, phenol biosynthesis, photosynthesis and several other functions. Manganese fertilization can control a number of pathogenic diseases such as powdery mildew, downy mildew and it inhibits the induction of aminopeptidase, an enzyme that supplies essential amino acids for fungal growth.
Zinc (Zn) was found to have a number of different effects as in some cases it decreased, in others increased, and in others had no effect on plant susceptibility to disease. Zinc plays an important role in protein and starch synthesis, and therefore a low zinc concentration induces accumulation of amino acids and reducing sugars in plant tissue. Zn is involved in membrane protection against oxidative damage through the detoxification of superoxide radicals and impairments in membrane structure caused by free radicals lead to increased membrane leakage of low-molecular-weight compounds, the presence of which favors pathogenesis. Application of zinc to the soil reduces infections caused by Fusarium graminearum and root rot diseases in wheat.
Boron (B) was found to reduce the severity of many diseases because of the function that B has on cell wall structure, plant membranes and plant metabolism. B has been shown to reduce the club root disease caused by Plasmodiophora brassicae, tobacco mosaic virus (TMV) in bean and tomato yellow leaf curl virus in tomato. Chlorine (Cl) application can enhance host plants’ resistance to disease. Silicon (Si) has been shown to control a number of diseases and it is believed that Si creates a physical barrier, which can restrict fungal hyphae penetration, or it may induce accumulation of antifungal compounds. Proper Iron (Fe) nutrition not only boosts plant vigour and health it indirectly affects disease in the rhizosphere where its availability may limit growth of pathogens.
Manipulating the various interactions of the plant, pathogen, and environment over time can reduce most diseases. Fertilizer application affects the development of plant disease under field conditions directly through the nutritional status of the plant and indirectly by affecting the conditions that can influence the development of the disease such as dense stands, changes in light interception and humidity within the crop stand. It is therefore important to maintain a balanced nutrition for disease control and higher yields.
Bolton Kudzai Kakava is a Plant Pathologist/Agronomist/Food and Agriculture Standards Consultant.