Agriculture is a man made ecosystem. In this system the occurrence of any type of losses in plant productivity depends on its interaction with a number of biotic and abiotic factors present in the ecological niche. Plant disease is one of the major factor hat causes huge yield losses. Plan disease is a dynamic process that occurs due to the interaction of host and pathogen when the environment is favorable for disease onset and spreads, for a reasonable time. The host factors like genetic makeup, age, nutritional status and population structure influence the disease development (Huber and Graham, 1999). Generally, host nutrition implies nutrient status of an organism. A healthy plant is an outcome of its nutrient status. Nutrients affect the rate of growth and state of readiness of plants to defend them against the pathogenic attack. Host plants derive their nutrition from the soil that is replenished by organic matters and minerals. Balanced nutrition is required for proper growth and vigor of plants. In the agro-ecosystem nutrition is provided to the plants by incorporating organic matters as well as inorganic fertilizers. Foliar pathogens like Alternaria spp. and Helminthosporium spp. cause severe damage to old and senescing as compared to young and vigorously growing plants. Good nutritional condition causes production of young, succulent growth, prolonged vegetative period and delayed maturity therefore are more prone to attack of obligate pathogens whereas poor nutritional condition causes slower growing plant so weak parasites attack more.
Nutrients required by the plants
Macronutrients- Carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, Magnesium and sulphur are major elements that are required in the large amounts (g/kg dry matter) to the plants.
Micronutrients- They includes iron, zinc, manganese, copper, boron, molybdenum and silicon etc. and are required in micro amount (<500 mg/ kg dry matter).
Role of various elements in the plants
| Macronutrients | Physiological function |
| Nitrogen | constituent of proteins, amino acids, enzymes, hormones, chlorophyll |
| Phosphorus | constituent of phospholipids, nucleic acids, ATP |
| Potassium | catalyst in nitrate reduction photosynthesis and cambium activity |
| Calcium | constituent of middle lamella, activator of enzymes |
| Magnesium | part of chlorophyll, structural integrity of cell components |
| Sulphur | component of amino acids |
| Silicon | cell wall component |
| Micronutients | |
| Boron | increase mobility of sugars ,essential for cell division, protein synthesis |
| Iron | constituent of cytochrome, haemetin enzymes |
| Zinc | constituent of enzymes, DNA polymerase |
| Copper | constituent of Plastocyanin, oxidation reduction reactions |
| Molybdenum | essential for nitrogen fixation process |
These elements are part of various fertilizers and organic matters incorporated into the soil. They influence the growth of the plant but as we know that green plants harbor many microbial parasites / pathogens therefore the nutrient status of the plant directly influence these microorganisms.
Factors influencing disease severity:
1. The amount and type of fertilizers have selective influence on the development of plant diseases.
2. Effect of organic amendments.
3. Concentration of minerals in resistant and susceptible cultivars.
4. Correlating conditions influencing mineral availability.
How nutrients application affect plant defense?
Plant nutrition influences the disease development through host metabolism and growth. Nutrient status of a plant influences disease development either by facilitating disease escape or by enhancing physiologic resistance of the plant. It increases tolerance to disease or pathogenic damage in the plant tissues through compensation. Several potash and phosphorus fertilizers have been known to reduce disease severity by inducing host resistance or accelerating maturity of crop plants thereby reducing pathogen virulence.
Effect of fertilizers application on plant pathogens and disease development
• Application of higher doses of nitrogen not only results in virourous growth of plants but also makes it prone to the attack of some specialized pathogens, powdery mildews and rust fungi. On the other hand, low nitrogen fertility creates stressed conditions for growth and hastens the senescence thereby favor infection by pathogens like Alternaria, Fusarium, Pseudomonas, Sclerotium and Pythium. The forms of nitrogen fertilizers used (nitrate or ammonium) also influence disease development. Disease caused by Fusarium spp., Plasmodiophora brassicae, Sclerotium rolfsii increase in severity by use of ammonium fertilizers. The nitrate nitrogen is reported to favor inection by Gaumenomyces graminis, Phymatotrichum and Streptomyces scabies. The forms of nitrogen are likely to influence disease/ pathogen through effect on soil pH and microbial activity.
• Phosphorus and potash fertilizers are known to reduce disease severity by inducing host resistance or accelerating plant maturity. Escape the infection of pathogen that prefers younger tissue. Phosphorus reduces severity of take â€"all disease of barley, potato scab whereas it increases severity of Cucumber Mosaic Virus on spinach and glume blotch of wheat.
• Potassium-alters the susceptibility of Cabbage to Club Root and Potatoes to Scab. Septoria glume blotch severity was positively correlated with Phosphorus rate while kernel weight and grain yield were negatively correlated.
• Increasing Potassium rates lessened the detrimental effect of applied Phosphorus. Potassium reduces severity of stem rust of wheat, early blight of tomato & stalk rot of corn but increase severity of rice blast & Root knot.
• Calcium fertilizers application reduces root and stem disease caused by Rhizoctonia, Sclerotium, Botrytis, Fusarium, Erwinia etc. by hardening the plant cell wall and made it resistant to pathogen penetration. Its excessive use increase back sank of tobacco and common scab of potato.
• Magnesium may reduce susceptibility to pathogens that produced macerating enzymes as long as Ca levels remain sufficient (Csinos and Bell, 1989). Application of Magnesium fertilizers decreases apple scab & late blight of potato.
• An adequate supply of several micronutrients (eg. Mangenese (Mn)) is needed to activate the Shikimate pathway, which leads to active defense mechanisms against various pathogens. Glycoproteins (lectin) which are associated with resistance of sweet potato to Ceratocystis fimbriota (black rot) and potato to Phytophthora infestans (late blight) require Mn for activity (Garas and Kuc, 1981).
• Silicon gives cell walls a greater strength and works as physical barriers to penetration by Pyricularia grisea (rice blast) and Erysiphe spp. (mildews).
• Iron fertilizers known to decrease Verticillium wilt of mango and silver leaf of tree.
• Application of silicon in soil or foliar spray reduces cucumber powdery mildew, brown spot and rice blast disease.
The application of micronutrients may correct the deficiency of these vital nutrients and thereby alter the host response.
Effect of nutrient deficiency on the plant
| Element | Deficiency symptoms |
| Nitrogen | Pale-yellow chlorosis near the tip of leaf blade, advances towards base in V shape pattern. Plants generally stunted, less flowers with shriveled grains. eg. Yellow berry of Wheat - Characterized by hard or flinty grains, partially or entirely starchy |
| Phosphorus | Plant develop poor root system, stunted, older leaves develop dark blue green colouration from tip backward. Spindly growth, reddish internodes and formation of anthocyanin. Purple coloring on underside of leaves. Reduced flower, seed and fruit production. Susceptible to cold injury. Poor quality fruit and seeds. |
| Potassium | Chlorosis from leaf margin, scorching and browning of tips and margins of Potato, wheat, oat, barley and maize. eg. Potash hunger of Potato; characterized by change in color during July from normal green to bronze, wilting and drooping of leaflets, premature death. |
| Calcium | Younger leaves retarded, short, distorted and torn. In cereals upper internodes become very small, rosette leaves, Weak stems and limited root development. e.g- Blossom end rot of Tomato, Apple bitter pit. |
| Magnesium | Interveinal chlorosis, development of purple lesion within chlorotic tissue. eg. Sand drown of Tobacco-Characterized by chlorosis on tips, advances towards base, reduces growth and commercial value. |
| Iron | Pale yellow interveinal chlorosis on younger leaves, brown lesion or pale red streaks in sorghum and maize |
| Zinc | In second or third fully mature young leaves, white yellow chlorosis between green mid vein and margins. Rosette appearance, stunted plant growth. e.g- White bud in maize and sorghum, khaira disease of rice |
Effect of over-nutrition:
For each crop there is an optimum concentration of food material required. Application of fertilizers in excess have some prominent effects on the plants as increased vegetative growth, deeper green than normal, more succulent and suppression of reproduction. Excessive accumulation of plastic substances results in the development of deformed structures. E.g. - Phyllody, Pistillody, Abnormal proliferation (Rose kings).
Excessive application of nitrogen delay maturity by encouraging vegetative growth weakens the straw and lodging the grains, impair the quality and decrease resistance. Phosphorus in excess amount increases soluble salt which can dry out roots by pulling water from the roots. Therefore, a balanced dose of organic or inorganic fertilizers is necessary for the proper development of plants.
The nutrition status of a plant determines its resistance or susceptibility to a disease. There is a dynamic relationship of the plant’s nutritional status with plant pathogens, environment and other organisms in the environment which could always influence severity of most diseases through nutrient management. In both intensive and integrated crop production systems the knowledge of the relationship of plant nutrition and disease development will be very helpful in deciding the fate of disease management in the crop production system.
References:
1. Csinos AS, Bell DK (1989) Pathology and nutrition in the peanut pod rot complex. In: Engelhard AW (ed) Soilborne plant pathogens : manage¬ment of diseases with macro- and microelements. St Paul, Minn : APS Press, pp 124-136.
2. Garas NA, Kuc J (1981) Potato lectin lyses zoospores of Phytophthora infestans and precipitates elicitors of terpenoid accumulation produced by the fungus. Physiol Mol Plant Pathol 18:227-238.
3. Huber DM, Graham RD (1999) The role of nutrition in crop resistance and tolerance to diseases. In: Rengel Z (ed) Mineral nutrition of crops : fundamental mechanisms and implications. New York : Food Products Press, pp 169-206.
About Author / Additional Info:
I am a Scientist in Plant Pathology at RARS, JNKVV, M.P.