Plant diseases are a major limiting factor in agricultural production. Most growers use high amounts of chemicals to control plant diseases, unaware to the fact that mineral nutrition has an important role in disease control. In addition to the economical consequences, the use of pesticides raises environmental and food safety concerns.
All essential plant nutrients influence the health of plants and their susceptibility to disease. Plants suffering a nutrient stress will be more susceptible to diseases, while adequate crop nutrition makes plants more tolerant of or resistant to disease.
The resistance of plants to diseases is mainly related to genetics. However, the ability of the plant to express its genetic resistance to a particular disease is affected by mineral nutrition.
Some nutrients have a greater impact on plant diseases than others. However, it should be noted that a particular nutrient may have opposite impacts on different diseases and in different environments, i.e. the same nutrient may decrease the incidence of one disease, but increase the incidence of others.
Soil pH, nitrogen forms and the availability of nutrients play a major role in disease management. Nutrient manipulation can be achieved directly by applying adequate fertilizers, or indirectly, through the use of different cultural practices, such as liming for pH adjustment, irrigation, organic amendments, tillage etc.
Integrating variety selection, cultural practices, chemical treatments and mineral nutrition is found to be the best approach to plant disease control.
Mineral nutrition can affect two primary resistance mechanisms:
Formation of mechanical barriers (e.g. thickness of cell walls).
Synthesis of natural defense compounds (e.g. antioxidants, phytoalexins and flavenoids).
Basically, all diseases have a cycle of 3-5 parts, and any disease can be prevented or suppressed if that cycle is interrupted. Different pathogens have different infection mechanisms.
Fungi penetrate the surface cells (epidermis), by passing between the cells or through them. The cell walls present a physical resistance to the fungus and stronger cell walls can prevent the infection. Certain nutrients, like Calcium, play a major role in the ability of the plant to develop stronger cell walls and tissues.
Plants exude various compounds. When some nutrients are below a certain level, the released compounds contain higher amounts of sugars and amino acids that stimulate the establishment of the fungus.
Mineral nutrition also affects the production of antifungal compounds in plants.
Bacteria invade the plant tissue through wounds, sucking insects and through the stomata. Then they spread within the intercellular spaces. The bacteria release enzymes that dissolve the plant tissue. Calcium is known in its ability to inhibit such enzymes.
The ability of bacteria to spread within the plant tissues depends on the strength of the internal cells, which is greatly affected by mineral nutrition.
Another mechanism in which bacteria spread within the plant is in the xylem (the vessels that transport water in the plant). The bacteria forms slime within the vessels and block them. As a result, stems and leaves wilt and die. Certain plant nutrients suppress the ability of bacteria to form this slime.
Viruses are transmitted to plants by sucking insects and fungi. It was found that silicon, although it is not a plant nutrient, inhibits the feeding ability of some sucking pests like aphids. As a result, virus infection is reduced.
Many plant diseases are categorized as high pH diseases or low pH diseases, high ammonium or nitrate types, high or low moisture etc. Cultural conditions that influence the form of nitrogen also affect soil pH, and therefore disease development.
For example, diseases such as Verticillium wilt of vegetables, Phymatotrichum root rot of cotton, Thielaviopsis root rot of tobacco, are associated with alkaline soils.
Scab is reduced in lower soil pH. Sulfur and NH4+ are being used to decrease soil pH and reduce scab, while Ca, K, and NO3- may increase scab.
Different oxidation forms of the same nutrient often have opposing effects on plant disease. This is true mainly for nitrogen, sulfur, manganese and iron. For example, Nitrate (NO3-) and Ammonium (NH4-) nitrogen forms have different metabolic pathways and therefore different effects on plant disease.
Adequate nitrogen levels are necessary for disease resistance. However, excess nitrogen may promote favorable conditions to plant disease. Some of the reasons for this are:
Potassium is a very important nutrient in plant disease prevention, as it is involved in many cellular processes that influence disease severity. It was also found effective in the prevention of bacteria, fungi and nematodes. However, excess potassium and high K:Ca ratio might result in calcium deficiency and reduced resistance to diseases.
Mineral nutrition and cultural practices, that affect it, play a very important role in the prevention of plant disease and in the resistance of plants to diseases. Balanced plant nutrition can help growers minimize chemical use and increase productivity. In this article this important issue was addressed in a nutshell.
Although disease cannot be totally eliminated by any particular nutrient, the severity of the disease can be greatly reduced by adequate nutrition. Growers must use proper fertilizer programs, based on soil tests, plant analysis and disease monitoring.