Plant diseases are a major limiting factor in agricultural production. Most growers use high amounts of chemicals to control plant diseases, but they are unaware that mineral nutrition has an important role in disease control.
Essential plant nutrients influence the health of plants and their susceptibility to disease. In other words, plants suffering a nutrient stress will be more susceptible to diseases, while adequate crop nutrition makes plants more tolerant of or resistant to disease.
(Note that the resistance of plants to diseases is mainly related to genetics, but 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 could have opposite impacts on different diseases and in different environments—for example, the same nutrient might decrease the incidence of one disease, but increase the incidence of others.
Soil pH, nitrogen forms and the availability of nutrients also 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.
How can mineral nutrition prevent plant disease?
Basically, all diseases have a cycle of three to five parts, and any disease can be prevented or suppressed if that cycle is interrupted. Mineral nutrition can affect two primary resistance mechanisms: first, the formation of mechanical barriers (e.g. thickness of cell walls), and secondly, the synthesis of natural defence compounds (e.g. antioxidants, phytoalexins and flavonoids).
Different pathogens have different infection mechanisms. Fungi penetrate surface cells (the epidermis) by passing between or through the cells. 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.
Mineral nutrition also affects the production of antifungal compounds in plants. However, when some nutrients are below a certain level, plants release compounds that contain higher amounts of sugars and amino acids, thus stimulating the establishment of the fungus.
Bacteria invade the plant tissue through wounds, sucking insects and through the stomata. Then they spread within the intercellular spaces, releasing enzymes that dissolve the plant tissue. 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. Calcium is known in its ability to inhibit such enzymes.
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, which blocks them and results in stems and leaves wilting and dying. 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 not a plant nutrient—inhibits the feeding ability of some sucking pests like aphids, thus reducing viral infection.
Cultural practices and soil condition
Many plant diseases are categorized as high-pH diseases or low-pH diseases, high-ammonium or high-nitrate types, high- or low-moisture varietals, etc. Thus, cultural conditions that influence the form of nitrogen also affect soil pH and, therefore, disease development.
For example, verticillium wilt of vegetables, phymatotrichum root rot of cotton, thielaviopsis root rot of tobacco are all associated with alkaline soils. Another example, scab is reduced in lower soil pH. Sulfur and ammonium are used to decrease soil pH and reduce scab, while calcium, potassium and nitrate might increase scab.
Same nutrient, different effect
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 and ammonium nitrogen forms have different metabolic pathways. Therefore, they have different effects on plant disease.
Adequate nitrogen levels are necessary for disease resistance. However, excess nitrogen could promote favorable conditions to plant disease. Reasons for this include:
- Excess nitrogen promotes thinner and weaker cell walls.
- Increased plants density creates high humidity and low light conditions.
- Excess nitrogen delays maturity of plant tissues and, therefore, increases the risk of disease infection and development.
- An unbalanced sodium–potassium ratio affects both yield and disease resistance.
Potassium and potassium–calcium ratio
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 a high potassium–calcium 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.
Although disease cannot be totally eliminated by any particular nutrient (growers must also use proper fertilizer programs based on soil tests, plant analysis and disease monitoring), balanced plant nutrition can help growers minimize chemical use, increase productivity and reduce the severity of disease.