Effect of fertilizers on plant diseases

S. Parthasarathy,

Ph.D Scholar, Department of Plant Pathology, Centre for Plant Protection Studies,

 Tamil Nadu Agricultural University, Coimbatore 641 003, Tamilnadu.

Adequate mineral nutrition is central to crop production. However, it can also exert considerable Influence on disease development. Fertilizer application can increase or decrease development of diseases caused by different pathogens, and the mechanisms responsible are complex, including effects of nutrients on plant growth, plant resistance mechanisms and direct effects on the pathogen. The effects of mineral nutrition on plant disease and the mechanisms responsible for those effects have been dealt with comprehensively elsewhere. The sections below will deal briefly with the influence of nitrogen, phosphorus, potassium, calcium and silicon on plant disease.

Nitrogen

Nitrogen fertilizer applied above the recommended rates can result in increased disease incidence and lesion area. This has been demonstrated for biotrophic fungal pathogens such as powdery mildews and rusts and necrotrophic pathogens such as Magnaporthe grisea, the cause of rice blast. It is commonly thought that application of nitrogen fertilizer can increase disease severity via effects on crop canopy development. Thus, large canopies with high shoot densities may be more conducive to spore transfer and pathogen infection than sparse canopies. For example, nitrogen has been shown to increase the severity of Fusarium head blight in wheat, and it has been suggested that this might be the result of a nitrogen-induced increase in canopy size, leading to an altered microclimate. In contrast, work on yellow rust on winter wheat suggested that the impact of nitrogen on disease was the result of effects of nitrogenous substances in wheat leaves on pathogen growth, rather than effects on canopy growth and microclimate. However, nitrogen fertilization is not always associated with increased disease.

            Several studies have reported no effect of nitrogen on disease severity, also the effect of nitrogen depended on the type of pathogen. Thus, nitrogen increased susceptibility of tomato to the powdery mildew pathogen Oidium lycopersicum and the bacterial pathogen Pseudomonas syringae pv. tomato, while it had no effect on susceptibility to the vascular wilt pathogen Fusarium oxysporum f. sp. lycopersici. In contrast, tomato plants were more susceptible to Botrytis cinerea when grown under low nitrogen conditions. It is clear therefore that generalizing about the effects of nitrogen on plant disease is unwise and practically, although manipulation or assessment of crop nitrogen status might be used as part of disease control strategies, the approach adopted will depend on the crop and the pathogens from which it is most at risk.

Phosphorus

            In an analysis of some studies of the effects of fertilizer on more than 400 diseases and pests, found that, in general, phosphorus fertilization tended to improve plant health, with reductions in disease recorded in 65% of cases. Nevertheless, phosphorus fertilization increased disease and pest problems in 28% of the cases examined. As with nitrogen, the effects of phosphorus on plant disease may be the result of direct effects on the pathogen, host plant metabolism, leading to effects on pathogen food supply, and effects on plant defenses. Indeed, foliar application of phosphate salts has been shown to induce resistance to pathogens in a range of crop plants, including cucumber, broad bean, grapevine, maize and rice. Clearly, an adequate phosphorus supply is important for crop growth and in turn, may well help to reduce disease. However, the regime of phosphorus fertilizer used will depend on a range of factors, including the crop and the pathogens likely to be important.  suggested that foliar-applied phosphate might be used as part of an integrated disease control programme, although grower adoption of such an approach will depend on the existence of other effective disease control measures and the economics of disease control in the particular crop.

Potassium

            In an analysis of 181 papers reporting effects of potassium on plant disease, found that 120 (66%) reported reductions in disease, while 49 (27%) reported an increase in disease. Although this suggests that in many cases, potassium is associated with disease reductions, point out that inadequate consideration has been given to the effects of associated anions, nutrient balance and nutrient status to allow the definitive role of potassium to be determined. For example, it has been suggested that in some cases, the effects of potassium, applied as potassium chloride fertilizer, might be due to the chloride ion rather than potassium. Further, chloride fertilization has been shown to suppress disease in cereal crops.

            Foliar-applied potassium chloride has been shown to control Blumeria graminis and Septoria tritici on wheat in field studies, probably due to osmotic effects on the fungal pathogens, disrupting pathogen development and subsequent infection. Application of potassium to deficient soils usually increases plant resistance to diseases. This might be partly related to the effect of potassium in increasing epidermal cell wall thickness or disease escape as a result of vigorous crop growth, although the mechanisms by which potassium affects plant disease are not well understood.

Calcium

            There are many reports that application of calcium to soils, foliage and fruit reduces the incidence and severity of a range of diseases of crops, including cereals, vegetable crops, legumes, fruit trees, as well as post-harvest diseases of tubers and fruits. For example, calcium has been shown to inhibit anthracnose caused by Colletotrichum gloeosporioides or C. acutatum in apples and to decrease post-harvest disease development on strawberry, while treatment of tomato with calcium carbonate reduced Fusarium crown rot disease. In contrast, could find no effect of calcium on anthracnose on strawberry. Because calcium increases resistance of plant cell membranes and cell walls to microbial enzymes, increasing calcium concentrations in storage organs could lead to enhanced resistance to pathogens. However, the form in which the calcium is applied can influence the mechanism by which calcium affects disease. For example, the addition of lime can affect disease by altering pH, while calcium salts (e.g. propionate) can be directly inhibitory to pathogens. Making general recommendations for the use of calcium in plant disease control would be unwise due to the range of crops and pathogens affected by calcium application. Instead, the appropriate amount and form of calcium to be applied needs to be determined for individual crop–pathogen interactions. The dwindling availability of fungicides, together with increasing public concern for the environment means that the use of calcium to control plant disease, especially post-harvest, is attracting increased attention.

Silicon

            Although the effects of silicon in reducing disease severity have been known since 1940, it was not until the 1980s that more detailed work was carried out in this area. In this work, cucumbers grown in nutrient solutions supplemented with silicon were found to have significantly less powdery mildew infection than plants not receiving silicon supplementation. Indeed, silicon has been shown to suppress both foliar and soil-borne pathogens in cucurbits and to reduce susceptibility of rice to various pathogens. Wheat grown in soil amended with silicon showed reduced infection by several pathogens, including B. graminis f. sp. tritici, S. tritici and Oculimacula yallundae.

            It has been suggested that the effects of silicon in providing disease control are due to the creation of a mechanical barrier to penetration. However, this has been disputed by studies which could find no evidence for the creation of a physical barrier following silicon treatment in wheat inoculated with powdery mildew and bitter gourd and tomato inoculated with Pythium aphanidermatum. Rather, several studies have suggested that silicon activates defences in plants. For example, in wheat inoculated with B. graminis f. sp. tritici, epidermal cells of silicon-treated plants were shown to react to attempted infection with specific defences, including papilla formation and callose production. In the rice M. grisea pathosystem, silicon-mediated resistance was found to be associated with accumulation of antimicrobial compounds at infection sites, including diterpenoid phytoalexins. In fact, phytoalexin accumulation occurs in silicon-mediated resistance in both dicots and monocots and since phytoalexins are highly specific to plant species, it has been suggested that silicon might be acting on mechanisms shared by all plant species, for example, those resulting in activation of plant stress genes.