Chemical disease control employs the use of chemicals that are either generally toxic and used as disinfectants or fumigants or chemicals that target specific kinds of pathogens, as in the case of fungicides, bactericides (or antibiotics) and nematicides. No substances have yet been found that effectively control plant viruses.
Ideally, a chemical control agent should be effective at concentrations that will not harm the plant, have low risk to humans and animals, and have minimal effect on the normal microflora on the plants and in the soil. Also, there should be little chance of the pathogen quickly developing resistance to it, and it should be suitable for long periods of storage in ambient conditions. These chemical agents can be sold as dusts, concentrated solutions, wettable powders, granules or emulsions.
Fungicides are chemicals used in the control of fungal diseases. They are often classified as either protectant or systemic. Protectant fungicides are usually effective against a broad range of fungi and protect the plant against infection on the surfaces of the plant to which they are applied. Often, they require multiple applications during the growing season to maintain coverage as new growth emerges and weathering removes past coverage. Systemic fungicides can be absorbed by the plant without harming it, and transported to other tissues where they are toxic to fungi. These compounds can control and eradicate established infections, but they are also vulnerable to fungi developing resistance, as they generally only target one step in a biosynthetic pathway to kill the fungus. To minimise the development of resistance by chemical overuse, fungicides are classified into groups based on their chemical activity. By alternating between the different classes of fungicides, the fungal population has less opportunity to build up resistance to one chemical. Link to Fungal Biology - Fungicides.
Relatively few antibiotics are routinely used to control plant diseases. Antibiotics are chemical produced by micro-organisms, which destroy or injure living organisms, in particular, bacteria. Streptomycin is effective against a few fruit pathogens, such as blights and cankers, and cyclohexamine can be used to control some fungal pathogens of crops, particularly powdery mildews and rusts. Bacteria, as well as fungi, have the ability to develop resistance to antibiotics, which is a major disadvantage of using these compounds, and one of the reasons that they are not widely used. Link to Fungal Biology - Fungicides.
The use of nematicides is confined largely to high-return horticultural crops, because they are expensive. Additionally, they are all highly toxic, and alternative measures for controlling nematodes are being investigated. Link to Fungal Biology - Nematodes.
Pathogen resistance to chemical agents has been a problem since the 1970s. Throughout the 1980s and 1990s, public concern has grown about the use of agricultural chemicals in general. The main concerns are the risk of poisoning humans or animals, contamination of livestock products, harm to beneficial insects, and the contamination of food products, waterways and soil. When properly used, fungicides should not cause problems in any of these areas, but they do have the potential to alter the balance of diseases on certain crops.
The main risk to humans is during the preparation or application of these chemicals, when they can be inhaled, ingested or absorbed through the skin, and upon consumption of plants or their products. To overcome these risks, safety precautions must be followed when working with the chemicals, and various regulations exist with regard to consumable plant products. For example, some chemicals may not be applied within a certain time before harvesting (the with-holding period), thus ensuring that chemical residues have fallen to an acceptable level by the time they reach the market. Maximum residue limits have been established to allow monitoring of the chemical residue on plant products.
Environmental concerns focus mainly on protectant fungicides. For example, copper and sulphur sprays have the potential to affect a broad range of organisms if they are washed off the leaves and accumulate in the soil or are washed into the waterways.
In response to public concern about these issues, many countries have initiated programs to reduced pesticide use. They can involve education of farmers on best practices in pesticide application, the use of lower doses of chemicals, better understanding of the threshold level of infection that causes economic loss and developing disease forecasting systems to allow optimal use of chemicals.