Fungicides that are also known as antimycotic are toxic substances that are used to kill or inhibit the growth of fungi. These are generally used to control parasite fungi as they damage agriculture resulting in damages to crops, yield and economic loss.
In other words, fungicide definition is that it is a special type of fungicide that is used to control the fungal disease in animals as well as decay of crops by killing or inhibiting the fungus causing the disease.
Fungicides are broadly classified into three categories, namely, contact, translaminar and systemic.
Contact fungicide, as the name suggests, is not distributed throughout all the plant tissues and just protects only the parts of the plants on which it is deposited while spraying.
Translaminar fungicides redistribute themselves from the upper part of the plant where they are sprayed to the lower unsprayed part of the plant.
Systemic fungicides are the most effective ones as they redistribute themselves to the vessels of the plant. Thus some fungicides are locally systemic, some spread throughout the plant tissues and some just stay on the surface on which they are sprayed.
Now, most of the fungicides used for farming today are chemical based fungicides. But there are few organic fungicides that are very effective for treating fungal diseases in plants and especially animals. Such organic fungicides examples are neem oil, horticulture oil and bicarbonates, jojoba oil, the good bacteria Bacillus subtilis and beneficial fungus Ulocladium Oudemansii.
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This is a sample of a dicarboximide fungicide N-(2-ethylhexyl)-5-norbornene-2,3-dicarbonyl bromide) whose molecular structure is shown in the following diagram.
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Fungicide chemicals are basically the most effective biocidal chemicals that are used to kill or prevent the growth of harmful fungus or their moulds development on plants, trees or animal skin like rusts, mildews or blights. The fungicide chemicals are also used to control oomycetes, though it is not considered as fungi, the mechanism by which it affects the plants is the same as that of fungus. Some of the class of fungicide chemicals with its example are mentioned below.
Now, most of these fungicides are in solid or liquid form. The major compound that is present in all the chemical fungicides is sulphur in different concentrations. In most of these chemical fungicides, sulfur is present at 0.08% in weaker concentrations but in a few of the potent fungicides, the concentration of sulfur goes up to 0.5%. In powder fungicides, the concentration of sulfur is as high as 90% and are therefore highly toxic in nature.
Benefits of Fungicide
Diseases and biological decay are common occurrences in plants that cause a significant impact on the economy due to the loss of crops as well as the values of ornamental plants. Therefore controlling the growth of fungus and bacteria and managing the diseases has become an essential part of the plantation as well as agriculture. The three most common reasons for the requirement of fungicides are
During the establishment and the development period of crops or plants, they are the most delicate in nature. So in order to prevent the fungus from spreading and causing biological decay of crops and plants in their initial stage of growth fungicides are needed.
It is important to reduce blemishes in order to promote healthy and abundant growth of crops and ornamental plants for a positive economic outcome. As blemishes reduce the crop yield because of improper photosynthesis due to the decaying of leaves. Also, the quality of edible crops reduces and for ornamental plants, their economic value drops. Hence fungicides are needed to control and reduce blemishes.
It is also needed to ensure the proper and appropriate condition of the crops, fruits and vegetables that are stored after their yield. Most of the time, the harvested crops acquire fungus due to climatic conditions like temperature that is suitable for fungal growth or the humidity in the air. This causes the biodegradation of crops. Also, a few which infect grains or fruits or even vegetables and seed produce toxic elements (mytoxines) that cause severe health damages as well as death if consumed by humans or animals. These days fungicides are used largely to reduce mycotoxin contamination in wheat that happens due to the presence of Fusarium Head Blight.
Types of Fungicides
Fungicides are categorized in many ways depending on their characteristics. Some of them are listed below.
Preventive or Curative (Role in Protection): Contacts are considered as preventive in action as they contact functional. They work on the surface where they are sprayed over or applied. The major drawback of the contacts is that they need to be repetitively applied over a certain period of time. It is because either they are washed away in rain or during irrigation or have a natural decay due to sunlight and other environmental factors. They are often referred to as “residual” products as they remain over the leaves or other areas and are visible in the form of residue for many days. On the other hand, systems are considered to be both preventive and curative as they move downwards after being sprayed over the top surfaces of the plant into its tissues. Therefore it affects the pathogen after infection.
Single Site or Multi-site (Breadth of Activity): Single fungicides have just a single point of action on a specific metabolism pathway of a pathogen or a single critical enzyme or a particular protein that is required by the fungus to grow. Since they are very specific in their actions and level of toxicity, they are not harmful to organism and are easily absorbed by the tissues of the plants. The effectiveness of this type of fungicide reduces over time as the fungi develop more resistance towards this fungicide because of the single mutation that happens inside the pathogen. Older fungicides were multi-site in nature and thus affected many types of fungi in different parts of plants. With the development of new rules and regulators on the use of fungicides and stringency on the number of tests required in order to register a new fungicide, scientists preferred to develop single-site fungicides.
Narrow Spectrum or Broad-spectrum (Breadth of Activity): Narrow spectrum fungicides are usually single-sited systemic fungicides that are effective on the fungus that is closely related to its pathogens. Whereas broad-spectrum fungicides are usually multi-sited and contact-based fungicides that are not just effective to the closely related pathogens but are also effective on the pathogens that are not related to it. But there are few broad-spectrum fungicides that are single sited in nature.
Organic and Inorganic Fungicides (Chemical Compositions): On the basis of the chemical compositions of the fungicides, they are classified as organic and inorganic fungicides. In chemistry, organic compounds are referred to as those which have carbon atoms in their molecules. Whereas, inorganic compounds refer to those that do not possess any carbon atom. Most fungicides that are used in a regular manner are inorganic compounds in which sulfur has the highest percentage followed by copper. Some of the fungicides use cadmium, mercury or tin as the inorganic fungicides. They are highly toxic in nature.
Fungicide refers to a situation where a particular fungicide, when used on a specific pathogen, was successful in controlling it but after one or two applications over the same pathogen, it fails to control the population due to the resistance development of that pathogen against the fungicide used.
This resistance is obtained through several revolutionary processes. Targeted pathogens are likely to develop resistance quickly against single-sited fungicides as compared to the multi-sited fungicides. Still, single-sited fungicides are preferred over multi-sited fungicides because of their low toxicity level and are less harmful towards non-targeted organisms. If the resistance develops due to the modification of a single major gene of the targeted pathogen, then the subpopulation of that pathogen will either be highly sensitive or highly resistant to that fungicide.
The resistance in the case of agriculture or plantation is considered as total loss of the pesticide as increasing the concentration of the pesticide or increasing the frequency of spreading pesticides will have absolutely no effects on the population. The most critical role in fungicide management is to increase the time of effectiveness of the fungicide which is at risk.
The main motto of fungicide resistance is to delay the development of the pathogen rather than managing the resistant strain of the pathogen after it is identified and targeted. Thus the best time to implement the resistance management programs is when the fungicide is first put into use after it is commercially available rather than when it is at risk. The main objective of fungicide management programs is to minimize the use of fungicide at risk without damaging the fungus control. This is accomplished when the at-risk pesticide is mixed with other pesticides for use or to incorporate a non-chemical method of controlling pathogens such as using disease resistance cultivators.