Fungicides (L. caedo = to kill) are the chemicals that kill fungi, the causal organisms of the majority of plant diseases, and are the most frequently used chemicals to control plant dusts, sprays, pastes or slurry, (1) to apply protective covers to kill the pathogen present and, by residual action, prevent subsequent infection, and (2) destroy pathogen on the dormant plant parts.
Properties of a Perfect Fungicide:
Following are the properties of a perfect fungicide:
i. It should be safe to store and transport.
ii. It should be simple to apply at a precise dosage level.
iii. It must offer effective and consistent disease control.
iv. It should not be toxic to plants at the recommended dose level.
v. It should not adversely affect other parts of the crop ecosystem.
vi. It must not constitute a hazard during application.
vii. Residues of the fungicide in the crop should not pose a problem for the consumer.
viii. The financial return should increase the cost of the fungicide and its application.
Types of Fungicides:
Fungicides are broadly classified as of two types:
1. Protectant and
2. Systemic.
1. Protectant Fungicides:
Protectant fungicides, when applied on plants or plant parts, supplement the defences of the plant by forming a superficial chemical barrier to prevent, or protect against, infection. Though the protectant fungicides are effective against a wide range of fungal pathogens, they have limitations in practical use.
These limitations are that:
(i) They must be applied before the pathogen initiates infection hence there should be a reliable, early warning of an infection risk if they are to be used effectively and economically,
(ii) They are subject to degradation and erosion by light, rain, and other environmental factor because they form surface coatings, and
(iii) Their applications to growing plants rapidly become ineffective as new leaves, flowers, and fruits continue to develop.
Due to these limitations the protectant fungicides need to be applied to a crop at regular intervals throughout the growing season.
Mode of Action:
Most of the protectant fungicides are known to be multi-site inhibitors, which interfere with the central metabolic processes of the target fungus. Indeed the majority of these fungicides appear to affect ATP (energy) production, either by inhibiting respiration or by uncoupling oxidative phosphorylation.
Metal-based protectant fungicides such as copper or mercury inhibit variety of-enzymes necessary for various metabolic pathways. Similarly, dithiocarbanates make a complex with thiol groups on proteins, thereby inactivating enzymes and ultimately causing cell death. Pyrenophora avenae is a remarkable exception to this rule as it manages to overcome the toxicity of mercury applied as a seed-dressing to oats.
2. Systemic Fungicides:
Systemic fungicides, when applied on plants or plant parts, are actually absorbed and translocated systemically by the plant and poison the pathogen from within. The systemic fungicides therefore are therapeutic (eradicant) in action against an established infection.
These fungicides, when absorbed by the plant, are translocated through apoplast (xylem, cell walls) and hence tend to travel from base to the top of the plant accumulating in leaves and shoot apices.
Once they accumulate in leaves and shoot apices, they fail to be translocated through phloem downward. For this reason, the systemic fungicides are usually ineffective against soil- borne pathogens infecting roots or other subterranean organs.
Mode of Action:
Most of the systemic fungicides are single-site inhibitors, i.e., they inhibit a specific enzyme or process in the cell. Benzimidazols inhibit cell division by acting upon β-tubulin, a polymeric protein present in microtubules that are the essential components of the cytoskeleton. Benzimidazols bind tubulin molecules and prevent its polymerization thus disrupting the normal activities of the cytoskeleton including spindle formation during cell division.
Azoles and imidazoles act upon enzyme sterol 14α – demethylase and interfere with the biosynthesis of sterols; the sterol 14α-demethylase catalyzes a single demethylation step in sterol biosynthesis pathway. Morpholines are also sterol biosynthesis inhibitors (SBIs) like azoles and imidazoles, but act upon different steps affecting isomerization and reduction of sterol.
Other examples of single-site inhibitors are oxanthiins (affect enzyme in citric acid cycle), phenylamides (affect RNA polymerase hence nucleic acid synthesis), organophosphorus fungicides (affect membrane function), and strobilurin fungicides, which are recently introduced and block mitochondrial electron transport.
A comparative account showing the contrasting properties of protectant and systemic fungicides is given below:
Protectant Fungicides:
(i) Protectant fungicides are multi-site inhibitors.
(ii) They are prophylactic, i.e., protective in function.
(iii) Many metabolic systems of fungal pathogens are affected.
(iv) Phytotoxicity is common, especially if applied to wrong tissue or an inappropriate host.
(v) Variety of pathogens and affected.
(vi) Pathogens rarely develop any resistance against these.
(vii) They are localized in action, i.e., effective only in the plant area to which they are applied.
(viii) They are confined to redistribution on plant surfaces.
Systemic Fungicides:
(i) Systemic fungicides are single-site inhibitors.
(ii) They are curative, i.e., therapeutic (eradicant) in function.
(iii) Few metabolic systems of fungal pathogens are affected.
(iv) Phytotoxicity is rare.
(v) Show variability as some are extremely specific, others are effective against a broad spectrum.
(vi) Pathogen resistance development commonly takes place.
(vii) They are systemic in action, i.e., effective throughout the plant.
(viii) They are translocated inside the plant usually through apoplast (xylem, cell walls).
Pathogen Resistance to Fungicides:
Protective fungicides, the broad specturm chemicals, were commonly used to control plant diseases prior to the discovery of systemic fungicides that are selective in nature.
There were very few instances when application of a protectant fungicide failed to control a pathogen if it was materialized at the correct time and recommended dose rate. The protectant fungicides, in general, provided long-term and durable control of many crop diseases.
For instance, sulphur, copper and dithiocarbamate fungicides have remained as effective as when they were first discovered. The reason behind the success of protectant fungicides is attributed to the fact that these chemicals are multi-site inhibitors and numerous mutations are required in the genes of the pathogen to develop resistance to these chemicals. However, such behaviour of protectant fungicides is not a rule as certain exceptions do prevail.
When systemic fungicides were discovered, they were preferred over the protectant fungicides for use to control plant diseases. But, the practical experience with them has been different. There have been numerous cases in which an initially highly effective fungicide has subsequently failed to control a pathogen in a crop.
Sometimes such failures have occurred within a short time of first use of the compound. The systemic fungicides are selective and mostly single- site inhibitors, they are much more likely to encounter problems of resistance since only a single mutation in the gene of the pathogen may be sufficient to counter the action of the compound.
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