4. Methods of plant disease control: i. Eradication ii. Chemical control iii. Biological control (employing bacterial and fungal cultures) iv. Integrated pest management v. Genetic engineering for disease resistant plants
4. Methods of plant disease control
i. Eradication
ii. Chemical control
iii. Biological control (employing bacterial and fungal cultures)
iv. Integrated pest management
v. Genetic engineering for disease resistant plants
i) Eradication:
Eradication is the term used to describe the process of removing all infected plant material after the outbreak of either a new disease in an area or an old disease in a new area.
The domestic quarantine policies of many countries and states have achieved notable successes in eradicating serious pathogens affecting important crop industries.
However, more cynical observers believe that eradication does not prevent the spread of a pathogen, merely delays it.
One Australian example of successful eradication of a disease is citms canker.
Citrus canker is a generic term that includes a number of diseases of citrus caused by strains of the bacterium Xanthomonas.
There have been three outbreaks of citrus canker on the Australian mainland.
The original outbreak in the Northern Territory in 1912 probably resulted from the introduction of plants from Japan or China.
Following this outbreak, all diseased trees and 'canker-free' trees close to infected trees were destroyed. However, this action was not successful in preventing spread of the disease so, in 1922, all citrus trees in the Northern Territory north of 19' S latitude were destroyed.
Symptoms of citrus canker were observed near Darwin again in 1991 and the affected block of trees was destroyed.
The disease was again observed at the same location in 1993 and more trees were destroyed. The disease has not been seen since
ii) Chemical control:
The most common method of pest control is the use of pesticides—chemicals that either kill pests or inhibit their development.
Pesticides are often classified according to the pest they are intended to control.
For example, insecticides are used to control insects; herbicides to control plants; fungicides, fungi; rodenticides, rodents; avicides, birds; and bactericides to control bacteria.
Pesticides also include chemosterilants and growth regulators, which are used to interfere with the normal reproduction or development of the pest.
Chemical control of pests probably began with poisonous plant compounds.
In the 18th and 19th centuries, farmers ground up certain plants that were toxic to insects or rodents—plants such as chrysanthemums or tobacco.
The plant “soup” was then applied directly to either the crops or the pests.
Chemists later discovered that they could extract the toxic compounds from these poisonous plants and apply the compounds as liquid sprays.
Such chemicals as nicotine, petroleum, coal tar, turpentine, and pyrethrum (obtained from a type of chrysanthemum) were eventually extracted for use as sprays.
Organic compounds such as these were eventually replaced by more effective inorganic chemicals, including arsenic, lime, sulfur, and cyanide.
With the advent of synthetic organic compounds during World War II, a dramatic change occurred in pest control.
The discovery of the insecticidal properties of the synthetic compounds DDT (dichlorodiphenyltrichloroethane)—which was widely used against disease-spreading insects—during the war and BHC (benzene hexachloride) made the notion of pest-free crops realistic.
The development of another synthetic organic compound, the selective herbicide 2,4-D (2,4-dichlorophenoxyacetic acid), led to the development of other selective herbicides.
With the discovery of DDT, 2,4-D, and BHC, researchers began to develop other synthetic organic pesticides, especially growth regulators, chemosterilants, pyrethroids (compounds with insecticidal properties similar to those of pyrethrum), and organophosphate chemicals.
This research expanded in order to develop other, nonchemical, methods of pest control after the harmful persistence of pesticides in the environment was recognized.
It was discovered in the 1950s that DDT and its related compounds are not easily broken down in the environment.
DDT’s high stability leads to its accumulation in insects that constitute the diet of other animals.
These high levels of DDT have toxic effects on animals, especially certain birds and fishes. Scientists also found that many species of insects rapidly develop populations that are resistant to the pesticide.
By the 1960s, the value of DDT as an insecticide had decreased, and in the 1970s severe restrictions were imposed on its use.
In the United States, the Federal Environmental Pesticide Control Act of 1972 and the Federal Insecticide, Fungicide, and Rodenticide Act passed in 1972 required pesticide manufacturers to conduct scientific tests on the biological activity, defectiveness, persistence, and toxicity of any new pesticide before the chemical could be marketed.
The Environmental Protection Agency (EPA) was created in 1970 to ascertain past damage and possible future damage that could occur to the environment as the result of widespread pesticide use, and to set up programs to combat environmental problems.
An alternative concept of integrated pest management was adopted for many agricultural pests.
This approach involves non-chemical pest-control methods, including crop exclusion, crop rotation, sanitation, and biological control. These methods augment other pest control programs designed to minimize pesticide usage
.
iii) Biological control (employing bacterial and fungal cultures):
Plant diseases need to be controlled to maintain the quality and abundance of food, feed, and fiber produced by growers around the world. Different approaches may be used to prevent, mitigate or control plant diseases.
Biological control is nothing but ecological management of community of organisms. It involves harnessing disease-suppressive microorganisms to improve plant health.
Disease suppression by use of biological agents is the sustained manifestation of interactions among the plant (host), the pathogen, the biocontrol agent (antagonist), the microbial community on and around the plant and the physical environment.
Advantages:
1. Biological control is less costly and cheaper than any other methods,
2. Biocontrol agents give protection to the crop throughout the crop period,
3. They are highly effective against specific plant diseases,
4. They do not cause toxicity to the plants,
5. Application of biocontrol agents is safer to the environment and to the person who applies them,
6. They multiply easily in the soil and leave no residual problem,
7. Biocontrol agents can eliminate pathogens from the site of infection,
8. Biocontrol agents not only control the disease but also enhance the root and plant growth by way of encouraging the beneficial soil microflora. It increases the crop yield also. It helps in the volatilization and sequestration of certain inorganic nutrients. For example Bacillus subtilis solubilizes the element, phosphorous and makes it available to the plant.
9. Biocontrol agents are very easy to handle and apply to the target,
10. Biocontrol agents can be combined with biofertilizers,
11. They are easy to manufacture.
Disadvantages Although biological control is advantageous in many aspects, it has the following disadvantages:
1. Biocontrol agents can only be used against specific diseases,
2. They are less effective than the fungicides,
3. Biocontrol agents have slow effect in the control of plant diseases,
4. At present, only few biocontrol agents are available for use and are available only in few places,
5. They are unavailable in larger quantities at present,
6. This method is only a preventive measure and not a curative measure,
7. Biocontrol agents should be multiplied and supplied without contamination and this requires skilled persons,
8. The shelf life of biocontrol agents is short. Antagonists (A substance that acts against and blocks an action.), Trichoderma viride is viable for four months and Pseudomonas fluorescens is viable for 3 months only,
9. The required amount of population of biocontrol agents should be checked at periodical interval and should be maintained at required level for effective use,
10. The efficiency of biocontrol agents is mainly decided by environmental conditions,
11. A biocontrol agent under certain circumstances may become a pathogen.
Microbial Biological Control Agents
Biological control of plant diseases is the suppression of populations of plant pathogens by living organisms .
Amongst beneficial microorganisms isolates can be selected which are highly effective against pathogens and can be multiplied on artificial media.
Application of such selected and mass produced antagonists in high densities once or several times during a growing season is called “augmentative biological control”.
For commercial augmentative biological control of diseases, growers use MBCAs containing living microorganisms, that are registered plant protection products produced by biocontrol companies.
In some cases, antimicrobial metabolites produced by selected microbial organisms are included in the product, and some products even contain only antimicrobial metabolites without living cells of the antagonist .
Also mycoviruses and bacteriophages can be potential MBCAs against plant pathogens.
Microbial biological control agents protect crops from damage by diseases via different modes of action .
They may induce resistance or prime enhanced resistance against infections by a pathogen in plant tissues without direct antagonistic interaction with the pathogen .
MBCAs may also interact directly with the pathogen by hyperparasitism or antibiosis.
Hyperparasites invade and kill mycelium, spores, and resting structures of fungal pathogens and cells of bacterial pathogens .
Production of antimicrobial secondary metabolites with inhibiting effects against pathogens is another direct mode of action .
In some cases, biocontrol agents have been selected which secrete already efficient secondary metabolites into the growth media during mass production that are applied together with or without living cells of antagonists in the biological control product.
iv) Integrated pest management:
Watch out for problems on your plants as they arise.
If a pest or disease causes unacceptable damage despite preventative efforts, choose an effective management method that will have the least amount of impact on other living creatures and the environment.
Steps to Make an IPM Control Decision
1.Confirm That There is a Pest Problem:
Look for pests and diseases and the evidence or signs they leave.
Look for symptoms the plant exhibits as a result of pest activity.
Examine your plants often:
Identify your plants to be sure that the twisted leaves, unusual coloration, or strange-looking structures you see are not a normal part of the plant.
Try to rule out site-related problems by making sure that the soil type, drainage conditions, fertility level, and other environmental conditions are favorable for the plant.
2. Identify the Problem :
Effective pest management depends on the accurate identification of the pest.
Insects and mites often are associated with specific plants, and they follow certain development and behavior patterns as the season progresses.
Use reference books from the library or garden center to identify pests.
If you can't find an accurate description there, consult someone in your local extension office.
Learn about the insect's life cycle, behavior, and natural enemies.
Plant diseases may be caused by pathogens including fungi, nematodes, bacteria, or viruses. Each pathogen is capable of infecting only certain plants.
Infection occurs under particular environmental conditions, with symptoms of the disease appearing later.
To identify plant diseases accurately, compare visible signs and symptoms of the disease with descriptions in reference books.
Some diseases are more difficult to identify, and you may need a laboratory analysis or the help of an expert. These services are available through landscape professionals or your local extension office.
Choose a Method
If a control is needed, consider physical or biorational methods first.
If they are unavailable or impractical, you may need to carefully use a conventional chemical control.
1. Physical methods:
Pests can be removed from plants physically.
For example, some aphids and mites can be knocked off by spraying the plant with water.
Bagworm larvae can be picked off an infested plant.
You can use traps to catch certain pests, and barriers to protect plants from insect attack or disease infection. One effective method for controlling gypsy moth larvae on small numbers of trees is to put a band of folded burlap around the tree trunk to provide an artificial resting site for the caterpillars, and then destroy the caterpillars that gather there.
Applying an anti-transpirant spray to lilac leaves in summer to prevent infection by the spores of powdery mildew is another example of a protective barrier.
In some cases, the best solution may be physically removing the plant and replacing it with one that will not be affected by the pest or disease.
Thinning crowded plants to improve air circulation can reduce many disease problems.
2.Biorational methods:
Biorational methods can be divided into two groups.
The first group includes living organisms that can kill the pest.
The second group includes naturally occurring biochemicals that are harmful to the pest yet often are harmless to other living organisms.
Insect pests frequently have natural enemies that are beneficial to the landscape.
These beneficial insects often exist in the landscape naturally, but they also can be introduced. "Beneficials" may be predators or parasites.
One common example of a beneficial predator is the lady beetle.
Both the larvae and adult lady beetles eat aphids and other soft-bodied insects.
Other predators include lacewings, spined soldier bugs, flower flies, and spiders.
Parasites live on and often kill another organism, called the host.
Some parasitic wasps use caterpillars, whiteflies, aphids, and soft scales as hosts.
An example of a method that uses a naturally occurring biochemical is the bacterium Bacillus thuringiensis (Bt). Bt contains a protein that is poisonous to specific insects, yet harmless to other organisms.
Bt can be sprayed on plants.
When the sensitive insect pest feeds on the sprayed leaves, it will ingest the protein and be killed.
3.Chemicals:
Conventional chemicals are used only as a last resort in an IPM program, but sometimes are the most effective means of control.
To have the greatest effect, these materials need to be applied on a specific part of the plant when the pest is most vulnerable.
Always apply chemical controls according to label directions.
In many cases, environmentally safe pesticides such as horticultural oil or insecticidal soap are effective choices. Again, applications must be timed carefully to have the greatest effect on the pest insect population.
Because they have no residual activity after they have dried, soaps and oils are usually the option that is the least disruptive to populations of beneficial organisms.
The label of every pesticide formulation displays a signal word that relates to the pesticide's level of toxicity. These words, from least toxic to most toxic are: "caution," "warning," and "danger."
Use these words as guidelines to help you choose the least hazardous material among the effective alternatives. For most landscape pests, you need to consider pesticides in only the first two categories.
Some pesticide formulations can be applied only by applicators with special training and who are certified by the state's department of agriculture.
A pesticide may be applied only to plants that are listed on the label.
Follow these procedures for your safety and success:
- Mix pesticides according to label instructions. Do not use more or less concentrate in the mixture than the label recommends. Mix only as much material as you need for the application.
- Wear protective clothing as specified on the label.
- Label a set of mixing and measuring tools that are used only for insecticides and fungicides, and store them with the products. Use a separate set of measuring tools and spray equipment for herbicides.
- Keep pets and people away from the area where you store, mix, and apply pesticides. Stay away from a treated area for as long as the label directs.
- Do not spray on a windy day or when air temperatures will be above 85°F before the spray solution dries.
- Clean equipment and mixing tools as soon as you finish spraying.
- After spraying, change your protective clothing and bathe. Wash the clothes you were wearing separately from your regular laundry.
- Keep records of where and when you sprayed, what pesticide you used, and how much you used. Give the treatment time to work, then evaluate and record your results.
v) Genetic engineering for disease resistant plants
Transfer and expression of foreign genes in plant cells, now routine practice in several laboratories around the world, has become a major tool to carry out gene expression studies and to obtain plant varieties of potential agricultural interest.
The capacity to introduce and express diverse foreign genes in plants, first described for tobacco in De Block (1984), has been extended to many species.
Transgenic crops such as tomato, papaya, cotton, maize, soybean etc., are now available for human consumption and by complementing traditional methods of crop improvement (and thus becoming an integral part of agriculture), they will have a profound impact on food production, economic development and on the development of a sustainable agricultural system during the 21st century.
Although the capacity to introduce and manipulate specific gene expression in plants provides a powerful tool for fundamental research, much of the support for plant transformation research has been provided because of the generation of plants with useful and rapidly discernible phenotypes which are unachievable by conventional plant breeding i.e., resistance to viruses, insects, herbicides, or postharvest deterioration.
Bt stands for Bacillus thuringiensis (Bt) a common soil bacterium so called because it was first isolated in the Thuringia region of Germany.
Bt produces a protein that paralyzes the larvae of some harmful insects, including the cotton bollworm and the Asian and European corn borers, all of which are common plant pests whose infestations produce devastating effects on important crops.
Mode of Action
When ingested by the larva of the target insect, the Bt protein is activated in the gut’s alkaline condition and punctures the mid-gut leaving the insect unable to eat. The insect dies within a few days.
It is because of its ability to produce the insecticidal protein that much research is being done to exploit the organism’s agronomic value.
To date, there are more than 200 types of Bt proteins identified with varying degrees of toxicity to some insects.
Advantages of Bt Crops
1.Improved pest management. Insect-protected Bt crops provide the farmer with season-long protection against several damaging insect pests, and reduce or eliminate the need for insecticide sprays.
2.Reduction in insecticide use.
3.Greater net return. Lower input costs often contribute to a higher net return compared to conventional crops.
4.Improved conditions for non-target organisms.
Reference links:
https://kids.britannica.com/students/article/pest-control/276378
https://www.appsnet.org/Publications/Brown_Ogle/23%20Control-exclusion%20&%20eradication%20(HJO).pdf
https://extension.psu.edu/pest-management-methods
https://www.isaaa.org/resources/publications/pocketk/6/default.asp
https://www.bighaat.com/blogs/kb/viral-diseases-in-plants-and-its-control
https://www.future-science.com/doi/10.2144/btn-2020-0098
https://www.researchgate.net/publication/319442989_RNA_interference-a_novel_approach_for_plant_disease_management
https://www.researchgate.net/publication/319442989_RNA_interference-a_novel_approach_for_plant_disease_management/link/59aa8472458515d09cd4d201/download
https://www.researchgate.net/publication/308625816_Mycoviruses_and_their_role_in_biological_control_of_plant_diseases
file:///C:/Users/KKW%20PC212/Downloads/Chapter_18.pdf
file:///C:/Users/KKW%20PC212/Downloads/Chapter_10.pdf
https://www.frontiersin.org/articles/10.3389/fpls.2019.00845/full
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