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Friday, October 31, 2008

BIOLOGICAL CONTROL OF PLANT DISEASES BY MICROORGANISMS


What is biological control?

Biological control can be defined as the use of natural enemies to reduce the damage caused by a pest population
• Biological control differs from "natural control
• Biological control requires intervention, rather than simply letting nature take its course
• It is an approach that fits into an overall pest management program, and represents an alternative to continued reliance on pesticides
Biological Control of diseases by microorganism

• Plant diseases are caused mainly by fungi, bacteria, viruses and nematodes

• Biocontrol of plant disease involves the use of an organism or organisms to reduce disease
Biological Control Targets
Agents that cause plant disease
Agents that cause plant damage
Weeds

What is a biological contol agent
means any living organism applied to or introduced into the environment that is intended to function as a destroying agent to control another organism ...

Biological control agents
Macroorganisms biological control agents

• Insects
• Man
• Animals

any living microorganism (Bacteria, Fungi, Nematodes & Viruses) applied to or introduced into the environment that is intended to function as a destroying to control another organism

Microbial biological control agents are
• Bacteria
• Fungi
• Nematodes
• Viruses
Bacterial Biological Control Agents
Xanthomonas campestris pv. poannua - postemergence activity on annual bluegrass in bermudagrass lawns (Johnson, 1994: Johnson, Wyse, Jones, 1996).
Pseudomonas syringae pv. tagetis - Canada thistle in soybean (Johnson, Wyse, Jones, 1996).

E.g.- Bacillus species

Bacillus spp have been widely exploited for the management of plant pathogens.

Bacillus is an ideal candidate for biocontrol of plant pathogens because of its ability to form endospores which are resting structures capable of surviving desiccation, heat, oxidising agents, UV & gamma radiations.

Many post-harvest diseases are successfully managed with Bacillus spp.
The broad spectrum activity, absence of phytoxigenic products, amenability for mass culturing, the ability to form endospores, availability of information on the genetics, physiology, r-DNA technology etc are some of the advantages of the genus Bacillus
Brown rot of potato caused by Ralstonia solanacearum is also amenable to management with Bacillus.

Bacillus spp, have been tested worldwide for management of pathogens causing spots, blights, rots etc. On the above ground parts of several plants.

Fungal Disease Control Agents

• Ampelomyces quisqualis.
• Candida oleophila.
• Coniothyrium minitans.
• Fusarium oxysporum.
• Gliocladium virens.
• Gliocladium catenulatum.
• Phlebia gigantea.
• Pythium oligandrum.
• Trichoderma harzianum and other spp.

Nematodes

There are over 300 species of nematodes that are known to attack insects.
Most of the research in biological control, however, has focused on only two genera, Steinernema and Heterorhabditis.
These nematodes are unique because they harbor symbiotic bacteria that are pathogenic to the nematode's insect host.

e.g.-nematode attacks insect pests
Nine families of nematodes (Allantone-matidae, Diplogasteridae, Heterorhabditidae, Mermithidae, Neotylenchidae, Rhabditidae, Sphaerulariidae, Steinernematidae, and Tetradonematidae) include species that attack insects and kill or sterilize them, or alter their development.
Viruses
• The use of entomopathic viruses for insect control is still in its infancy.
• Many of these pathogens appear to have good potential as bio control agents because they are relatively host-specific.
• Viral-induced mortality is usually caused by toxic proteins that accumulate during the reproductive cycle of the virus.

NPV or CPV (Nuclear or Cytoplasmic

Polyhedrosis Virus) -- Clusters of virus particles are embedded within polyhedral inclusion bodies (crystals) that develop inside the nucleus or cytoplasm of infected cells.

They usually attack larvae of Lepidoptera or Hymenoptera (sawflies).

There is also commercial interest in developing NPVs for use against corn earworms, cotton bollworms, cabbage loopers, and alfalfa butterflies.

Granulosis virus -- These pathogens typically infect the fat body in Lepidopteran larvae and pupae.

E.g.-A granulosis virus has been developed for use in apple orchards against larvae of the codling moth (Cydia pomonella).

Non-inclusion viruses -- These pathogens (entomopox virus, for example) do not produce granules or polyhedral bodies. The cause of their toxicity is not well understood, but they are usually less virulent than other types of viruses.

Biological Control Mechanisms
Competition
Antibiosis
Parasitism
Induced resistance
enzymes
Competition
The Biological control agent more efficiently utilizes space and nutrients.


Antibiosis
the Biological control agent produces one or more deleterious compounds.

Production of antibiotics seems to be the most important way in which many spp of Bacillus bring about bio control of plant pathogens.

Two antibiotics, iturin and surfactin have been B. subtilis indicated in the suppression of R. solani induced damping off of tomato by B. subtilis


Parasitism
The Biological control agent utilizes the target for food or for reproduction.
Biological control agents parasite on
• Larvae
• Eggs
• Pupa

Induced resistance
The Biological control agent indirectly stimulates the plant to be resistant.
Several beneficial rhizosphere bacteria induce systemic resistance against one or more diseases.
Increased Phenylalanine ammonia lyase (PAL) activity and increased lignification acting as barrier to the fusarial wilt pathogen in pigeon pea treated with the above bacterisl stain.
Enzymes
Mycelial lysis by enzyme production is one another mode of antagonism by many Bacillus spp.

Hydrolytic enzyme produced and released by bacteria contribute to the suppression of phytopathogens
Using Biological control agents
Advantages of Bio control

 Most natural enemies used in biocontrol attack on target species & less possibility to destroy non- target organisms
 Environmental friendly
• No any waste remain in the environment
 Economically friendly
• Reduce cost for pesticides
 Useful in Integrated pest management

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