Definitions used in biological control

Biological control of PLANT DISEASES

Overview

• What is biological control, what are the benefits to its use

• Mechanism of biological control

• Requirements of successful biocontrol

• Working example of biocontrol

Concept of biological control

Idea was first developed by entomologists as natural control of insect population – “the action of parasites, predators or pathogens in maintaining harmful insect population density at a lower average than would occur in their absence”.

Later – terms like biological methods, parasitic control, natural control etc. have been used for this type of control.

What is biological control?

• First coined by Harry Smith in relation to the biological control of insects– Suppression of insect populations by

native or introduced enemies

• Generic terms– A population-leveling process in which the

population of one species lowers the number of another

Definitions

“Any condition under which or practice whereby survival or activity of a pathogen is reduced through the agency of any other living organism (except man himself) with the result that there is a reduction in the incidence of the disease caused by the pathogen” - Garret (1965)

“Reduction of inoculum density or disease producing activities of a pathogen or parasite in its active or dormant state, by one or more organisms, accomplished naturally or through manipulation of the environment, host, or antagonist, or by mass introduction of one or more antagonists” – Baker and Cook (1974)

Other definitions

“Use of microbial antagonists to suppress diseases as well as the use of host-specific pathogens to control weed populations”.

“Use of natural or modified organisms, genes, or gene products, to reduce the effects of undesirable organisms and to favour desirable organisms such as crops, beneficial insects, and microorganisms”.

“The suppression of damaging activities of one organism by one or more other organisms, often referred to as natural enemies”.

“The exploitation by humans of the natural competition, parasitism and/or antagonism of organisms for management of pests and pathogens”

More specific definitions

• The purposeful utilization of introduced or resident living organisms, other than disease resistant host plants, to suppress the activities and populations of one or more plant pathogens.

• The suppression of a single pathogen (or pest), by a single antagonist, in a single cropping system.

• Biological control is the total or partial destruction of pathogen populations by other organisms.

• The exploitation by humans of the natural competition, parasitism and/or antagonism of organisms for management of pests and pathogens.

Related Terms

• Ecology- the study of the interactions that occur between individual organisms, groups of organisms and organisms and their environment

• Ecosystem- a community of living things and their environment

• Biodiversitythe existing genetic variability among living organisms

• Antagonisma general term for interference between organisms that may include antibiosis or competition for nutrients or space (contrasts with synergism)

• Symbiosis (adj. symbiotic; n. symbiont)the living together of two different kinds of organisms that may, but does not necessarily, benefit each organism

• Antibiosisan association between organisms, or between an organism and a metabolic product of another organism, that is harmful to one of them

• Antibiotic a chemical compound produced by one microorganism that inhibits growth or kills other living organisms

• Bacteriocina protein antibiotic, one or more types of which can be produced and excreted by certain strains of bacteria

• Bacteriophage a virus that infects a bacterium

Related Terms

Mycorrhiza (pl. mycorrhizae; adj. mycorrhizal) a symbiotic association between a nonpathogenic or weakly pathogenic fungus and the roots of plants.Ectomycorrhiza (pl. ectomycorrhizae) a symbiotic association between a nonpathogenic or weakly pathogenic fungus and the roots of plants with fungal hyphae between and external to root cells. (contrasts with arbuscular mycorrhiza, endomycorrhiza)Endomycorrhiza (pl. endomycorrhizae) a symbiotic association between a nonpathogenic or weakly pathogenic fungus and the roots of plants in which fungal hyphae invade cortical cells of the root.Arbuscular mycorrhiza (acronym AM) a symbiotic association between a nonpathogenic or weakly pathogenic fungus and the roots of plants in which fungal hyphae invade cortical cells of the root and produce vesicles and arbuscles.Arbuscule (adj. arbuscular) a branched haustorial structure of certain endomycorrhizal fungi that forms within living cells of the root.

Related Terms

• Endophyte - a plant developing inside another organism;

also used for endoparasitic fungi found in grass species• Fungistasis - the inhibition of fungal growth, sporulation,

or spore germination but not death;

used to describe the nonspecific phenomenon in natural soils where spore germination is inhibited and often overcome by rhizosphere nutrients

• Hyperparasite- an organism that is parasitic upon another parasite

• Hypovirulence- the reduced ability to cause disease• Mycoparasite- a fungus that attacks another fungus• Mycovirus- a virus that infects fungi

Related Terms

• Induced systemic resistance (acronym ISR)the reduced disease symptoms on a portion of a plant distant from the area where the inducing agent is active, caused by the triggering of active plant defenses against a variety of pathogens; used to describe increased resistance in plants induced by certain rhizobacteria

(acronymn SAR-systemic acquired resistance)• Mycoherbicide

a pathogenic fungus used as a biological control agent to manage weeds or other undesirable plants

• Mycotoxin a poisonous compound produced by a fungus

Related Terms

• Pathogenesis-related (PR) proteinsthe proteins, such as antimicrobial proteins and hydrolytic enzymes, that are synthesized in the early events of the plant defense response

• Rhizosphere-competentused to describe microorganisms adapted to living in the rhizosphere of a plant

• Suppressive soil a soil in which various diseases are naturally at lower levels than expected due to biological factors in the soil;

an example of natural biological control

Related Terms

Importance of Biological Control

• Control of plant diseases is required to maintain quality and abundance of food, feed and fiber

• For this growers mostly rely on chemicals for last 100 years

• Excessive use of agrochemicals caused environmental pollution, health hazards etc.

• Appearance of resistant strains of pathogens to chemical fungicides and antibiotics

• Biological control provides an alternative and if effective is more stable

Why use biological control?

WHEN :• Biological control agents are

– Expensive– Labor intensive– Host specific

WHILE :• Chemical pesticides are:

– cost-effective– easy to apply– Broad spectrum

Why use biological control?

WILL:• Chemical pesticides

– Implicated in ecological, environmental, and human health problems

– Require yearly treatments– Broad spectrum

• Toxic to both beneficial and pathogenic species

BUT:• Biological control agents

– Non-toxic to human– Not a water contaminant concern– Once colonized may last for years– Host specific

• Only effect one or few species

Mechanisms of biological control of plant pathogens

• Antibiosis – inhibition of one organism by another as a result of diffusion of an antibiotic– Antibiotic production common in soil-dwelling

bacteria and fungi– Example: zwittermicin A production by B.

cereus against Phytophthora root rot in alfalfa


• Nutrient competition – competition between microorganisms for carbon, nitrogen, O2, iron, and other nutrients– Most common way organisms limit growth of

others– Example

• P. fluorescens, VITCUS, prevents bacterial blotch by competing with P. tolaasii


• Destructive mycoparasitism – the parasitism of one fungus by another– Direct contact– Cell wall degrading enzymes– Some produce antibiotics– Example

• Trichoderma harzianum, BioTrek, used as seed treatment against pathogenic fungus

Requirements of successful biocontrol

1. Highly effective biocontrol strain must be obtained or produced

a. Be able to compete and persist

b. Be able to colonize and proliferate

c. Be non-pathogenic to host plant and environment


2. Inexpensive production and formulation of agent must be developed

a. Production must result in biomass with excellent shelf live

b. To be successful as agricultural agent must be

i. Inexpensive

ii. Able to produce in large quantities

iii. Maintain viability


3. Delivery and application must permit full expression of the agent

a. Must ensure agents will grow and achieve their purpose

Coiling of Trichoderma around a

pathogen. (Plant Biocontrol by Trichoderma spp. Ilan Chet, Ada Viterbo and Yariv Brotman)

Plant pathogen control by Trichoderma spp.

• Trichoderma spp. are present in nearly all agricultural soils

• Antifungal abilities have been known since 1930s

• Mycoparasitism• Nutrient competition

• Agriculturally used as biocontrol agent and as a plant growth promoter

http://www.ars.usda.gov/is/pr/2002/021231.trichoderma.jpg


Why buy/develop a product that is readily available in the soil?


• Genetic Modification– Wild strains

• Heterokaryotic – contain nuclei of dissimilar genotypes within a single organism

– Biocontrol strains• Homokaryotic – contain nuclei which are similar or

identical• Allows genetic distinction and non-variability

– IMPORTANT FOR QUALITY CONTROL


• Most strains have innate resistance to some agricultural chemicals– Resistance is variable

• Strains available for commercial use are selected or modified for resistance to specific chemicals


How is it applied?

• Favored by presence of high levels of plant roots

• Some are highly rhizosphere competent– Capable of colonizing the expanding root

surface– Can be used as soil or seed treatment

http://www.nysaes.cornell.edu/ent/biocontrol/pathogens/images/trichoderma3.jpg


• Action against pathogenic fungi

1. Attachment to the host hyphae by coiling

a. Lectin-carbohydrate interaction

(Hubbard et al., 1983. Phytopathology 73:655-659).


• Action against pathogenic fungi

2. Penetrate the host cell walls by secreting lytic enzymes

a. Chitinases

b. Proteases

c. Glucanases

(Ilan Chet, Hebrew University of Jerusalem).


• Some strains colonize the root with mycoparasitic properties– Penetrate the root tissue – Induce metabolic changes which induce

resistance• Accumulation of antimicrobial compounds


• Commercial availability

T-22• Seed coating, seed pieces, transplant starter • Protects roots from diseases caused by Pythium,

Rhizoctonia and Fusarium • Interacts with the Rhizosphere, near the root hairs

and increases the available form of nutrients needed by plants.


• Future developments Transgenes

• Biocontrol microbes contain a large number of genes which allow biocontrol to occur

• Cloned several genes from Trichoderma as transgenes

– Produce crops which are resistant to plant diseases

• Currently not commercially available

Principles of plant disease management through bioagents

•Eradication

•Protection

References

• Current Microbiology Vol. 37 (1998), pp.6-11 Target Range of Zwittermicin A, and Aminopolyol antibiotic from B. cereus

• Trichoderma for Biocontrol of Plant Pathogens: From Basic Research to Commercialized Products Gary E. Harman Departments of Horticultural Science and of Plant Pathology ,Cornell University

• Plant Biocontrol by Trichoderma spp. Ilan Chet, Ada Viterbo and Yariv Brotman. Department of Biological Chemistry

• Trichoderma spp., including T. harzianum, T. viride, T. koningii, T. hamatum and other spp.by G. E. Harman, Cornell University, Geneva, NY 14456

• The Plant Cell, Vol. 8, 1855-1869, October 1996 O 1996 American Society of Plant Physiologists Biocontrol of Soilborne Plant Pathogens. Jo Handelsman‘ and Eric V. Stabb

• BioWorks products http://www.bioworksbiocontrol.com/productsections/agprod.html

• Trichoderma image http://www.ars.usda.gov/is/pr/2002/021231.trichoderma.jpg

• Trichoderma colonization image http://www.nysaes.cornell.edu/ent/biocontrol/pathogens/images/trichoderma3.jpg

• www.weizmann.ac.il/Biological_Chemistry/scientist/Chet/Chet.html

Documents

Definitions used in biological control