Biological control

The Plant Disease Triangle

Path

ogen H

ost

Environment

Take home message: Microorganisms, whether indigenous or introduced are an important component of the environment.

Biological control

Reduction of the amount of inoculum or disease-producing activity of a

pathogen accomplished by or through one or more organisms

other than humans.

Biological control gained strength as a subdiscipline of Plant Pathology in the 1960s, when a group of scientists recognized that epidemics of soilborne plant diseases could not be understood without considering the the ecology of soil fungi and Oomycetes and the resident soil microflora.

Biological Control of Plant Diseases

Kinds of Biological Control

• Conservation- cultural practices Suppressive soils

General suppression

Specific suppression• Classical – self sustaining following a single release of a “natural enemy”

• Augmentative – periodic introduction to supplement natural reproduction Chestnut Blight and hypovirulence

• Innudative – mass introduction of biocontrol agent Crown gall Heterobasion root rot Fire blight Postharvest diseases

Suppressive Soils

• The pathogen does not establish or persist

• The pathogen establishes but causes little or no disease

General Suppression

A fixed level of the pathogen causes less disease in the presence of indigenous soil organisms.

Propagule level

Dis

ease

sev

erit

y

field soilsterilized soil

Take all of wheat caused by Gaeumannomyces graminis var tritici

Take-all decline with monoculture of wheat

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Years of Monoculture

Disease severity

Associating populations of organisms with soil suppression

Pseudomonas fluorescens produces an antibiotic that is toxic to the take-all pathogen

The antibiotic 2-4-diacetylphloroglucinol

is toxic to the take-all pathogen

CH3 CH3

OHOH

OHO O

No seed treatment Seed treatment with P.f.

Wheat seed treatment with Pseudomonas fluorescens for control of take-all

Seed treatment with mutant that does not produce an antibiotic

Borneman and Becker evaluated theMicroorganisms associated with cysts in suppressive and conducive soils

Associating populations of organisms with soil suppression

Olatinwo, R., Borneman, J., and Becker, J. O. 2006. Induction of beetcystnematode suppressiveness by the fungi Dactylella oviparasitica andFusarium oxysporum in field microplots. Phytopathology 96:855-859.



General suppressionSpecific suppression

• Classical – self sustaining following a single release of a “natural enemy”

• Augmentative – periodic introduction to supplement natural reproduction

Innudative – mass introduction of biocontrol agent Crown gall Heterobasion root rot Fire blight Postharvest diseases

Chestnut Blight and hypovirulence

Biological control with Introduced Antagonists

Biological control agent:

Hypovirulent isolates of Cryphonectria parasitica

Disease: Chestnut Blight

Pathogen: Cryphonectria parasitica

Mechanism: hypovirulence

History of the Chestnut Blight Pathogen in the United States

Transmission of hypovirulence decreases with diversity of vegetative compatibility groups in the pathogen population






• Innudative – mass introduction of biocontrol agent

Crown gall Heterobasion root rot Fire blight Postharvest diseases

Biological control with Introduced Antagonists

Biological control agent:

Agrobacterium radiobacter

Disease: Crown gall

Pathogen: Agrobacterium tumefaciens

Mechanism: antibiosis

Crown gall caused by

Agrobacterium tumefaciens

Infection byAgrobacteriumtumefaciens

Short period ofSusceptibility:Wound typicallyheals over after about 24 hours andis no longer an opening for infection

In nurseries growing woody perennials, wounds are inducedby root pruning. These can betreated with the biocontrol agent immediately

Treated with K84 Untreated

Why does biological control of crown gall work so well???

• Limited time of host susceptibility to disease This means the biocontrol agent doesn’t have to persist for a long time in the environment

• The infection court is defined and easily treatedThis means the biocontrol agent can be applied directly to the infection court, and doesn’tHave to move there on its own• There are no chemical controls available • The sensitivity of the pathogen population can be

predictedFor example, strains pathogenic to cherry are sensitive to agrocin 84, whereas strains pathogenic to apple are not uniformly sensitive.


• Conservation- Suppressive soils




• Innudative – mass introduction of biocontrol agent Crown gall

Heterobasion root rot Fire blight Postharvest diseases

Heterobasidion root rot of pine

• The fungus Heterobasidion annosum is the most damaging root pathogen of coniferous trees in the Northern hemisphere.

• It progresses from the roots into the base of a tree, causing an economically important butt rot. • Once established in a site, the fungus is almost impossible to eradicate; it spreads

progressively by contact of healthy roots with infected roots

http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/heterob.htm

• Fruiting bodies release air-borne basidiospores that can spread the infection to new sites.

• Basidiospores land on freshly cut stump surfaces, and the fungus grows down through the stump tissues to the dead roots, from which it can infect the roots of adjacent healthy trees.


• Fresh pine stumps can be colonised by another fungus, Phlebiopsis gigantea (previously called Peniophora gigantea), which is weakly parasitic but poses no danger to healthy trees.

• If Phlebiopsis is applied first then it can prevent invasion by H. annosum, protecting the stump surfaces without the need for phytotoxic chemicals.

• P. gigantea is commercially available in Britain, Sweden, Norway, Switzerland and Finland, as either a spore suspension or a dry product (named "Rotstop").

• It was available in the USA until 1995 when the Environmental Protection Agency required it to be registered officially as a biological pesticide - a relatively expensive process that probably would not be cost-effective for the commercial producers.

http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/heterob.htmhttp://www.nysaes.cornell.edu/ent/biocontrol/pathogens/p_gigantea.html



• Conservation- Suppressive soils




• Innudative – mass introduction of biocontrol agent Crown gall Heterobasion root rot Fire blight

Postharvest diseases

Decay management product

for • Citrus• Stone fruits• Pome fruits• Potatoes

a.i.: Pseudomonas syringae - ESC-10: EcoScience strain - ESC-11: USDA strain

Bio-Save

untreated

Why Postharvest Biocontrol?

• Market need:– few labeled chemicals– fungicide resistance problem

• System characteristics

bin dump

chlorine spray

Chlorine bath or spray

Clean Rinse or Fungicide Spray

fungicide in wax

Wax application

Bio-Save Application

Usage History of Bio-Save by Crop

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

Ca

rto

ns

Citrus

0

1,000,000

2,000,000

3,000,000

4,000,000

1999 2000 2001 2002 2003 2004 2005

Sa

cks

Potato

0

2,000,000

4,000,000

6,000,000

8,000,000

1999 2000 2001 2002 2003 2004 2005

Lug

s

Cherry

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

Ca

rto

ns

Pear and Apple

From Stockwell and Slack. Phytopathology 97: 244-249.

The Plant Disease Triangle

Path

ogen H

ost

Environment

Take home message: Microorganisms, whether indigenous or introduced are an important component of the environment.

Education

Biological control