biological methods of weed control

1. Bio-Herbicide
and Allelo-chemicals
Presented by:
Uday Kumar Lodh
M.Sc. Forestry (Plantation Technology)
ICAR JRF (AIR-2)

2. Weed
Any unwanted plant that interferes or tends to interfere with
the growth of the individuals of favored species.
(Any plant whose virtue is not known)
Benefits
Resist soil erosion and used as fencing material
 Has Medicinal value and also used as green manures, mulch
etc.
Limitations
Reduce crop yield and productivity by competing for light,
space, water and nutrients.
Increases cost of cultivation, harmful to livestock & human
beings
 Reduces efficiency of irrigation and drainage etc.

3. Biological Method of
Weed Control
1. Bioherbicide
2. Allelochemicals
Methods
1. Mechanical methods
2. Cultural methods
3. Biological methods
4. Chemical methods
5. Integrated weed
management

4. Bio-herbicide
Biologically based weed control agents derived from microbes
such as fungi, bacteria etc.
The first bio-herbicide happened to be was Mycoherbicide
(Bio-herbicide based on a fungus is called as Mycoherbicide).
Phytophthora plamivora in 1981 , a fungus which
inhibited growth of Milkweed vine to grow in citrus orchard
Eichhornia crassipes (Water hyacinth) is controlled by
Alternaria eicchorniae in India.
.

5. Advantages of bioherbicides:
High degree of specificity of target weed
 No effect on non-target and beneficial plants or man
Absence of residue build-up in the environment
 Effectiveness for managing herbicide-resistant (HR) weed
populations
Limitations of bioherbicides:
 Biological constraints
 Environment constraints
 Technical constraints and
 Commercial limitations

6. Bio-herbicides :
How relevant?
They have remain a novelty
and miniscule component of
weed control options
Have small market value when
compaired to chemical herbicide
Used locally or regionally on a
small scale.

7. Fusarium oxysporium
S asiatica (91.3%) S hermonthica (94.3%) S gesneroidis (81.8%)
Elzien., 2004

8. Myrothecium verrucaria
Sesbania exaltata Senna obtusifolia Datura stramonium
Hoagland et al., 2007

9. Phomopsis amaranthicola
Amaranthus sps
Colletotrichum truncatum
Sesbania exaltata

10. Synergism between bioherbicides and chemical
herbicides
Combinations of some bioherbicides and synthetic
herbicides can be synergistic resulting from lowered weed
defense responses caused by the herbicides, thus making
the weeds more susceptible to pathogen attack
Synergism
Xanthomonos campestris

11. Bentazone
Colletotrichum truncatum
Sesbania exaltata
Acifluorfin Myrothecium verrucaria Senna obtusifolia

12. Different limitations about bioherbicides use
1. Environmental limitations
Environmental factors influence formulation performance of
bioherbicides as inoculum production is dependent on sporelation of the
formulation.
A requirement for more than 12 h of dew period for severe
infection by a pathogen, has been reported for several potential
bioherbicides and this may limit the efficacy of the bioherbicide in the
field.
Biological limitations
It is desirable for a bioherbicide to act relatively quickly and
have sufficient efficacy to control weeds. Unfortunately, many of the
weed pathogens discovered may provide only partial control of only one
weed species, even under ideal conditions.
Host specificity is related to the basic biology of the pathogen and to
host resistance.

13. Technological-commercial limitations
Several technological limitations have been
identified that could prevent the widespread use of
bioherbicides
Pathogenical strains, formulation method and the
interaction of these two parameters significantly affect the
shelf life of the formulations at room temperature.
The most challenging aspect of formulating
bioherbicides is to overcome the dew requirement that
exists for several of them.

14. Allelopathy
Process of plants producing secondary metabolites, capable of initiating
chemical warfare among neighbouring plants growing in plant community.
The chemicals are called Allelochemicals.
The term allelopathy derived from the Greek word compounds allelo- and -
pathy (meaning "mutual harm" or "suffering")
It was first time used by Austrian professor Hans Molisch in 1936 in his
book “Der Einfluss einer Pflanze auf die andere – Allelopathie” (The
Effect of Plants on Each Other) published in German.
For weed management we are interested in the
inhibition of one plant (the weed or weeds) by
another (usually the crop or weed) through the
production of Allelochemicals

15. The source of allelochemicals in agricultural fields can be
the weeds, crops or microorganisms.
Many crops have been reported as showing allelopathic
properties at one time or another …
Some crops such as oats seem to clean fields of weeds
better than others. The list also includes:
Lucerne Rice Sorghum Red clover
sweet
clover
Barley Wheat

16. Factors..
Varieties
There can be a great deal of difference in the
strength of allelopathic effects between different crop
varieties.
Specificity
A crop which is strongly allelopathic against one
weed may show little or no effect against another.
Autotoxicity
Allelopathic chemicals may suppress same species.
Lucerne is particularly known for this kind of autotoxicity.

17. Forms of Allelopathic interactions
Weed
against
weed
Crop
against
crop
Crop
against
weed
Weed
against
crop

18. Crop Weed sp Source of
inhibitors
Maize Chenopodium album,
Amaranthus retroflexus
Roots
Sorghum Abutylontheophrasti,
Amaranthus hybridus
Shoots and foliages
Rye Digitaria sanguinalis,
Ambrosia artimisifolia
Shoots and foliages

19. Weeds against weeds
Parthenium hysterophorus
 Effect of several waste land weeds on parthenium
suppression
Sl.no Species Extent of suppression (%)
1 Cassia auriculata Moderate(26-50 )
2 Sida spinosa Moderate(26-50 )
3 Cassia occidentalis High(51-754)
4 Amaranthus spiinosus High(51-754)
5 Mirabilis jalappa Very high(76-100)
6 Ipomea carnea Very high(76-100)

20. Sources
Stem
Leaves
&
Flowers
Fruits&
seeds
Roots &
Rhizome
Sources of Allelochemicals and Chemical Nature
Secondary metabolites
Phenyl propanone
Terpenoids
Acetogenins
Alkaloids
Steroids

21. • Volatilization
• Leaching
• Exudation
• Weathering
Liberation of Allelochemicals

22. Volatilization
Arid and semi arid environment
Mostly terpenoid group.
Released from special glands on stems or leaves
Susceptible plants absorbs through cuticle directly from air
or adsorbed on dry soil and taken up.
Leaching
Through aqueous solutions (rain,dew)
Mature leaves are more susceptible
Exudation
Metabolites exuded from roots to surrounding atm.
Potential source of allelopathic effect
Decomposition
Leaves and stubbles
By weathering and micro organisms.

23. • Celldivision and elongation
• Gibberllin or IAA(growth hormones)
• Mineral uptake
• Nitrification (nitrosomonas-furilic acid)
• Respiration
• Stomatal opening
• Protein synthesis and org.acid metabolism
• Specific enzymatic activities.
• Retardation of photosynthesis
I
N
H
I
B
I
T
I
O
N
Mechanisms of action

24. Inhibition of plant species through allelopathy
• The leaf litter and root exudates of some Eucalyptus species
are allelopathic for certain soil microbes and plant species.
• The tree of heaven , Ailanthus altissima, produces
allelochemicals in its roots that inhibit the growth of many
plants.
• Continuous cropping of legume crop alfaalfa has been created
to auto toxicity in low land weed.
• The black walnut (Juglans nigra) produces the allelochemical
juglone, which affects some species

25. Constrains in research..
1.Difficulty in exudate collection
2.Poor knowledge of conditions under which exudates are
released
3.The exact concentration in which allelochemicals are
released is unknown.
4.Difficulty to identify which weed is responsible for the
observed allelopathy.
5.Presence of autoallelopathy.
6.Lab to field gap.
7. Information about which crops are effective against
which weeds is limited
8. Information about which are the most allelopathic
varieties of a particular crop is not available.

26. Sources of plant and microbial phytotoxins with
promising herbicidal activity
Phytotoxins Source
Coumaric acid
Vanillic acid
Ferrulic acid
Maize
Vanillic acid,
P -hydrobenzoic acid
Wheat
Caffeic
clorigenic
sunflower
Ferrulic acid Soybean
Dhurrin Sorghum
caffeine coffee
Acetaldehyde Sugar beat
Sinigrin Mustard

27. Conclusion…
New bioherbicides will find place in irrigated lands,
wastelands as resistant weed control
Complimentary components in successful IWM systems,
and in the discovery of novel phytotoxins with new chemistries
and new molecular sites of action research will benefit in
future.
Though attempting to use allelopathy as a practical tool
still if we overcome them to some extend allelopathy is the best
“Natural herbicide”
Bioherbicides will not solve all of the environmental and
weed management problems associated with synthetic
herbicides, nor will replace the current or future arsenal of
synthetic herbicides

28. Thank You….