Environmental Engineering Reference
In-Depth Information
3.2.2 Biotic Stress
3.2.2.1 Protection Against Pests
and Pathogens
In agriculture, pest and pathogen infestations
severely damage the crops, resulting in a decline
in crop yield. In spite of constant efforts to eradi-
cate these pests and pathogens using chemical
agents, little and temporary success has been
achieved. Alternatively cost-effective biological
methods involving microbes could be used to
improve host plant resistance against pests and
pathogens. There is a direct competition in host
roots for nutrient uptake and proliferation
between AM fungi and pathogens as they colo-
nize the same niche. Some of the recent studies
do provide evidence that AM fungi and their
interaction with plants could substantially reduce
the damage caused by soilborne pathogens
(Whipps
2004
; St-Arnaud and Vujanovic
2007
;
Smith and Read
2008
). Further, the extent of pro-
tection imparted by AM fungi could vary with
the pathogens and the host plant involved.
Nevertheless, the degree of protection imparted
by AM symbiosis against pests and pathogens
could be modifi ed by soil and other environmental
conditions. Mechanisms by which AM fungi con-
trol root pathogens include (i) improved nutrient
status of the host, (ii) damage compensa-
tion, (iii) competition for host photosynthates,
(iv) competition for infection sites, (v) anatomi-
cal and morphological changes in the root
system, (vi) microbial changes in the mycorrhi-
zosphere and (vii) activation of plant defence
mechanisms. In some cases, the direct biocontrol
potential of AM has been demonstrated, espe-
cially for plant diseases involving pathogens
like
Phytophthora
,
Rhizoctonia
and
Fusarium
(Abdel-Aziz et al.
1997
; St-Arnaud et al.
1997
;
Vigo et al.
2000
). A recent study by Singh et al.
(
2013
) has clearly demonstrated the AM fungal
ability to effi ciently control
Fusarium
wilt dis-
ease under all conditions in three chickpea (
Cicer
arietinum
) varieties tested. Further, several studies
have also confi rmed the existence of synergism
between AM fungi and biocontrol agents such as
Burkholderia cepacia
(Ravnskov et al.
2002
),
Pseudomonas fl uorescens
(Edwards et al.
1998
),
Trichoderma harzianum
(Datnoff et al.
1995
) and
Verticillium chlamydosporium
(Rao et al.
1997
).
These interactions suggest that AM might affect
plant and soil microbial activity by stimulating
the production of root exudates, phytoalexins and
phenolic compounds (Norman and Hooker
2000
;
Bais et al.
2005
). A small increase in the activity
of plant defence genes, especially those involved
in the production of chitinases, glucanases, fl avo-
noid biosynthesis and phytoalexins, has been
observed during mycorrhizal growth; however,
these mycorrhizal defence induction mechanisms
remain transitory (Guillon et al.
2002
; Harrier
and Watson
2004
). Further, AM-mediated resistance
to biotic stress could vary with the mycobiont
involved. For example, Ozgonen and Erkilic (
2007
)
used three different species of Glomeraceae
[
Funneliformis mosseae
(
=Glomus mosseae
)
,
Claroideoglomus etinicatum
(
=Glomus etuni-
catum
)
, Rhizophagus fasciculatus
(
=Glomus
fasciculatum
)] and a Gigasporaceae species
(
Gigaspora margarita
) to control blight disease
caused by
Phytophthora capsici
in pepper. The
results of the study clearly showed a signifi cantly
higher plant growth and reduced disease severity
in AM-inoculated plants. Of the different species
of Glomeraceae
screened,
F. mosseae
was found
to be more effi cient than others.
Many studies have also reported the suppressive
effect of AM fungi on sedentary endoparasitic
nematodes (Elsen et al.
2003
; de la Pe
a et al.
2006
). In some crops, this effect is signifi cant
enough to the level, to consider AM fungi to be
more or less an effi cient means of biological con-
trol (Castillo et al.
2006
). With migratory endopar-
asitic nematodes, studies have demonstrated a
decrease in nematode population development
like
Meloidogyne incognita
on cucumber (
Cucumis
sativus
) (Zhang et al.
2008
),
Radopholus similis
on
banana (
Musa
spp.) (Elsen et al.
2004
; Jefwa et al.
2010
),
Pratylenchus
on dune grass (
Ammophila
arenaria
) (de la Peña et al.
2006
) and
Rhizoctonia
solani
(Yao et al.
2002
) on potato. Recently,
Affokpon et al. (
2011
) evaluated native and com-
mercial AM fungi for their effi cacy to protect
plants against root-knot nematode,
Meloidogyne
spp. The results of this study indicated that the
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