Allelochemicals Biologica... - Name

DISEASE RESISTANCE IN PLANTS THROUGH
ALLELOCHEMICALS 183
2. MYCORRIZA IN DISEASE RESISTANCE
Safir (1968) found that inoculation of onion with Glomus mosseae could significantly
reduce pink root disease due to Pyrenochaeta terrestris. Later studies indicate that
AM fungi can induce resistance or increase tolerance to some root-borne pathogens
(Azcon - Aguilar and Barea, 1996, 1997; Caron et al., 1986; Caron, 1989; Cordier et
al., 1997; Dehne, 1982; Hooker et al., 1994; Trotta et al., 1996). Glomus mosseae
protected peanut plants from infection by pod rot fungal pathogens Fusarium solani
and Rhizoctonia solani (Abdalla and Abdel-Fateh, 2000).
The Glomus intraradices increased P uptake and reduced disease development
of Aphanomyces euteiches in pea roots (Bodker et al., 1998) Mycorrhization with
Glomus mosseae and G. intraradices induced local or systemic resistance to
Phytophthora parasitica in tomato roots (Cordier et al., 1996, 1998; Pozo et al.,
2002). Decreased pathogen development in mycorrhizal and non-mycorrhizal parts
of inoculated roots is associated with accumulation of phenolics and plant cell defense
response. The protective effects induced by AM fungi against a phytoplasma is reported
in tomato (Lingua et al., 2002).
AM protects an annual grass from root pathogenic fungi in the field (Newsham
et al., 1995). Inoculation of onion with Glomus sp. Zac-19 delayed onion white rot
epidemics caused by Sclerotium cepivorum Berk by two weeks and increased the
yield by 22% under field conditions (Torres-Barragan et al., 1996). Diospyros lotus
inoculated with Glomus mosseae, Glomus intraradices, Glomus versiforme
significantly increased the plant growth and decreased the disease caused by Cercospora
kaki under field conditions. The AM fungal inoculum even suppressed postharvest
disease of potato dry rot (Fusarium sambucinum) in prenuclear minitubers (Niemira
et al., 1996).
Root rot caused by Fusarium solani significantly contributes to crop yield decline,
up to 50%. The inoculation of common bean (Phaseolus vulgaris) with Glomus
mosseae, besides decreasing propagule number of F. solani in the rhizosphere,
decreased root rot by 34 to 77% (Dar et al., 1997). In the presence of the root nodulating
symbiont Rhizobium leguminosarum, mycorrhizal inoculated plants were more tolerant
to the fungal root pathogen. This indicates that interactions between mycorrhizal
fungi and other rhizosphere microbes might have greater effects on soil-borne
pathogens than mycorrhizal fungi alone. Davis and Menge (1980) found that Glomus
fasciculatum reduced Phytophthora root rot of citrus at low level of soil phosphorus
but had no effect in high phosphorus soil. The VAM fungi has also been employed as
biocontrol agents for Macrophomina root rot of cowpea and Fusarium wilt of tomato
(Ramaraj et al., 1988). The understanding of the mechanisms of plant disease resistance
in mycorrhizal plants would provide better directions towards the development of
efficient crop production and sustainable agriculture.
3. MECHANISM OF DISEASE CONTROL BY MYCORRHIZAL FUNGI
The mycorrhizal symbiosis involves several mechanisms in control of plant diseases.
(i) Creating a mechanical barrier for the pathogen penetration and subsequent spread