Agriculture Reference
In-Depth Information
2003), compost teas (Larkin, 2008), and crop sequencing (Larkin, 2008). Such approaches
have been shown to provide site-specific reductions in disease and pest incidence. The
advantage of this approach is that it relies on locally available resources to maximize soil
health in a sustainable way. The disadvantage is that outcomes tend to be more variable
depending on soil type and climate, so knowledge of soils and available inputs is essen-
tial. Therefore, for the most part recommendations must be developed and adapted on a
regional basis.
The second approach involves inoculation of disease-suppressive microorganisms.
All soils harbor detectable populations of disease-suppressive organisms; however, the
diversity, relative abundance, and activities of these organisms can vary substantially
from site to site. Historically, researchers recovered microbes and then screened them for
disease-suppressing activity. However, molecular tools now allow us to identify microbes
that are suppressive in situ and recover them in a directed fashion (Borneman et al., 2007).
Such an approach has already proven useful at collecting indigenous fungi that acted as
effective inoculants to suppress soilborne diseases caused by nematodes (Olatinwo et al.,
2006). In addition, analyses of the genetics and genomics of disease-suppressive microbes
(Paulsen et al., 2005; Bangera and Thomashow, 1999; Koumoutsi et al., 2004) have led to
new methods to isolate disease-suppressive microbes in a directed fashion from any loca-
tion (McSpadden Gardener et al., 2001; Joshi et al., 2006). This approach also has led to the
development of effective and low-cost inoculants (McSpadden Gardener et al., 2005).
Endophytic fungi or bacteria that colonize the host can stimulate systemic resistance to
pathogens by producing compounds or causing changes in plant morphology or physiol-
ogy that can control or prevent plant diseases (Faeth, 2002; Harman et al., 2004; Bailey et al.,
2006; Melnick et al., 2008; Hanada et al., 2010). Inoculation with disease-suppressing micro-
organisms has been extended to harvestable products. This was shown by Shi et al. (2011)
with endophytic
Pseudomonas putida
MGY2, which reduced anthracnose (
Colletotrichum
gloeosporioides
) infection in harvested papaya fruit. Fruit treated with MGY2 showed a sig-
nificantly lower disease index, disease incidence, and lesion diameter than the control.
Potential control mechanisms were related to enhanced activity of phenylalanine ammo-
nia-lyase, catalase (CAT), and peroxidase and increased the phenolic content. These results
suggested that papaya fruit is capable of responding to the endophyte
P. putida.
In Taita District, Kenya, integrated soil management interventions (cow manure, inor-
ganic fertilizers, and
Trichoderma
inoculant) controlled the
Fusarium
root rot of maize and
bean cropping systems (Okoth and Siameto, 2011). The use of mavuno fertilizer (contain-
ing 11 nutrients in balanced proportion) recorded the least root infection, followed by tri-
ple superphosphate plus calcium ammonium nitrate, mavuno plus
Trichoderma,
and cow
manure plus
Trichoderma,
implying that low soil fertility could be the reason for high infec-
tion of
Fusarium
in soil (Okoth and Siameto, 2011).
Considerable progress has been made in identifying specific organisms and managing
organic inputs to suppress diseases or protect plants from infection in developed coun-
tries, but there has been limited progress in developing countries
.
Methods that transform
resident microbial communities in a manner that induces natural soil disease suppression
have potential as components of environmentally sustainable systems for management
of soilborne plant pathogens to reduce the need for pesticides. A high priority should be
devoted to manipulating organic amendments from on-farm sources to naturally suppress
diseases and protect plants, particularly for developing countries where farmers operate
under infrastructure and economic constraints.
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