Allelochemicals Biologica... - Name

ALLEOPATHIC ORGANISMS AND
MOLECULES
into potatoes (Solanum tuberosum). Several bacterial species possessed activity against
root-lesion nematodes (Pratylenchus penetrans) in soils around the root zone of
potatoes, namely: Microbacterium esteraromaticum, Tsukamurella paurometabolum,
isolate TP6, Pseudomonas chlororaphis, Kocuria varians and K. kristinae. Of these,
M. esteraromaticum and K. varians depressed the population densities of root-lesion
nematodes without incurring any yield penalty (tuber wet weight). No significant
differences were found in the total numbers of P. penetrans nematodes, rhabditid
nematodes or ‘other’ parasitic nematode species within the root tissues of bacterized
potato plants compared to the unbacterized check. Overall, tuber fresh weights and
tuber number were equal to or significantly lower (P < 0.05) in bacterized plants than
their unbacterized counterpart (Sturz and Kimpinski, 2004). The authors of this study
conclude that endoroot bacteria from Tagetes spp. can play a role in nematode
suppression through the attenuation of nematode proliferation, and proposed that
these nematode control properties are capable of transfer to other crops in a rotation
as a beneficial ‘residual’ microflora – a form of beneficial microbial allelopathy.
In relation with this same type of study, Hallman et al. (1998) performed a
greenhouse experiments with cotton and cucumber to determine the effects of
inoculation of the parasitic nematode Meloidogyne incognita on population dynamics
of indigenous bacterial endophytes and introduced endophytic bacterial strains JM22
(Enterobacter asburiae) and 89B-61 (Pseuedomonas fluorescens) applied as seed
treatments. Internal communities of endophytic bacteria in roots were generally largest
in the presence of M. incognita. Recovery of JM22 from cucumber roots was positively,
but not significantly, associated with soilborne nematode inoculum size, except at 2
weeks after inoculation. The internal populations of 89B-61 applied to seed also
increased with nematode applications. The diversity of indigenous bacterial endophytes
changed within 7 d after M. incognita inoculation. Species richness and diversity of
endophytic bacteria were slightly, but not significantly, greater for nematode-infested
plants than for non-infested plants. Alcaligenes piechaudii and Burkholderia pickettii
occurred only in nematode-infested plants, whereas Bievundimonas vesicularis was
mainly isolated from nematode-free plants. Agrobacterium radiobacter and
Pseudomonas spp. were the most common taxa found in both treatments, accounting
for a total of 41% and 37% of the community for non-inoculated and inoculated
plants, respectively. JM22 colonized cotton roots internally and was also found in
high numbers on the root surface around nematode penetration sites and on root galls
where the root tissue had been disruptured due to gall enlargement. Single cells of
JM22 were attached to the cuticle of M. incognita juveniles. Sturz et al. (2000) assesses
that endophytic bacteria and M. incognita form complex associations and an
understanding of these associations will aid efforts to develop and manage microbial
communities of endophytic bacteria for practical use as biocontrol agents against
plant-parasitic nematodes and soil-borne pests and pathogens.
In addition, Postma et al. (2003) found that compost amended soil has also been
found to be suppressive against plant diseases in various cropping systems. The level
and reproducibility of disease suppressive properties of compost might be increased
by the addition of antagonists. In this study, the establishment and suppressive activity
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