Agriculture Reference
In-Depth Information
Earthworm activity has also been shown to influence the distribution and activity of bacteria,
fungi, and protozoa in soils. In laboratory experiments, the earthworm
A. trapezoides
was fed sheep
manure containing
Rhizobium trifolii
in pots in which subterranean clover was growing (Doube et
al. 1994c). Earthworm activity dispersed the surface-applied rhizobia through the soil, resulting in
a fivefold increase in the total number of root nodules on the clover plants and a four- to sixfold
increase in the number of nodules on the primary roots 2 to 8 cm below the surface. Stephens et
al. (1994d) further showed that both
A. trapezoides
and
M. dubius
could disperse
Rhizobium melliloti
through soil, and root colonization of alfalfa by this species increased more than 100-fold in the
presence of
A. trapezoides
. However, the survival of
R. melliloti
in soil in pots was related inversely
to the earthworm population density and, at the highest density tested, was reduced by 99% after
40 days in the presence of
A. trapezoides
. The reasons for this are unclear, and these results need
to be confirmed in the field before there is confidence that such processes occur in natural envi-
ronments.
The majority of root diseases of agricultural crops are caused by soil-borne fungi, and the
possibility that earthworm-dispersed biological control agents (bacteria and fungi) can enhance the
biological control of these fungal diseases prompted several studies in Australia (e.g., Doube et al.
1994d,e). One such study concerned the bacterium
P. corrugata,
which has been shown to control
Ñtake-all,Ò a serious fungal root disease of wheat plants caused by
Gauemannomyces graminis
var.
tritici
. Using columns filled with soil, Doube et al. (1995) showed that both
A. trapezoides
and
A.
rosea
dispersed
P. corrugata
up to 20 cm in 8 days, and that large numbers (ca. log 6 g
−1
) were
recovered from both the earthworm casts and the burrow walls. This work was supported by
Stephens et al. (1993), who placed straw pellets (inoculated with
P. corrugata
) on the soil surface
in the presence and absence of
A. trapezoides
. In the presence of the earthworm, the bacteria were
dispersed through the soil (>log 4 g
−1
soil at 9 cm after 9 days), and there was a substantial increase
in the level of colonization of the roots of seedling wheat plants by the bacteria. However, attempts
to evaluate experimentally the capacity of this earthworm-dispersed inoculum to control take-all
have been frustrated because the earthworm presence alone also controlled the disease (Doube et
al. 1994d).
Stephens et al. (1994a,b) thoroughly examined such interactions in greenhouse and field studies
using the earthworms
A. trapezoides
and
A. rosea
, the two most common species in the cropping
soils of southern Australia. Both species caused significant reductions in the severity of the disease
symptoms (root lesions) in both laboratory and field trials. These effects were also observed in two
soils of contrasting texture (a red-brown earth and a calcareous sandy loam). Reduced severity of
disease was associated with a corresponding increase in plant growth, but in some cases, the
earthworm activity had no effect on plant growth. In pot trials, the level of disease control increased
with higher earthworm populations, but in the field trials, the level of disease control (30 to 40%
reduction in lesions) with the equivalent of 100 earthworms m
−2
did not increase by tripling the
earthworm population (300 m
−2
).
Similar results were obtained for the effects of the activity of the earthworm
A. trapezoides
on the bare patch root disease of wheat caused by
Rhizoctonia solani
(Stephens et al. 1994c,e;
Stephens and Davoren 1995, 1997). Laboratory and field trials demonstrated reduced numbers of
root lesions (20 to 40% reduction) and, by inference, reduced activity of
Rhizoctonia
species in
soil. Again, earthworm populations of 100 m
−2
and 300 m
−2
resulted in similar degrees of disease
suppression. Why additional earthworm activity failed to increase the level of control of root
disease is unknown. Although the earthworm populations used in these experiments were greater
than those commonly found in cropped soils in southern Australia, it seems likely that earthworms
have potential to help control root diseases of crops under some conditions. For example, Doube
et al. (1994a) reported
A. trapezoides
at populations of 410 m
−2
and 140 m
−2
in soils under canola
and wheat, respectively, in New South Wales, Australia. Moreover, from their results, it seems
probable that earthworm activity can modify the microenvironment in soils in such a manner that
its suitability for root-associated fungi (including pathogens) is affected. This has important
