Agriculture Reference
In-Depth Information
successive susceptible crops, and therefore several years in many cases. Such
epidemics are described as polyetic (Zadoks, 1999). Crop rotation is recognised as
the best way to keep levels of soil-borne diseases low, although it is not always
acceptable to farmers for economic reasons. The mode of action of crop rotation was
long thought to involve only the breakdown of pathogen inoculum build-up
associated with decay of the inoculum during the cropping of non-host plants.
However, different break crops have been shown to have different impacts on
disease levels in the following host crop. In a study on the influence of crop rotation
on foot and root diseases (take-all, sharp eyespot and eyespot) of wheat, Colbach
et al. (1994) showed that host crops (wheat and barley) tended to increase the risk of
the diseases whereas some non-host crops (alfalfa, peas, sunflower) decreased
disease risk and others (maize, and sorghum for sharp eyespot) had an intermediate
effect. These findings are consistent with those of Lucas et al. (1989), who showed
that a soil cropped continuously with maize or cultivated under a wheat-maize
rotation was far more conducive to take-all than the same soil cultivated under a
wheat-beet rotation. The soil inhibiting disease development most strongly was
wheat monoculture, providing evidence for take-all decline, which is known to be
due to changes in soil microbial populations. The plant species grown is therefore a
significant factor determining the composition of microbial soil communities living
in soils and the rhizosphere. This applies not only to pathogens, but also to
antagonistic and deleterious microbes. Lemanceau et al . (1996) demonstrated that
two plant species, flax and tomato, modified in different ways the genetic and
phenotypic diversity of the fluorescent pseudomonad community resident in the soil.
The cropping of resistant plants is limited by the fact that, curiously, cultivar
selection has produced abundant examples of useful genetic resistance to above-
ground but not to below-ground pathogens. Cook et al. (1995) suggested that the
selection imposed by soil-borne pathogens may favour a different defence strategy
which is for the plants to support and respond to populations of rhizosphere micro-
organisms antagonistic to their pathogens. Attempts have been made to breed wheat
cultivars able to react to hypovirulent strains of G. graminis var. tritici used as a
biocontrol agent against take-all (Lemaire et al ., 1982). However, more attention has
been paid to selecting bacteria displaying a combination of efficient root
colonisation and beneficial effects on the activity of a given plant (Kuiper et al .,
2001) than to breeding plants able to exert beneficial selection pressure on microbial
communities, although there are some reports of genotype-specific induction of soil
microbial communities inhibiting soil-borne diseases such as rhizoctonia root rot
in winter wheat cultivars (Mazzola and Gu, 2002; Mazzola, 2004). This approach
is supported by studies in other areas demonstrating that the sensitivity of wheat
(Rengel, 1997) and oat (Timonin, 1965) genotypes to manganese deficiency,
for example, depends on the number of Mn-reducing microorganisms in the
rhizosphere.
Search WWH ::




Custom Search