Agriculture Reference
In-Depth Information
14.2 THE SOIL-BORNE DISEASE EPIDEMIC
Soil-borne plant pathogens affect crops throughout the world and have been
extensively studied. Research efforts have been justified by the economic impact of
these diseases on crop production and by specific difficulties associated with
studying and controlling soil-borne diseases, mostly due to the complex
environment in which these diseases occur (Lucas and Sarniguet, 1998).
14.2.1 A closed environment limiting dispersal
Soil is a closed environment in which propagules capable of initiating epidemics
(e.g. spores, sclerotia, mycelia and hyphae) cannot disperse over long distances, with
the exception of certain spores or bacteria transported in run-off water or in soil
flowing within the soil matrix. It is rare for horizontal dispersal to extend beyond the
field margins.
For some soil-borne pathogens, infection is also transmitted by the growth of the
pathogen on or through the soil, from a source of inoculum to a susceptible host.
This situation mostly applies to fungi, which form mycelia capable of growing
through a heterogeneous medium of pores, cracks and aggregates, although this
growth is affected by many other physical, biological and chemical factors (Otten
and Gilligan, 1998).
Thus, during the crop cycle, soil-borne pathogen propagules are naturally
dispersed over short distances (from a few centimetres to a few decimetres). For this
reason, diseased plants show up as patches within a field at the start of epidemics
(e.g. take-all of winter wheat caused by Gaeumannomyces graminis var. tritici )
(Hornby et al ., 1998; Cook, 2003). For soil-borne pathogens with an aerial phase,
disease may extend to the whole field during the cropping season if climatic
conditions are favourable for the disease (e.g. spore production and dispersal) as is
the case for eyespot on wheat, caused by Tapesia yallundae .
Most soil-borne pathogens require oxygen. They are therefore mostly located
towards the top of the soil profile and their vertical dispersal depends largely on
water infiltration pathways and root progression.
14.2.2 A complex, opaque environment with intense biotic and abiotic interactions
Soil is a complex substance with solid, liquid and gaseous components. The
organisation and interconnection of these components depend on soil texture, soil
structure and external factors, such as climate. Soil structure, and its effects on the
relationship between the liquid and gaseous phases, is a major feature determining
microbial survival and development in soil. High soil moisture content generally
favours microbial activity, but too much water may result in a high prevalence of
water-filled pores, resulting in changes in the concentration of O 2 , CO 2 or other
gases, with consequences for the aerobic or anaerobic microbial communities of the
soil microflora (McDonald, 1994).
The effects of the physicochemical characteristics of the soil on the behaviour of
microorganisms have been investigated in detail but are still only imperfectly
Search WWH ::




Custom Search