Diseases and Management of Crops under Protected Cultivation

(Diseases and Management of Crops under Protected Cultivation)
Absorption of solar radiation in different soils varies according to the colour, moisture, and
texture of the soil. In general, the soil has high thermal capacity and is a poor heat conductor thus
resulting in a very slow heat penetration in soil. The energy is lost from the soil in the form of long
wave radiation through conduction, convection, and water evaporation. The principles of solar
heating in polyethylene mulched soil were demonstrated by Waggoner et al., 1960. If thermal
processes occurring in mulched soil are considered, then soil temperatures at the desired depth
can be predicted. Mahrer, 1979 developed a one dimensional numerical model for such
predictions. As per this model in wet, polyethylene mulched soil, increased temperatures are due
primarily to the elimination of heat loss by evaporation and heat convection during the day time
and partially to the green house effect (preventing part of the long wave radiation from leaving the
ground). By predicting the temperatures at any depth of the mulched soil, the model enables us to
select the suitable climatic regions and the time of year most adequate for solarization of soil,
providing data on the heat sensitivity of the pathogens and their population density at various
depths are available. Relative importance of type of mulching material, soil type, moisture and
climatic factors can also be evaluated. Analysis of the spatial soil temperature regimes in mulched
soil showed that heating at the edges of the mulch is lower than at the center, and that a narrow
mulch strip is less efficient in heating than a wider one ( Mahrer and Katan, 1981).
Mechanisms
Reduction in disease incidence occurring in solarized soils, results from the effects exerted
on each of the three living components involved in disease (host, pathogen, and soil microbiota)
as well as the physical and chemical environment which, in turn affects the activity and
interrelationships of the organisms. Although these processes occur primarily during solarization,
they may continue to various extents and in different ways, after the removal of the polyethylene
sheets and planting. The most pronounced effect of soil mulching with polyethylene is a physical
one, i.e. an increase in soil temperatures, for several hours of the day. However, other
accompanying processes such as shifts in microbial populations, changes in chemical composition
and physical structure of the soil, high moisture levels maintained by the mulch, and changes in
gas composition of the soil, should also be considered while analyzing mechanisms of disease
control. The following equation proposed by Baker (1968), for relating the various factors involved in
biological control, should be adopted for this analysis:
Disease severity =inoculum potential x disease potential, where inoculum potential is the energy
available for colonization of a substrate (infection court) at the surface and disease potential is the ability
of the host to contract disease. More specifically the equation becomes:
Disease severity = (inoculum density x capacity) x (proneness x susceptibility), where
capacity is the effect of the environment on energy for colonization, and proneness is the effect of
the environment on the host. Of these four components, inoculum density (ID) is the one most
affected by solarization either through the direct physical effect of the heat or by microbial
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