Diseases and Management of Crops under Protected Cultivation

(Diseases and Management of Crops under Protected Cultivation)
processes induced in the soil. The other components, however (except for susceptibility which is
genetically determined) might also be affected.
Whenever microorganisms are subjected to moist heat, at temperatures exceeding the
maximum for growth, their viability is reduced. The thermal death rate of a population of an
organism depends on both the temperature level and exposure time, which are inversely related.
At a given temperature and time of exposure, mortality rate is related to the inherent heat
sensitivity of the organisms and to the prevailing environmental conditions. In general, populations
of soil borne fungal pathogens are drastically reduced at temperatures of 40-50 0 C, exposure time
ranging from minutes to hours for the higher temperatures, and up to days for the lower
temperatures. The response of the population to elevated temperatures depends on propagule
type, age and on environmental factors like pH, presence of ions etc. Presence of moisture is a
crucial factor since microorganisms are much more resistant to heat under dry conditions. The
effect of water can be explained by the dependence of the heat stability of proteins on hydration. In
the presence of water less energy is required to unfold the peptide chain of proteins, resulting in a
decreased heat resistance. Heating dry soils is therefore not effective in pathogen control (Katan
et al., 1976).
Microbial processes, induced in the soil by solarization, may contribute to disease control,
since the impact of any lethal agent in the soil extends beyond the target organisms. If induced by
solarization, biological control may affect the pathogen by increasing its vulnerability to soil
microorganisms or increasing the activity of soil microorganisms toward pathogen or plant, which
will finally lead to a reduction in disease incidence, pathogen survivability, or both. Thus both short
and long term effects might be expected. Biological control may operate at any stage of pathogen
survival or disease development during or after solarization, through antibiosis, lysis, parasitism, or
competition.
Disease Management
Soil solarization has been demonstrated to control diseases caused by many fungal
pathogens such as Rhizoctonia solani, Fusarium spp., Pythium spp., Phytophthora spp.,
Verticillium spp., Sclerotium rolfsii etc., bacterial pathogens such as Agrobacteria and
Pseudomonas and many species of nematodes in many crops both under field conditions and
plastic houses (Katan et al., 1983; Abdul et al., 1995; Raoof and Rao,1997; Chellemi et al., 1994).
The m,ethod has also been used to control many species of nematodes. Diseases caused by
Meloidogyne spp., Heterodera spp. etc.have been successfully controlled by soil solarization (Rao
and Krishnappa, 1995; Grinstein et al., 1995).
Polyhouse farming is an alternative emerging technique in agriculture which is becoming
increasingly popular . It reduces dependency on rainfall and makes the optimum use of land and
water resources. Polyhouse farming enables, raising off-season nurseries, maintenance and
multiplication of self incompatible line for hybrid seed production, cultivation of flowers, vegetables
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