Diseases and Management of Crops under Protected Cultivation

(Diseases and Management of Crops under Protected Cultivation)
biocontrol agents or reduced doses of soil fumigants/fungicides has shown a remarkable
destructive effect on most soil borne plant pathogens.
Various terms like solar heating, plastic or polyethylene tarping, polyethylene or plastic
mulching of soil have been used to describe this method. Since this method involves repeated
daily heating at relatively mild temperatures, the term solar pasteurization has also been
suggested.
Principles
Heat is used as a lethal agent for the control of plant pathogenic organisms through the
use of transparent polyethylene soil mulches (tarps) for capturing solar energy. Polyethylene
covering of soil induces green house effect and raises soil temperature. The following
recommendations are made to bring about effective solar heating of soil:
Transparent (clear) not black polyethylene should be used since it transmits most of the
solar radiation that heats the soil. Black polyethylene, though it is greatly heated by itself, is
less efficient in heating the soil than transparent sheet.
Soil mulching should be carried out during the period of high temperatures and intense
solar irradiation.
Soil should be kept wet during mulching to increase thermal sensitivity of resting structures
such as sclerotia, chlamydospores, etc. and to improve heat conduction.
The thinnest possible polyethylene tarp (25-30 µm) is recommended, since it is both
cheaper and more effective in heating, due to better radiation transmittance, than the
thicker one. Polyethylene reduces heat convection and water evaporation from the soil to
the atmosphere. As a result of the formation of water droplets on the inner surface of the
polythene film, its transmissivity to long wave radiation is highly reduced, resulting in better
heating due to an increase in its greenhouse effect. An ideal plastic mulch is that which is
100% transparent to solar radiation and completely opaque to long wave radiation. This
ideal mulch can increase soil temp. by 6-8 0 c over ordinary polyethylene.
Since temperatures at the deeper soil layers are lower than at the upper ones, the
mulching period should be sufficiently extended, usually 4 weeks or longer, in order to
achieve pathogen control at all desired depths.
The solar heating method for disease control is similar, in principle, to that of artificial soil
heating by steam or other means. There are, however, important biological and technological
differences: (i) With soil solarization there is no need to transport the heat from its source to the
field. (ii) Solar heating is carried out at relatively low temperatures as compared to artificial heating;
thus its effects on living and nonliving components are likely to be less drastic. Negative side
effects observed with soil steaming such as phytotoxicity due to release of manganese or other
toxic products and a rapid soil reinfestation due to the creation of a biological vacuum have not
been reported so far with solar heating.
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