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LECTURE 14
PHYSICAL METHODS: Physical methods include soil solarization and hot water
treatments.
i. Soil solarization: Soil solarization or slow soil pasteurization is the hydro/thermal soil
heating accomplished by covering moist soil with polyethylene sheets as soil mulch
during summer months for 4-6 weeks. Soil solarization was developed for the first time in
Israel (Egley and Katan) for the management of plant pathogenic pests, diseases and
weeds.
ii. Soil sterilization: Soil can be sterilized in green houses and sometimes in seed beds by
aerated steam or hot water. At about 500C, nematodes, some oomycetous fungi and other
water molds are killed. At about 60 and 720C, most of the plant pathogenic fungi and
bacteria are killed. At about 820C, most weeds, plant pathogenic bacteria and insects are
killed. Heat tolerant weed seeds and some plant viruses, such as TMV are killed at or
near the boiling point (95-1000C).
iii. Hot water or Hot air treatment: Hot water treatment or hat air treatment will
prevent the seed borne and sett borne infectious diseases. Hot water treatment of certain
seeds, bulbs and nursery stock is done to kill many pathogens present in or on the seed
and other propagating materials. Hot water treatment is used for controlling sett borne
diseases of sugarcane [whip smut, grassy shoot and red rot of sugarcane (520C for 30
min)] and loose smut of wheat (520C for 10 min).
Biological methods:
Def: Biological control of plant disease is a condition or practice whereby survival or
activity of a pathogen is reduced through the agency of any other living organism (except
human beings), with the result that there is reduction in incidence of the disease caused
by the pathogen (Garett, 1965).
Def: Biological control is the reduction of inoculum density or disease producing activity
of a pathogen or a parasite in its active or dormant state by one or more organisms
accomplished naturally or through manipulation of the environment of host or antagonist
by mass introduction of one or more antagonists (Baker and Cook, 1974)
Mechanisms of biological control
1. Competition: Most of the biocontrol agents are fast growing and they compete with
plant pathogens for space, organic nutrients and minerals. Most aerobic and facultative
anaerobic micro-organisms respond to low iron stress by producing extracellular, low
molecular weight (500-1000 daltons) iron transport agents, designated as Siderophores,
which selectively make complex with iron (Fe3+) with very high affinity. Siderophore
producing strains are able to utilize Fe3+ - Siderophore complex and restrict the growth of
deleterious micro-organisms mostly at the plant roots. Iron starvation prevents the
germination of spores of fungal pathogens in rhizosphere as well as rhizoplane.
Siderophores produced by
Pseudomonas fluorescens (known as pseudobactins or pyoveridins) helps in the control
of soft rot bacterium, Erwinia caratovora.
2. Antibiosis: Antagonism mediated by specific or non-specific metabolites of microbial
origin, by lytic agents, enzymes, volatile compounds or other toxic substances is known
as antibiosis.
a. Antibiotics: Antibiotics are generally considered to be organic compounds of low
molecular weight produced by microbes. At low concentrations, antibiotics are
deleterious to the growth or metabolic activities of other micro-organisms.
Ex: Gliocladium virens produces gliotoxin that was responsible for the death of
Rhizoctonia solani on potato tubers.
Ex: Colonization of pea seeds by Trichoderma viride resulted in the accumulation of
significant amount of the antibiotic viridin in the seeds, thus controlling Pythium
ultimum.
