January 2005 The CRS CommPoster 5
|
Yes. We do Have a Choice: Chemical or Biological Warfare Dr.
Lakshmi Sridharan Spring heralds the birth of a new season. All
creatures great and small, visible and invisible, good and bad emerge out of
their hibernating grounds to continue with their cosmic dance of life. Roses put forth hundreds of new
shoots. Garden is lush and green. Hungry insects can not resist the
temptation of feeding on juicy tender plant parts. Pathogens (bacteria, fungi, and viruses)
have to live and reproduce. Pests, pathogens, plants, and people demand their
fair share of Nature’s bounty. This
creates intense competition in the garden.
Survival of the fittest is the name of the game. Through billions and billion's years of
organic evolution, pests, pathogens, and plants have evolved ingenuous
offensive and defensive strategies for their survival. Should we let Nature take its own course
of action or intervene and take charge of the situation? Are we not the Masters in our own garden?
Don’t we have the right to launch a chemical or biological warfare against
fungal, bacterial, and viral plant pathogens?
Yes, we do have the right to defend our territory. However, we should weigh the pros and cons
of our combat strategies before we make a choice between chemical and
biological warfare. Understanding the basics of
microbial interaction with plants, the offensive mechanisms of pathogens and
defensive mechanisms of plants is the key to successful control. Pathogen virulence and plant resistance
are not only under the control of their respective genes but also under the
interaction of pathogen and plant genes.
For successful parasitization, a pathogen has to recognize the host,
penetrate the host and establish within the host during infection. A biological or chemical control agent
should be able to counteract the offensive mechanisms of the pathogen at
several points (recognition, penetration and establishment) during infection. |
|
A chemical or biological control agent should be able attack one or more of the pathogen offensive mechanisms and thereby disarm the pathogen. One
should take several factors into consideration in the choice of control
methods. It is good to ask the
following questions: (1) Will the use
of the agent lead to formation of resistant pathogen strains? Will the agent of our choice be able to
destroy or debilitate pathogen without harming the plant? (2) How much would
it cost? (3) Is it easy to apply? (4)
Will the protective effect of the agent last long? (5) Finally what are the long-term effects
of the agent on environment, soil microorganisms, and the gardeners? Currently no chemical or biological agent
has all the desirable characteristics. Chemical
controls: A variety of fungicides are commercially available to combat fungal
diseases (black spot, powdery mildew, downy mildew, rust, etc. The widely used fungicides are either
single site fungicides or Multisite fungicides. Single site fungicides target
one specific site or function of pathogen.
Single site fungicides such as Funginex, Bayleton, Rubigan, Tilt etc.
are sterol biosynthesis inhibitors (SBI).
They inhibit ergosterol synthesis in fungi. Ergosterol is one of the chemical
constituents of fungal cell wall. SBI weaken the fungus because of the
inferior fungal membrane and inhibit fungal growth. However, the fungus can synthesize other
sterols. Fungus may use an alternate pathway and become resistant. Repeated
uses of the same fungicides favor selection of resistant strains from the
existing minor resistant population in a large population of fungi. Continued use of SBI also leads to cross
resistance, resistance to more one SBI with similar mode of action. SBI, therefore, is not a good choice for
disease control. Multi-site
fungicides adversely affect a number of fungal biochemical pathways. They target Continued
on Page 6 |
Top of
Page