Introduction- In irrigated agriculture, weed control through chemical herbicides, creates spray drift hazards and adversely affects the environment. Besides, pesticide residues (herbicides) in food commodities, directly or indirectly affect human health. These lead to the search for an alternate method of weed management, which is eco-friendly. In this regard the biological approach is gaining momentum. Utilization of plant pathogens for weed control was first reported in the early 1900s, but the concept of using bioherbicides to control weeds attracted wide interest among weed scientists and plant pathologists after the Second World War.

A bioherbicide is a biologically based control agent for weeds. Bioherbicides are made up of microorganisms (e.g. bacteria, viruses, fungi) and certain insects (e.g. parasitic wasps, painted lady butterfly) that can target very specific weeds. The microbes possess invasive genes that can attack the defense genes of the weeds, thereby killing it. Bioherbicides may be compounds derived from microbes such as fungi, bacteria or protozoa or phytotoxic plant residues, extracts or single compounds derived from other plant species. A bioherbicide based on a fungus is called a mycoherbicide. In the industry, bioherbicides and other biopesticides are often referred to as “naturals”.

A bioherbicide is a preparation of living inoculum of a plant pathogen, formulated, and applied in a manner analogous to that of a chemical herbicide in an effort to control or suppress the growth of weed species. The use of bioherbicides is based on the fundamental epidemiological principles of plant pathology. Plant disease is the result of the interaction among the host plant, the pathogen and the environment, commonly referred to as the disease triangle. Although serious, devastating disease epidemics of crop plants occur, they are the exception rather than the rule and many factors can limit disease development. Pathogen factors such as low inoculum levels, weakly virulent pathogens, and poor spore dispersal mechanisms, environmental factors such as unfavorable moisture and temperature conditions and plant factors such as low susceptibility of the host, and widely dispersed host populations often limit disease.

The bioherbicide approach is an attempt to bypass many of these restraints on disease development by periodically dispersing an abundant supply of virulent inoculum uniformly onto a susceptible weed population. The application is timed to take advantage of favorable environmental conditions and the most susceptible stage of plant growth. Bioherbicides offer many advantages. They include a high degree of specificity of target weed, no effect on non-target and beneficial plants or man, absence of weed resistance development and absence of residue build-up in the environment.

Basis of the Bioherbicide Approach
In the development of any new pest control strategy, safety and efficacy are the two primary concerns. As a consequence, safety and efficacy are the major criteria in the selection of suitable plant pathogens. The better understanding of the genes of both microorganisms and plants has allowed scientists to isolate microbes (pathogens) whose genes match particular weeds and are effective in causing a fatal disease in those weeds.

The preferred characteristics of a potential bioherbicide pathogen include:-
– Growth and sporulation on artificial media
– Highly virulent
– Genetic stability
– Restricted host range
– Broad tolerance range
– Prolific propagules production
– Capacity to damage its host plant
– Innocuous in ecological effects

Limitations in use of Bioherbicides
Besides many advantages of bioherbicides, certain factors have been reported to limit the development of bioherbicides into commercial products.

These include biological constraints (host variability, host range resistance mechanisms and interaction with other microorganisms that affect efficacy), environment constraints (epidemiology of bioherbicides dependent on optimum environmental conditions), technical constraints (mass production and formulations development of reliable and efficacious bioherbicide), and commercial limitations (market size, patent protection, secrecy and regulations). The bioherbicides approach is gaining momentum. New bioherbicides will find place in irrigated lands, wastelands as well as in mimic parasite weeds or resistant weed control.

Research on synergy test of pathogens and pesticides for inclusion in IPM, developmental technology, fungal toxins, and application of biotechnology, especially genetic engineering is required. However, bioherbicides should not be viewed as a total replacement to chemicals, but rather as complementary in integrated weed management systems.

Dr. Monika Karnawat, Associate Professor, School of Agriculture Sciences, Career Point University, Kota

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