Biological control of weeds: Strategies, current status and feature prospects of mycoherbicides

Weeds, the plants growing rapidly where not desired, are a major kind of pests. Weeds constitute 3 % of the total 3,50,386 species of plants identified worldwide. They are troublesome and a major threat to human health, biodiversity, and agricultural productivity. Annually, they cause billions of US dollars of damage. It is estimated that they can reduce crop yields by as much as 12 % which results to US $ 32 billion as a whole. Management of weeds by herbicides account for over US $ 14 billion every year. Controlling weeds through herbicides/ weedicides is not an ideal option in organic cropping system since they cause significant damage to the environment by polluting/ contaminating air, soil and water, thereby posing severe health risks to humans (cancer, neurological disorders), reducing biodiversity and soil microbiota (essential in biomass cycling, increasing soil fertility), and creating superweeds.

Control  of  agricultural  and  forest  weeds  using  foliar pathogens has gained acceptance as a safe and environmentally sound approach. The microbe- based formulations used to control weeds are termed as microbial herbicides/ bioherbicides. Based upon the microbial control agent used in a formulation, the bioherbicides are classified as: mycoherbicides (fungi), bacterial herbicides (bacteria) and virus herbicides (viruses).

Biological weed control using fungal phytopathogens is carried out by three strategies: classical, mycoherbicidal and manipulated mycoherbicide strategy. Mycoherbicidal strategy involves the usage of host-specific, indigenous, virulent fungal plant pathogens, which are mass produced and sprayed in the fields, the way chemical herbicides are used, to control specific weeds without harming to the crop and other non-target hosts in the environment. Globally 26 bioherbicides have been developed so far. The pace of their adoption by the users (farmers, foresters, horticulturists) is, however, slow because of an array of biological, economic and regulatory constraints. The future of bioherbicides appears to be promising. The need of the hour  is  working  of  plant  pathologists,  weed  scientists, and biotechnologists, in collaboration with industrial houses to resolve the issues in their production and successful application in fields to control weeds within short time comparable to chemical herbicides.