Biocontrol Mechanisms

Abstract

It has been estimated that various plant diseases reduce plant yields by ~10 % per year in the more developed countries and by >20 % per year in the less developed countries of the world. From the perspective of many farmers in the developed world, the control of plant diseases is often not considered to be problematic. Plant disease control may be achieved by use of plants that have been bred for good resistance to many diseases, and by plant cultivation approaches such as crop rotation, the use of pathogen-free seed, appropriate planting date and plant density , control of field moisture, and, perhaps most importantly, the use of pesticides. Independent of human intervention, plants can synthesize a variety of phytochemi-cals as secondary metabolites and many of these chemicals have antimicrobial properties and can, to some extent, protect plants against diseases caused by different pathogens. Interestingly, some of these secondary metabolites are useful in treating a wide range of human diseases. A wide variety of infectious organisms can cause plant diseases including fungi, oomycetes, bacteria, viruses, phytoplasmas, protozoa, and nematodes. Fungi and bacteria cause the majority of common plant diseases. Most of the phytopathogenic fungi belong to the Ascomycetes and the basidiomycetes. Some important disease-causing Ascomycetes include Fusarium spp., Thielaviopsis spp., Verticillium spp., Magnaporthe grisea, and Sclerotinia sclerotiorum. Some common disease-causing basidiomycetes include Ustilago spp., Rhizoctonia spp., Phakospora pachyrhizi, and Puccinia spp. Fungal pathogens that are biotrophic colonize living plant tissue and obtain nutrients from living plant cells. On the other hand, fungal pathogens that are necrotrophic infect and kill plant tissue, and subsequently extract nutrients from the dead plant cells. Fungal diseases are typically controlled through the use of fungicides. A smaller number of bacteria than fungi can cause disease in plants. Pathogenic bacteria include some species of