Biopesticides are currently used to manage inverte brate pests, plant pathogens, and weeds. The microorganisms used include bacteria, yeasts, fungi, and viruses. Most research has been conducted on fungal species; as many are easily cultured, their spores are environmentally stable and can be easily managed, amended, and manipulated - they can hence be applied with convention spray systems. To date, much more research effort has been spent on the development, registration, and commercialization of products for plant pathogens and pest species than of weed species, so only the former two will be covered here.
Biofungicides attack plant pathogens through several mechanisms, including antibiosis, hyperparasitism, and competition, with the latter usually being the predo minant and most important mechanism. There are many products available for a wide range of crop/ pathogen systems which are effective on both foliar and root diseases. Many agents are highly competitive for space and nutrients but usually have a low cap ability of displacing developed microorganisms. These characteristics make biofungicides preventative treat ments but some have limited curative potential. Biofungicides share the characteristics of the best bio logical control agents of being highly reproductive, environmentally hardy, and highly antagonistic. Many of the current products contain a series of species from the same genus that operate through different mechanisms to obtain higher levels of con trol. To date, most products for foliar and soil plant pathogens are Trichoderma species and can help to manage economically important plant pathogens such as botrytis rot, powdery mildew, and Sclerotinia species. Products have also been developed as seed dressings from the genus Bacillus and Pseudomonas to control several soil borne plant diseases. Species have also been investigated as potential biofungicides when they are suspected to have a role in suppressive soils, a concept where plants grown in soils high in parti cular soil flora are more resistant to disease. Fluorescent pseudomonades are one of these species and can inhibit germination of fungi through competi tion for ferrous minerals.
Microorganism biopesticides for the management of invertebrate pests
More than 1500 species of fungi, viruses, bacteria, and protozoa can infect arthropod species. The first attempt to use them as bioinsecticides was in 1884 when a Russian scientist, Elie Metchnikoff, used Metarhizium anisopliae to infect two invertebrate pests, the cereal cockchafer, Anisoplia austriaca, and the sugar beet weevil, Bothynoderes punctiventris. The first commercially available bioinsecti cide, Bt, was produced in the 1930s and many bioinsecticides have been produced since, the majority of which are entomopathogenic hyphomycete fungi. Bioinsecticides have not always provided consistent con trol of insect pests, as the relationship between the agent, the pest, the timing, and the environment is inherently complex. Of these variables, the environment is the most important and the one most likely to lead to a breakdown in the system. Consequently, bioinsecticides have been most successful in the warm, humid conditions of the tropics. When environmental conditions change, both the pest and the biological control agent are affected. Both species have an optimum range of temperatures and humidities. In temperate climates, the agents' opti mum temperature and humidity conditions are usually higher than those of the pest species, so the agents' per formance can decline and the pests' fitness can increase with a decline in these environmental variables. Pest health also has impacts on resistance to fungal attack, especially pest nutrition, age, genetics, and physical damage through mechanical, chemical, or biological means. Agriculturalists can influence pest health and, as a consequence, increase pest susceptibility to bioinsecti cides, by using resistant crop cultivars or manipulating the habitat to alter pest nutrition. Other variables that influ ence the success of the technique are the virulence and hardiness of the agent, its compatibility with the host, and the viability of the propagule after storage. To increase the effectiveness of the products, agriculturalists are urged to monitor environmental conditions to assist in deciding when bioinsecticides should be applied and when to apply them to optimize environmental condi tions. Spray machinery should also be calibrated to ensure good spray coverage at the correct rate, as this will increase the likelihood of exceeding the pest propagule threshold and lead to effective management of the pest as a consequence.
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