Agriculture group symposium microbial inoculants in agriculture
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J Sci Food Agric 1990, 50, 127-140
Agriculture Group Symposium Microbial Inoculants in Agriculture
The following are summaries of papers presented at a joint meeting of the Agriculture Group and the Biotechnology Group of the Society of Chemical Industry held on 14 February 1989 at the Society of Chemical Industry, 14-15 Belgrave Square, London SWlX 9 P S . The papers published here are entirely the responsibility of the authors and do not reflect the views of the Editorial Board of the Journal of the Science of Food Agriculture.
Microbial Inoculants: A Promise Deferred?
P Bernard Tinker Terrestrial and Freshwater Sciences Directorate, Natural Environment Research Council, Swindon, Wiltshire SN2 IEU, UK
Over many years there have been attempts to use microbiological inocula to improve crop growth and agricultural productivity. It is a contention of this paper that, when surveyed overall, the results are deeply unsatisfactory, in that each group of new materials is greeted with enthusiasm but usually fades into disappointment and neglect.
Among the earliest such interests were nitrobacterin and phosphobacterin, which were very popular during the 1930s. Despite many claims from within the USSR, and careful tests by USDA and Australian scientists, it was finally concluded that no dependable and repeatable positive results could be obtained.
The main type of inoculum which has established itself as a major product is Rhizobium. It is well established that certain crops require inoculation, because of the low population of the specific Rhizobium species which are required for particular crops. Despite this well established commercial success, it is notable that the huge amount of research work on Rhizobium over the last 15 years has produced little of applied value. Strains of Rhizobium in commercial production and distribution are still substantially similar to those used before that time, and have been obtained by simple selection and testing from natural isolates.
Mycorrhizal inocula have generally proved disappointing on a field scale. Numerous experiments have shown extremely large responses to inoculation, but usually when carried out in sterilised soil most often in the glasshouse. The potential for the future may be better, the most promising application probably being in tree nursery inoculation. A large range of other organisms have been considered, and are
J Sci Food Agric (50) (1990)-62 1989 Society of Chemical Industry. Printed in Great Britain
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now popular subjects of research. Pseudomonads have been suggested for root disease control and Azotobacter for nitrogen production in the rhizosphere.
The reasons for this rather disappointing history are discussed, together with prospects for the future.
Fate of Microbial Inoculants in Soil
Martin Wood Department of Soil Science. University of Reading, London Road, Reading GRI 5AQ, U K
In order to survive, multiply and be effective in soil, an introduced microorganism should be tolerant of chemical factors such as acidity and physical factors such as extremes of temperature. It should also be able to move through soil to its site of action. Bacteria are the main organisms of current interest, and the required destination for most inoculants is the plant root. At present it is impossible to predict the fate of an organism introduced into soil. This paper therefore considers some of the factors which may influence the survival, multiplication and movement of bacteria in soil, in particular the soil around plant roots.
Bulk soil measurements of acidity, salinity and water content are of little use in understanding the fate of microorganisms in soil, because they survive in microsites which may be quite different from the bulk soil conditions. Microsites are also extremely difficult to measure. The soil around plant roots may differ by as much as 2.5 pH units from the non-rhizosphere soil, will have a much lower concentration of phosphate, and may be wetter or drier than the bulk soil. A successful inoculant must be able to tolerate these localised conditions. There may also be a higher population of predatory protozoa and nematodes in the rhizosphere, which will reduce the population of introduced microorganisms.
For appreciable movement of bacteria through soils there must be enough water- filled pores of the required diameter to provide a continuous pathway. Movement will be restricted at water potentials of approximately - 100 kPa, which is much higher than potentials which cause plants to wilt. Motile organisms (those possessing flagella) may have a better chance of meeting a plant root, and chemotactic responses may assist in avoiding unfavourable conditions and locating a plant root. Bacteria may also move relatively rapidly by mass flow through cracks and channels formed by earthworms and dead roots. This may play an important part in determining the fate of bacteria introduced into soil.
The fickle nature of most current soil microbial inoculants such as Rhizobium sp and Pseudornonas sp is probably a reflection of the complexity and hostility of the environment into which they are introduced. Improvements in the reliability of soil inoculants will depend upon a greater understanding of soil microbial ecology. In the meantime the development of soil inoculants will remain empirical.
Microhiul irioculants in agriculture 129
Biological Control of Diseases Using Microbial Inoculants
Richard Campbell Department of Botany, University of Bristol, Bristol BS8 lUG, UK
Microbial inoculants for disease control have existed, and been available commercially on a small scale, for many years. They have not, however, been widely used in general agriculture. The main reason for this is that a need for them has not been perceived by the industry, and such research as has been done has had difficulty in developing inocula which have consistent control of disease. There has recently been an increased interest in biocontrol agents, and research and development effort has increased.
Most of the research programmes have concentrated on soilborne diseases, especially of intensively grown crops, or on situations where conditions especially favour introduced inocula. Areas of interest include: (1) disease where there is RO chemical control or varietal resistance available at present; (2) horticultural crops where sterilised or specially prepared composts may be used and where plants are transplanted; (3) diseases where entry is by defined infection routes (eg pruning wounds on tree).
There are many possible modes of action of biocontrol agents, but those which produce antibiotics, siderophores or lytic enzymes are receiving most attention at present.
Future developments are likely to concentrate on integrated control, using chemicals and agriculture practices to enhance biological control or allow establishment of inocula. Genetically engineered organisms with enhanced biocontrol ability and improved colonisation and survival are possible. The problem is to determine the exact characteristics, which could be manipulated genetically, to confer these properties on the organism.
The concept of biological control is attractive, but there are clearly problems which were not foreseen in early work. These problems are not insuperable, but much more information is needed on basic microbial ecology before the behaviour of biocontrol agents can be monitored and predicted in natural and agro- ecosystems.
The Development and Use of Viral Insecticides
David H L Bishop NERC Institute of Virology, Mansfield Road, Oxford OX1 3SR, UK
A variety of biological agents for the control of insect pests have been developed in the twentieth century. Both bacterial toxins, such as Bacillus thuringiensis toxin, and insect-specific viruses have found favour as alternatives to chemical insecticides,
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particularly for environmentally sensitive cultivars (such as forests that are in water catchment areas).
Unlike many chemical insecticides these biological agents are specific and limited in their effect to particular (or a few) insect species. There are, however, disadvantages to the extensive use of viral insecticides. For example, they are of limited use against pest complexes that include susceptible and non-susceptible species. Viruses are much slower than chemicals, so that crop damage may occur while the viruses produce an effect. On the whole, biological control agents are more expensive to manufacture than chemicals. Despite these drawbacks there is a role for biological insecticides both as stand-alone agents and in concert with others in integrated programmes of pest management.
Three areas illustrate the use of viral insecticides: the control of the leipidopteran pest Panolis Jammea, pine beauty moth, in Scotland; the control of the coleopteran Oryctes rhinoceros beetles in palm groves in the Seychelles; and the control of the hymenopteran Neodiprion sertijer, pest pinesawfly, in Scottish forests. Effective control procedures have been developed for these pests by staff of the Institute of Virology.
The opportunities for the development of novel insecticides using genetically engineered viruses are described, including insecticides that can control pests that are of concern not only to agriculture but also to human health.
Past, Present and Future Uses of Rhizobiurn Inoculants
Penny R Hirsch AFRC IACR Rothamsted Experimental Station, Harpenden, Herts AL5 UQ. UK
The beneficial effects of including legumes in crop rotations was known to the Romans and was exploited in European agriculture long before it was realised that they enriched