SOIL PRODUCED ANTIBIOTICS—PLANT DISEASE AND INSECT CONTROL

Abstract

The soil is the natural habitant of coutless diseases live in the soil, and it has long been hordes of microorganisms, consisting of many known that the composition of the soil micro-kinds and types, living in a constantly changing flora can exert a controlling influence on them. environment. Most of them are active one way This effect is connected closely with microbial or another in reducing complex organic substances antagonism. When a soil has been sterilized, to simpler forms. Usually their existence pro-competition between microorganisms is reduced duces a healthy soil and a more favorable en-or eliminated; when such a soil is invaded later vironment for plant growth. The latter is brought by a parasite, the disease spreads much more about in part by increasing the food supply and quickly and is more virulent than in a natural improving the structure of the soil. Some of the nonsterilized soil. Thus, the microflora of the soil organisms are injurious, however. They can cause acts as a barrier to the invader. a variety of diseases among cultivated plants. Organic substances high in carbonaceous mate-During recent years it has been discovered that rials appear to be particularly favorable to the the organisms which produce the "wonder" development of antibiotic producing microorgan-drugs (e.g., penicillin) used in treating certain isms in the soil. The limited data available on human ills also produce these antibiotics in the this point suggest that it may have practical soil. Certain of these antibiotics have been found possibilities under field conditions in controlling capable of destroying or inhibiting the develop-plant diseases and insects by using suitable ment of a number of plant pathogens, viruses, cropping systems and the proper utilization of the and insect pests, both in the laboratory and under resultant plant residues. field conditions. It has been found that the most intensive Presumably these antibiotics are produced in microbiological activity occurs in the rhizosphere. the soil for the protection of the microorganisms The microbiological population and activity also producing them. The apparent purpose is to vary widely within different parts of the rhizo-enable the particular organisms to hold their own sphere. The antibiotic and associative inter-in competition with numerous others, perhaps by actions between groups of microorganisms are insuring the production of an uninterrupted sup-thought to be intensified in the rhizosphere by ply of food and energy. This is accomplished the effect of root growth and decay on the num-through the elimination or control of plant patho-bers and kinds present. This intensified activity gens, viruses, and insect pests that prey on their is believed to account for the production of sub-food producing plants. stances beneficial to higher plants when absorbed These antibiotics may destroy the plant patho-by their roots. This intensified microbial activity gens either directly by attacking them in the soil in the rhizosphere is thought to be related closely or by acting systemically. In the latter case, the to other conditions existing in soils. There is a antibiotics, after being absorbed by the roots, vast difference in the microflora of the rhizo-are translocated to other parts of the plant, sphere of disease resistant and disease susceptible rendering the penetrated tissues toxic (1, 2, 3). varieties and strains of crops grown in the same Certain plant pathogens, viruses, and insect soil under identical conditions. Antibiotics pro-pests which invade or attack the plants are thus duced in the soil have prevented some plant destroyed. diseases by inhibiting or destroying pathogenic Many of the organisms which cause plant microorganisms, either in the soil or systemically 1 J. H. Stallings, formerly research specialist when absorbed by roots of higher plants.