Biotechnology and Soilborne Plant Diseases

George C. Papavizas, plant pathologist, and Joyce E. Loper, microbiologist, soilborne Diseases Laboratory, Plant Protection Institute, Beltsville Agricultural Research Center, Agricultural Research Service.

The soil nourishing the crops that provide our food and fiber is a complex environment where microbes live in numbers estimated to reach 2 billion per ounce of soil.

Millions upon millions of individual bacteria and fungi live on root surfaces and the soil particles that surround them. Bacteria are the smallest and the most abundant. The fungi, usually larger than bacteria, are found throughout the world, not only in fertile soils but also in deserts and drylands, in polar lands and mountain highlands, and in forests and marshes.

Beneficial Soil Microbes

Great numbers of soil microbes are beneficial to humans and other animals, and to cultivated plants. They convert atmospheric nitrogen, which plants cannot use, to ammonia or other useful nitrogenous compounds in small nodules on the roots of legumes such as peas, beans, soybeans, and clovers; oxidize chemicals and assist plants in absorbing nutrients and trace elements such as iron, cobalt, manganese, and molybdenum from soil; and decompose plant and animal organic matter into simpler organic products that plants can absorb and use to sustain their growth. Soil microbes also help to form and maintain arable soils rich in complex organic materials through which roots easily grow and absorb water and nutrients. Were it not for microbial activity to transform organic matter released by living animals, plants, and microbial cells and dead organisms, nutrients for plants would be locked away from them in large, unmanageable piles.

Soil microbes help transform organic matter into a form useful to plants.

Harmful Soil Microbes

Harmful molds and other soil fungi are responsible for many serious root diseases and aboveground diseases of plants. No major economic crop escapes damage from soilborne fungi and bacteria. Root rots, collar rots, wilts, seed decay, seedling blights, fruit rots, root browning, and damping-off take a heavy toll year after year. More than 50 percent of the total estimated annual ll losses of economic crops due to all plant diseases, or about $4 billion annually in the United States alone, are caused by soilborne diseases.

Many plant diseases caused by soil-borne plant pathogens (disease-causing organisms) are difficult to control by conventional procedures. Plants with resistance to most diseases caused by harmful soilborne microbes have not yet been developed by scientists. Growers currently depend on pesticides to fight some soilborne diseases. Aside from the environmental damage caused by many pesticides and public pressure not to use them, some of them are expensive, difficult to apply, or not completely effective against soilborne pathogens. Moreover, pesticides may indiscriminately kill both harmful and helpful soil microbes or present a health risk to humans and animals. Cultural control methods such as crop rotation may affect soilborne diseases very little since the pathogens that cause them attack a wide range of crops and can live in soil for a long time.

The thread-like hypha of the beneficial fungus Gliocladium virens coils around and destroys the Rhizoctonia solani, a soilborne pathogen that attacks more than 200 plant species (magnified 2,000 times).

Biological Control Using Soil Microbes

Fortunately for our crops and soil environment, every ounce of soil harbors numerous species of microbes, including many of the plant pathogens' natural enemies. These natural enemies, which include bacteria and fungi, could be exploited to reduce or curb the diseases caused by plant pathogens. This method of controlling plant diseases biologically is gaining in stature as a feasible technology of the future.

The concept and practice of biological control, or biocontrol, can be advanced not only by discovering and using new disease-fighting microbes, but also by improving their effectiveness with conventional genetics and modem biotechnological approaches such as genetic engineering, and by improving production and delivery systems.