USDA ARS Appalachian Fruit Research Station, 45 Wiltshire Rd, Kearneysville, WV 25430, USA

Background and objectives
Plant pathologists have perhaps been overly influenced by entomologists in defining the biological control of plant diseases. The primary object for biocontrol of insects is an organism (the insect). Plant pathologists, on the other hand, are targeting a process (a disease), as well as an organism (the pathogen). Strategies to control plant diseases biologically should take into account the disease process, as well as the pathogen.

As we have gained more insight into the biological control of post-harvest diseases it became apparent that a broader definition of biological control was needed to encompass the complex interactions that occur. My objective is to define biological control of plant diseases in a manner that clearly distinguishes it from physical and chemical control, and that takes into account the multifaceted nature of biological control. The inclusiveness and exclusiveness of a definition affect relationships among the components of a definition and the subsequent evolution of scientific thought. For example, if genetic resistance is not considered biological control, scientists and concepts in biological control and genetic resistance will evolve independently.

Results and conclusions
In our studies of yeast antagonists that control post-harvest decay of fruits and vegetables [3], we have discovered that their mode of action is mediated both by the antagonist and the host. In the classical sense of biological control, some yeast antagonists attach to pathogens and degrade their cell walls. They also compete at the wound site with the pathogen for space and nutrients. It has also been found that yeasts antagonistic to post-harvest pathogens can 'turn on' host defense reactions to disease, such as defense enzymes and anatomical barriers.

I would like to present a definition of biological control which accounts for the unique nature of plant diseases and which encompasses all the elements of biological control that occur naturally. Therefore, I would like to define biological control of plant diseases as:

"The control of a plant disease with a natural biological process or the product of a natural biological process."

If we take this broad definition of biological control, chemicals extracted from living organisms would be 'biological', as well as those 'delivered' by living organisms. Also, host resistance (constitutive and elicited) would be biological control. Biological control under this definition would be clearly distinguishable from physical and synthetic chemical control of plant diseases.

Utilizing this broad definition of biological control we have pursued three avenues to control post-harvest diseases of fruits and vegetables biologically: (i) antagonistic microorganisms; (ii) natural plant- and animal-derived fungicides; and (iii) induced resistance [2]. We have also combined these different types of biological control into a multifaceted biological control strategy [3]. For example, when peaches are first subjected to LTV-C light to induce resistance, and subsequently treated with antagonistic yeasts, rot control is greater than the sum of the control by UV-C light and the antagonist alone [1].

1. Stevens C, Khan, V A, Wilson et al., 1997. Biological Control 10, 98-103.
2. Wilson CL, 1997. Journal of Industrial Microbiology and Biotechnology 19, 159-159.
3.Wilson CL, Wisneiwski ME, eds, 1994. Biological Control of Postharvest Diseases of Fruits and Vegetables. CRC Press, Boca Raton, FL.