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ALAN K. WATSON
Department of Plant Science, Macdonald College of McGill University, 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, Quebec, H9X lCO, Canada
Significant advances are being made in the discovery and development phases of novel ~ products, but various factors have limited the deployment of bioherbicides in crop production systems. Many of the limitations to bioherbicide advancement have been suggested with low pathogen virulence and fastidious environmental conditions identified as the key restraints to overcome. Improvements in strain selection, bioherbicide formulation, and field use are being made and bioherbicides are being integrated with other weed control strategies to provide effective weed control. Overcoming these biological and technological limitations will significantly reduce the economic questions and concerns which should translate into increased industrial commitment and involvement, culminating in the deployment of effective, economically viable bioherbicide solutions to some of our major weed problems.
This paper will attempt to address the current advances in bioherbicide research through a brief review and discussion of the basis, the progress, the restraints, and the prospects of this approach to weed control. Although fungi, bacteria, mycoplasmas, viruses and nematodes incite plant disease, the use of pathogens other than fungi as bioherbicides is limited. Therefore, the term "mycoherbicide" has often been used interchangeably with "bioherbicide." In addition the term "bioherbicide" is generally restricted to the use of plant pathogens and does not include attempts to augment populations of beneficial insects, nor does it generally include the use of naturally occurring compounds (phytotoxins) produced by microorganisms. Phytotoxins, however, may play an important role in bioherbicide development.
BASIS OF THE BIOHERBICIDE APPROACH
A bioherbicide is a preparation of living inoculum of a plant pathogen, formulated, and applied in a manner analogous to that of a chemical herbicide in an effort to control or suppress the growth of weed species. The use of bioherbicides is based on the fundamental epidemiological principles of plant pathology. Plant disease is the result of the interaction among the host plant, the pathogen and the environment, commonly referred to as the disease triangle. Although serious, devastating disease epidemics of crop plants occur, they are the exception rather than the rule and many factors can limit disease development. Pathogen factors such as low inoculum levels, weakly virulent pathogens, and poor spore dispersal mechanisms; environmental factors such as unfavorable moisture and/or temperature conditions; and plant factors such as low susceptibility of the host, and widely dispersed host populations often limit disease. The bioherbicide approach is an attempt to bypass many of these restraints on disease development by periodically dispersing an abundant supply of virulent inoculum uniformly onto a susceptible weed population. The application is timed to take advantage of favorable environmental conditions and/or the most susceptible stage of plant growth. Similarly the bioherbicide is formulated to avoid unfavorable environmental conditions and to facilitate application. As a consequence, the development of an effective bioherbicide requires a comprehensive understanding of the pathogen(s) involved, the biology and population dynamics of the target weed(s), the optimum requirements for disease initiation and development, and the complex interactions within the hostpathogen system.
In the development of any new pest control strategy, safety and efficacy are the two primary concerns (Watson & Wymore, 1989b). As a consequence, safety (in relation to crop plants, the environment, and human health) and efficacy (in relation to environmental tolerance, level of damage to the weed, and ability to be integrated within the crop production system) are the major criteria in the selection of suitable plant pathogens. The preferred characteristics of a potential bioherbicide pathogen include: 1) growth and sporulation on artificial media, 2) highly virulent, 3) genetic stability, 4) restricted host range, 5) broad tolerance range, 6) prolific propagule production, 7) capacity to damage its host plant, and 8) innocuous in ecological effects (Templeton et al., 1979).
In determining the suitability of a particular weed species as a target for bioherbicide development, native or naturalized weed species should have a larger complement of indigenous pathogens to select from as compared to fewer pathogens associated with recently introduced weeds. Templeton et al. (1986) suggest that bioherbicides have greatest potential for control of: a) weeds infesting small specialized areas where chemical herbicide development would be too costly, b) weeds that have been intransigent to chemical control, c) crop mimics, and d) parasitic weeds. Since potential return on investment is critical to industrial involvement in bioherbicide development, major weeds, presently not controlled by available technology, in major crops are perhaps the ideal targets for the bioherbicide approach.
Intuitively, annual weed species may be considered preferred targets when compared to perennial weed species. However, the growth habit, growth rate and other biological parameters which determine susceptibility and subsequent disease development are more critical than whether the weed is an annual or a perennial. For example, annual weeds such as velvetleaf (Abutilon theophrasti) with their erect habit of growth and rapid rate of stem elongation may be less susceptible to disease development of foliar pathogens when compared to vigorous perennials such as field bindweed (Convolvulus arvensis) and dandelion (Taraxacum officinale) with their prostrate habits of growth.
STEPS IN BIOHERBICIDE DEVELOPMENT
The development of a biological herbicide involves three major phases or stages: 1) discovery, 2) development, and 3) deployment (Templeton, 1982). The discovery phase involves the collection of diseased plant material, isolation of the causal organism, demonstration of Koch's postulates, identification of the pathogen, culture of the pathogen on artificial media, and maintenance of the pathogen cultures in short-term and long-term storage. The development phase involves the determination of optimum conditions for spore production, determination of optimum conditions for infection and disease development, determination of host range and elucidation of mechanism of action of the pathogen. The final phase, deployment, often involves close collaboration between nonindustrial and industrial sectors through the formulation, scale-up, field evaluation, and marketing stages of commercialization process of a new bioherbicide product.
The proposed close collaboration between industrial and non-industrial sectors is not always easy, especially when the objectives of the two groups are often not completely compatible. Both Baker (1986) and Scher and Castagno (1986) point out that despite intensive research and numerous apparently successful biological control agents, very few have reached the marketplace. Baker (1986) suggests the need for more research related to understanding the basic mechanisms of biological control, whereas Scher and Castagno (1986) suggest the reason for the paucity of marketable biocontrols is because most biocontrol agents have been developed from a scientific point of view only, without an industrial perspective.
To date, two bioherbicides have been registered for weed control, both in the United States. DEVINER, a liquid formulation of Phytophthora palmivora was registered in 1981 for control of stranglervine (Morrenia odorata) in Florida citrus groves. COLLEGOR, a dry powder formulation of Colletotrichum gloeosporioides f. sp. aeschynomene, was registered in 1982 for the control of Northern Jointvetch (Aeschynomene virginica) in rice and soybeans in Arkansas, Louisiana and Mississippi. To my knowledge no other bioherbicides are as yet registered for use, but active research programs in various laboratories within North America, Europe and elsewhere are making rapid progress towards the development and registration of additional bioherbicide products for specific weed problems.
Copyright © 1989.
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