A bioherbicide is a biologically based control agent for weed. In irrigated agriculture, weed control through chemical herbicides, creates spray drift hazards and adversely affects the environment. Besides, pesticide residues (herbicides) in food commodities, directly or indirectly affect human health. These lead to the search for an alternate method of weed management, which is eco-friendly.
Commercial bioherbicides first appeared in the market in USA in early 1980s with the release of the products Devine ,Collego and Biomal. Devine, developed by Abbott Laboratories,USA, the first mycoherbicide derived from fungi (Phytophthora palmivora Butl.), is a facultative parasite that produces lethal root and collar rot of its host plant Morrenia odorata (stangler wine)and persists in soil saprophytically for extended periods of residual control. It was the first product to be fully registered as a mycoherbicide.
The initiative for using pathogens, phytotoxins from pathogens, and other microorganisms as biological weed-control agents began about three decades ago. Since then, numerous microbes have been screened for phytotoxic potential, and several dozens evaluated as bioherbicides as reported by various researchers and summarized (e.g., Hoagland, 1990, 2001; TeBeest, 1991). Due to the interest in this area, many other weed pathogens and phytotoxins (from pathogenic and non- pathogenic microorganisms) will be discovered that possess bioherbicidal activity. Most bioherbicides have been targeted toward agronomic weeds, but these agents may also be useful to control weeds in nonagronomic areas (recreational areas, forests, rights-
Approach Classical – agent selection, inoculation, self- perpetuating, long term protection. Inundative – mass production, application at high inoculum levels over a localized area, short term, repeated application. Augmentation – re-establishment of a classical agent. Classes Mycoherbicide – fungal pathogen Bioherbicide – fungi and bacteria
Bruckart and Dowler, 1986: rust fungi are effective biological control agents - USDA Templeton, 1988: predicted widespread adoption can be achieved Strobel, 1991: predicted that bioherbicide use will realize a tremendous increase in agriculture
Demand for decreased use of pesticides Large areas where herbicide application not possible or not cost effective Damage to the environment Contamination of our water supply High yield losses still occur $619 million in vegetable, $441 million in fruit and nut crops in the US
Produce abundant and durable inoculum in culture Be target specific Be genetically stable Be capable of killing a significant portion of the weed population under a variety of environmental conditions (weed densities) Boyetchko, 1997
Herbicide resistant weed population Detrimental effects on non target organisms Native plants
Trait Bacteria Fungi VirusCulture Easy Easy HostSpecificity Excellent Good ExcellentField Performance Variable Variable UnknownFormulation Variable Excellent UnknownEffectiveness Variable Variable ExcellentGenetic stability High Medium Unknown
Pathogens of plant (i.e. bioherbicides) are generally host specific. Cherrington, C. A. and L. F. Elliott. 1987. Incidence of inhibitory pseudomonads in the Pacific Northwest. Plant and Soil 101:159- 165. Isolated pseudomonads from downy brome, winter barley, winter wheat, pea, lentil roots Found 106 - 108 CFU per gram dry weight Found isolates that reduced downy brome root growth but not wheat root growth
Kennedy, A. C., L. F. Elliott, F. L. Young and C. L. Douglas. 1991. Rhizobacteria suppressive to the weed downy brome. Soil Sci. Soc. Am. J. 55:722- 727 1000 isolates, 18 inhibitory to downy brome and not wheat Reduced DB population up to 30% Reduced DB shoot weight up to 42% Increased winter wheat yields 35% Both in greenhouse and in field trials in eastern Washington
Many fungi have been shown to exhibit broadspectrum weed control ranges.
WEED PATHOGEN REFERENCE1. Velvet Colletotrichum Hodgson et al., leaf coccodes, 1988 Fusarium lateritium Walker, 19812.Wild oat Septoria tritici Madariaga and Scharen, 19853.Water Alternaria eichhorniae Shabana, 1987hyacinth4.Sickle pod Pseudocercospora Hofmeister and nigricans Charudattan, 19875.Barnyard Cochliobolus lunatus Scheepens, 1987grass
PATHOGEN WEED SPECIES OR REFERNCE FAMILYAlternaria cassiae Sicklepod Boyette, 1988; Coffee senna Charudattan et Showy crotalaria al., 1986; Walker, 1982, 1983Amphobotrys ricini Members of Holcomb et al., 1989; Euphorbiaceae Whitney and Taber, 1986Colletotrichum Members of Daniel et al., 1973;gloeosporioides Leguminosae, Mortensen Malvaceae, and Makowski, 1997 Convolvulaceae (dodders)Myrothecium Sicklepod; various Walker and Tilley, 1997verrucaria species of other plant families
In most instances the potential risks associated with the use of Bioherbicides may includes certain concerns such as:- worker exposure and safety Plant host range effects to nontarget organisms (competition/displacement of beneficial microbes in the community) production of chemicals that are persistent or toxic to mammalian systems
Puccinia melampodii, a rust fungus isolated in Mexico, was approvedfor release in Australia in an integrated strategy to managethe highly allergenic weed, Parthenium hysterophorus, even thoughit could also sporulate on several marigold and sunflower cultivars(Evans, 2000). The Australian Quarantine and Plant InspectionService concluded that the actual and potential hazards involvedin not attempting to control this weed were significantly greaterthan the perceived risks to nontarget plants.One of the major hurdles in the use of bioherbicides is the risk associates with that of secreted metabolites.
Fungi secrete a wide range of metabolites, some of which are important medicines or research tools (Vey et al., 2001). Some of these metabolites are highly toxic (fumonisins, ochratoxins, patulin, zearalenone) or carcinogenic (moniliformin, aflatoxin). A large amount of data has accumulated on mycotoxin contamination of foodstuffs and the risk these metabolites (mostly from saprophytic fungi) pose to human and animal health (Abramson, 1998). In contrast less is known about metabolites from fungal biocontrol agents, particularly those from commercialized mycoherbicides, mycoinsecticides, and mycoparasiticides (Strasser et al., 2000; Vey et al., 2001).
There are two particular areas where there appears tobe cause for optimism in the mycoherbicides field;the use of virulent pathogens for the treatment of thecut stumps of weedy trees in forest ecosystems, andweed control targeted at the leisure industry (Evanset al., 2001). A recent example of the former concernsusing the silver leaf fungus, Chondrostereumpurpureum, for control of black cherry (Prunusserotina: Rosaceae); an invasive North Americanspecies which is a serious threat to coniferplantations, as well as to native woodlands in theNetherlands (De Jong et al., 1990)
The bioherbicide, Biochons, is currently being marketed by Koppert Biological systems as an environmentally friendly solution to undesirable tree regrowth. The use of this pathogen for management of weedy, endemic, deciduous trees in conifer plantations and amenity areas is also being evaluated in Canada(Prasad,1994).
In complete contrast, advanced technology and largecompanies are currently involved in the development ofbioherbicides in Japan, not only in crop protection butalso in the highly lucrative leisure industry. The mosttroublesome weed in golf courses is annual bluegrass(Poa annua) and chemical herbicides are either nonselectiveor now considered to be environmentallyundesirable. A highly specific, bacterial endophyte,Xanthomonas campestris pv. poae, has recently (1997)been registered under the name Campericos, andconstitutes the first bacterial herbicide to reach thecommercial market (Imaizumi and Fujimori, 1998).
There is no doubt that formulation has played a keyrole in the marketing of bioherbicides, such as Campericos, in order to overcome problems with storage,establishment and efficacy in the field. Essentially formulation is mixing the active ingredient, in this casethe biological propagule, with a carrier or solvent andother adjuvants in order to develop a product which can be stored, for at least 1 year, effectively applied to the target weed with safe and consistent results.
The development of bioherbicides are less expensive thanfor chemical herbicides (Templeton et al., 1986). Forexample, the cost of developing COLLEGO wasapproximately $1.5 million in research and developmentin the late 1970s and early 1980s (Heiny and Templeton,1993), and the cost of developing BIOMAL was estimatedto be about $2.6 million(J.R. Cross, Philom Bios, personalcommunication).These development costs, compared tothe $30 millionore more to discover and develop achemical herbicide, make bioherbicide development quitefavourable (Heiny and Templeton, 1993).
The role of biomicrobial herbicides in agriculture,however, is still problematic and insignificant.Nevertheless, because of pressures to reduce thereliance on chemical herbicides, bioherbicides couldmake a significant contribution to weed control. Inthe future, once the well-documented constraintshave been overcome, particularly through improvedtarget selection, formulation and marketing.
Prospects for the development and utilization of bioherbicide technology for majorrice weeds are very good. Work in this area is preliminary for the most part, butvirulent pathogens of some potential weed targets have been identified and initiallaboratory and field results are encouraging. Increased activity in basic and appliedscience and in biotechnology have a definite role to play in development,implementation, and advancement of this weed control strategy in tropical andsubtropical regions. Virulence, efficacy, fermentation, formulation, and applicationare aspects of prime importance. Industry must become more involved in small nichemarkets, and techniques must be developed for subsistence farmers as well as modernones. There is likely to be increased pressure from public and governmental bodies toreduce the use of chemical herbicides. We are challenged to find acceptable, effectivecomplementary weed control tactics.
Advances in bioherbicides development—an overview: R. Mohan Babu, , A. Sajeenaa, K. Seetharamana, P. Vidhyasekarana, P. Rangasamy, M. Som Prakash, A. Senthil Raja, K.R. Biji BIOHERBICIDES: RESEARCH AND RISKS- ROBERT E. HOAGLAND, C. DOUGLAS BOYETTE, and MARK A. WEAVER Southern Weed Science Research Unit, USDA- ARS, Stoneville, Mississippi, USA
HAMED K. ABBAS Crop Genetics and ProductionResearch Unit, USDA-ARS,Stoneville, Mississippi, USA.•CURRENT STATUS OF BIOHERBICIDEDEVELOPMENT AND PROSPECTS FOR RICE IN ASIA- Alan K. Watson.Plant Science Department, McGill University, 21,111Lakeshore Road,Canada.•Boyetchko, S. M. 1997. Principles of biological weedcontrol with microorganisms. HortSci. 32(2):201- 205.•Cherrington, C. A. and L. F. Elliott. 1987. Incidence ofinhibitory pseudomonads in the Pacific Northwest. Plantand Soil 101:159-165.
Kennedy, A. C., L. F. Elliott, F. L. Young and C. L. Douglas. 1991. Rhizobacteria suppressive to the weed downy brome. Soil Sci. Soc. Am. J. 55:722-727 Heiny, D.K., Templeton, G.E., 1993. Economic comparisons of mycoherbicides to conventional herbicides. In: Altman, J. (Ed.),Pesticide Interactions in Crop Production. CRC Press, Boca Raton, FL, pp. 395–408.
Bayot, R., A.K. Watson, and K. Moody.1992. Control of paddy and aquatic weeds by pathogeninPhilippinesIn: IntegratedManagement of Paddy and Aquatic Weeds and Prospects for Biological Control. Food and Fertilizer Technology Center, Taipei, Taiwan.