Use of Plant Pathogens as Bioherbicides to Manage Weeds in Horticultural Crops

Certain fungal, bacterial, and viral pathogens can be mass-produced and used as biological herbicides to control weeds in crops. This approach, referred to as the “bioherbicide” or the “inundative” biological control strategy, is based on our ability to manipulate certain weed-pathogen systems to cause highly damaging levels of disease epidemics during critical periods of weed interference and by doing so minimize present and future weed impacts in crop fields. Worldwide about 10 bioherbicide products have been developed and used commercially to manage weeds in various crops, including several horticultural crops. In fact, one of the first bioherbicides registered by the EPA was developed for control of a weed in Florida citrus. Currently, we are developing bioherbicides to manage weeds in citrus, vegetables, pastures, and natural areas, targeting pigweeds ( Amaranthus spp.), purple nutsedge ( Cyperus rotundus L.), several invasive grasses, dodder ( Cuscuta spp.), and tropical soda apple ( Solanum viarum Dunal). The following is a brief overview of this topic. The U.S. Environmental Protection Agency (EPA) regards any microbial pathogen offered for public use as a biological weed control agent a biopesticide (i.e., a bioherbicide). Generally, pathogens that can be industrially mass-produced and host-pathogen systems that can be manipulated to create managed epidemics are suited for the bioherbicide approach. Bioherbicides can be integrated into intensively managed horticultural production systems because of their potential to yield quick and reliable levels of weed control. They can be used to manage invasive weeds in natural areas and in situations where nonchemical alternatives to weed control are needed. In addition, bioherbicides based on naturally occurring pathogens that are not genetically modified, should be acceptable in organic agriculture. Although bioherbicides can be used as the sole option for the management of certain weeds in particular situations, typically they have been used as a minor supplement to conventional chemical herbicides. A list of pathogens that have been developed and registered or sanctioned for use as bioherbicides since the early 1980s is given in (Table 1). One of two early bioherbicides registered was DeVine ( Phytophthora palmivora ), which was developed to control strangler vine (aka mildewed vine, Morrenia odorata ) in citrus in Florida. This was followed in the next quarter century by the development and use of several pathogenic fungi and a bacterium to control weeds in horticultural crops, turf, and forestry. These bioherbicides have fulfilled highly specialized, small market needs, and several of them are no longer in use due to economic reasons. Nonetheless, their development represents an important contribution to weed science, plant pathology, and horticultural sciences. I predict that several additional bioherbicides and new breakthroughs in bioherbicide technology are likely to follow. Some Prospective Bioherbicides under Development in Florida Bioherbicide research is being carried out at three locations in Florida: the University of Florida/IFAS, Gainesville; the USDA-ARS-U.S. Horticultural Research Laboratory, Fort Pierce; and to a limited extent at the USDA-ARS-Invasive Plants Research Laboratory, Fort Lauderdale (where the principal focus is on classical biological control agents). Working as a team, scientists at these locations are attempting to develop bioherbicides and application technologies for five major weeds or weed complexes, as described below. Phomopsis amaranthicola Rosskopf et al., a broad-Spectrum Bioherbicide for Amaranthus spp. A fungal pathogen, Phomopsis amaranthicola, was discovered in Gainesville and described as a new species pathogenic to Amaranthus spp. (Rosskopf, 1997; Rosskopf et al., 2000a, b) and was shown to have potential for use as a bioherbicide (Rosskopf et al., 2000a). The fungus is pathogenic only to plants in the genus Amaranthus (Rosskopf et al., 2005) and it could be grown on common laboratory media such as V8juice agar, tomato-paste agar, potato-dextrose agar, and several inexpensive substrates (DeValerio et al., unpublished). Phomopsis amaranthicola initially causes leaf spots that promote extensive leaf abscission. Subsequently, stem lesions form that ring the stem and cause shoot death (Fig. 1). Symptoms appear in about one week after the fungus is sprayed over susceptible Amaranthus plants and the disease progresses steadily in the following weeks. The fungus sporulates profusely on infected plants, which promotes the development rapid secondary disease cycles and intra-field disease spread. Given its wide host range to several weedy Amaranthus spp., P. amaranthicola could be developed as broad-spectrum bioherbicide for pigweeds of which many are problematic weeds. Morales-Payan et al. (2002, 2003b) have shown that one or two early applications of P. amaranthicola made 10 and 20 d after weed emergence (DAE) can significantly reduce interference of Amaranthus lividus and A. dubius with bell pepper, Caribbean-bonnet pepper, and eggplant and significantly improve crop yields. Wyss et al. (2004) and Morales-Payan et al. (2003c, 2005) have screened chemical pesticides, surfactants, and adjuvants for their inhibitory activity with P. amaranthicola and determined that it is possible to integrate the use of this bioherbicide with crop-protection chemicals. Collectively This research was supported by the Florida Agricultural Experiment Station and grants from USDA-CSREES-TSTAR (2004-34135-14666 and P0016615), Florida Department of Agriculture and Consumer Services (008152), USDA-APHIS Coop. Agreement, and USDA-CSREES-IR-4