Tenuazonic acid (TeA), a naturally occurring product of Alternaria alternata, a pathogen to croftonweed, was discovered to be a novel natural photosystem II (PSII) inhibitor. However, herbicidal activity of AAC-toxin, a metabolite of this fungus containing TeA as the main active ingredient, has not been evaluated systematically. In this study, we conducted activity-evaluation experiments in the laboratory, greenhouse, and field trials to assess the herbicidal potential of this fungal metabolite. AAC-toxin had high herbicidal activity on all species tested: croftonweed, large crabgrass, barnyardgrass, redroot pigweed, and eclipta. The AAC-toxin caused brown, leaf spot symptoms and leaf necrosis, subsequently killing the seedlings. When AAC-toxin was applied POST at 83 ml ai/ha, more than 95% of large crabgrass, barnyardgrass, and redroot pigweed plants were controlled 2 d after treatment in field trials. It can be concluded that AAC-toxin has broad-spectrum, rapid, and high herbicidal activity similar to that of paraquat and may have the potential to be developed as a microbe-based herbicide.

Pathogens were not seriously considered as biological control agents for aquatic plants in the United States until the Chesapeake Bay Eurasian watermilfoil decline occurred in the 1960s. The decline and suggestion that it was induced by pathogens spawned interest in the use of pathogens as biological control agents for nuisance aquatic species. In the years that followed, emphasis was placed on finding pathogen agents for some of the most problematic aquatic weeds, including waterhyacinth, Eurasian watermilfoil, and hydrilla. The scientist that has contributed the most to our knowledge of pathogen biological control in aquatic plants has been Dr. Raghavan Charudattan (University of Florida, Gainesville, FL). For the past 40 yr, he has authored or coauthored more than 50 manuscripts devoted to the subject in peer-reviewed journals, books, and proceedings.

When I began my foray into the field of biological control of weeds in 1971, the concept of deliberately using pathogens to control weeds was novel and untested and met with skepticism and resistance. Soon, a worldwide network of plant pathologists, weed scientists, microbial technologists, formulation specialists, and regulatory personnel came together to study, develop, and apply pathogens in safe and effective ways of control of a variety of weeds in crops and natural areas. Several new weed–pathogen systems were studied; a few dozen products and pathogens were brought to use, albeit on a very small scale compared to conventional weed-control products; and along the way, some valuable lessons were learned in phytopathology and weed ecology. A seminal body of information was published on the etiology and epidemiology of several diseases of weeds, many new pathogens were discovered and described, and methods were developed for mass production, formulation, and storage of pathogens. Numerous pathogen-produced herbicidal metabolites were discovered and characterized. Protocols were developed, tested, and applied for safe importation and release of exotic pathogens and for registration of microbial herbicides. Spectacular success was achieved with some pathogens used as classical biocontrol agents, and a new class of herbicide, the bioherbicides, came on the scene. Yet some key opportunities were missed. Notably, weed biocontrol research remained largely preoccupied with agent or product development and deployment while great strides were made during this period in phytopathology to understand the genetic–molecular basis of virulence, host range, host specificity, host response to infection, cell death, and pathogen population structure. Nevertheless, the accomplishments in the field of weed biocontrol by pathogens are truly significant. Certainly, we are poised to apply the knowledge gained toward discovery and development of additional weed-control pathogens, but increased effort should be directed also at using pathogen genes, gene products, and genetic mechanisms for weed control. An investment in the latter could help us gain insights into genetically programmed host–pathogen interactions that may be exploited to kill weeds, restrain weed growth, or knock out traits for invasiveness. In our continuing struggle to manage weeds, biocontrol with pathogens should remain a major thrust. Here I present perceptions I have gained from the work that my students, postdoctoral and technical associates, colleagues, and I have done with several weed–pathogen systems.