and Weed Biocontrol
Ann C. Kennedy
Public concern over surface water, groundwater, and food contamination has resulted in stricter pesticide regulations, fewer approved pesticides reaching the market, and the withdrawal of previously registered pesticides. As a result, growers are being forced to reduce the use of synthetic chemical pesticides and to rely on cultural and biological methods to control pests. Biological control of weeds is based on the premise that biotic factors have a significant influence on the competitive abilities of plant species. Phytotoxic effects of microorganisms are often plant species and cultivar specific. Microorganisms that selectively suppress weed species may alter competition among plants. These plant pathogens potentially may be used to regulate the growth of unwanted plant species growing simultaneously with more desirable plants ( Templeton 1982). This would be true especially if competitive weed growth coincided with environmental factors conducive to microbial growth and weed-suppressive activity. Biological control offers alternative means of suppressing weed growth and establishment in agricultural and range systems. Before biological control methods can be fully integrated into weed management systems, however, we need a greater understanding of the processes of host recognition, specificity, and colonization. Awareness of the ecological constraints of the pathogen and the weed is critical to successful biological weed control.
Most of the research on microbial control of weeds has concentrated on fungal plant pathogens for broadleaf weed control. Most notable is the use of rusts (Puccinia jaceae Otth.) for the control of diffuse knapweed (Centuria diffusa Lam.) ( Mortensen 1986; Watson and Clement 1986), and skeleton weed (Chondrilla juncea) control with the use of Puccinia chondrillina ( Cullen et al. 1973). Mycoherbicides, such as the fungal pathogens of weeds sold under the trade names of DeVine and Collego, are commercially available. DeVine is being used to control stranglervine (Morrenia odorata) in citrus ( Ridings 1986), and Collego (Colletotrichum gloeosporioides f. sp. aeschvnomene) ( Daniel et al. 1973) is used for the