Foliar Pathogens in Weed Biocontrol:
Ecological and Regulatory Constraints
Roberte M.D. Makowski
On a global scale, over half of all pesticides used are for weed control. In the United States, herbicides accounted for 65 percent of domestic pesticide sales in 1990 ( Standard & Poor 1991). It is a formidable task for biologicals to displace or complement some of this market in an integrated approach for weed control, even with the increasing public concern over the use of pesticides. The use of pathogens as biocontrol agents for weed control is relatively new in comparison to their use in insect control. Yet with little over two decades of research in this area, many advances have been made in both the classical and inundative strategies ( TeBeest 1991). There are now three commercially available mycoherbicides in North America ( Templeton 1982b; Bowers 1986; Kenney 1986; Makowski and Mortensen 1992) and approximately five or six others in practical use worldwide ( Charudattan 1991). In addition, eight pathogens are in use worldwide as classical control agents ( Watson 1991).
One of the first and most noteworthy examples of the classical approach was the successful introduction of Puccinia chondrillina Bubak & Syd. into Australia in 1971 for the control of rush skeletonweed (Chondrilla juncea L.) ( Hasan 1972; 1981; Hasan and Wapshere 1973). It was also the first exotic plant pathogen intentionally introduced into North America in 1976 ( Supkoff et al. 1988). Bioherbicide research was initiated in the United States in the late 1960s and early 1970s ( Daniel et al. 1973), and in Canada in the mid-1970s ( Watson 1975), with practical use not occurring until a decade later. Two bioherbicides have been registered for use in the United States: DeVine in 1981, consisting of chlamydospores of Phytophthora palmivora (Butler) Butler for control of strangler vine (Morrenia odorata Lindl.) in citrus groves ( Burnett et al. 1974; Ridings et al. 1976; Kenney 1986; Ridings 1986); and Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. f. sp. aeschynomene in 1982 as Collego for control of northern jointvetch (Aeschynomene virginica (L.) B.S.P.) in rice and soybean ( Daniel et al. 1973; Templeton 1982a; TeBeest and Templeton 1985; Bowers 1986; Smith