HERBICIDES in Asian rice - IRRI books - International Rice ...

ange need not be excluded as it may have a selective niche for weed control
where other potential nontarget hosts are absent.
2. Obtain pathogens from a climatic region similar to that in which the target
weed is growing. The importance of secondary infection consequent to the
primary application of the biocontrol agent should not be overlooked; understanding
the epidemiology of the pathogen may contribute to increased disease
severity (Yang and TeBeest 1993).
Such an approach to acquiring potentially inundative agents need not be as risky
as may first appear. National and regional centers such as ASEAN PLANTI already
take a unified approach to the introduction of biological control agents, dissemination
of information, and training of personnel (Sastroutomo 1992). Such centers could
facilitate involving the countries of Southeast Asia in an expanded approach to
biocontrol based on sound quarantine practices.
Assessing bioherbicide efficacy
Evaluation of a bioherbicide’s efficacy on a target weed should include studying its
effect at different growth stages of the plant. Measurements should include not only
weed mortality but also plant growth rates, biomass production, and reproductive
potential. Testing should include isolates from different hosts and/or geographic locations,
in order to compare variation within the pathogen; that would increase the chances
of selecting the most virulent isolate. Host range studies conducted with potential
pathogens should be as realistic as possible (Cother 1975) for the plant, the pathogen,
and the environment (Watson 1985), and should challenge taxa that represent the full
range of plant genetic diversity within the area of intended use (Weidemann and TeBeest
1990). Performance under appropriate conditions in the field should be the final determinant
of efficacy (Charudattan 1988). If an inundatively applied inoculum is packaged
in any adjuvant (i.e., nutrient sources, wetting agents, ultraviolet protectants,
etc.), host range assessments should take into consideration that natural dispersal of
inoculum from diseased plants to beyond the ricefield may not pose a threat to other
plant species because of the absence of specific enhanced requirements for infection.
Herbicide industry involvement
For bioherbicides to become a practical reality, the herbicide industry must become
involved in small niche markets (Watson 1992a). These markets, however, are not
economically attractive to many agrochemical companies (Hess 1992). Even while
recognizing the usefulness of bioherbicides in small niche markets, the chemical industry
has narrow, fixed perceptions of the future value of commercial bioherbicides
(Wilson 1990) and, in some cases, unrealistic expectations (Powell 1993). Commercialization
is influenced primarily by the views of the larger chemical pesticide companies.
That relatively few marketable biocontrol agents are available appears to be
due to the absence of a broad industry perspective (Watson and Wymore 1990).
Charudattan (1988) and Watson and Wymore (1990) discussed perceived limitations
to the development of biocontrol agents. Those limitations can be classified as
Bioherbicides and weed management in Asian ricefields 189