Application Technology

Delivery and retention of sufficient number of fungal propagules on weeds can be very challenging. Most initial studies on mycoherbicide efficacy spray the inoculum to the weed foliage till runoff using aerosol sprayers. This spray method generally applies excessive volumes (up to 3000 l/ha) that can maximize the retention and exaggerate the potential of biocontrol agents (Greaves et al. 2000). Disregard for appropriate methods of application can contribute to poor or inconsistent field performance (Smith and Bouse 1981). In most field crops, application volumes over 600 l/ha are considered high (Matthews 1992), and the trend is generally toward lower volumes.

Use of high spore concentrations can potentially reduce application volumes without compromising the efficacy of weed control (Peng et al. 2001). Increased propagule number may help improve the efficiency of foliar coverage by reducing the proportion of "empty" droplets (Jones 1998), but also pose high requirements on quantity and quality of the inoculum necessary to achieve the desired level of weed control. Mycelium clumps or other impurities in extremely concentrated formulations can also easily plug up the spray system. Inoculum concentration, carrier volume, and other spray parameters need to be studied jointly to optimize spray results (Jones and Burges 1998). Nordbo et al. (1993) suggested use of fast travel speed to improve spray retention on vertical leaf surfaces. In a recent study, Peng et al. (2001) observed that a finer droplet spectrum combined with more horizontal trajectories enhanced retention efficiency on green foxtail. Richardson (1987) and Spillman (1984) made similar suggestions on application of herbicides. These retention characteristics have also been discussed by Jones (1998) and Reichard (1988), and have been used to explain improved spray results in a number of herbicide studies (Knoche 1994). It needs to be recognized that there are limitations with manipulation of certain spray parameters. For instance, too fine a spray may not be practical in every case depending on the size of mycoherbicide propagules and due to potential spray drift concerns (Jones 1998). Interpretation of droplet size spectra for optimal dose transfer of biopesticides is required, and this will vary with the mycoherbicide agent, particularly in relation to the spore size and morphology (Bateman 1999). Although fine tuning of application parameters can improve spray characteristics, it is more important to determine if these improvements can be translated into meaningful enhancements of weed control efficacy.

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