5.2.8
ADDRESSING DEW PERIOD, THE MAIN CONSTRAINT TO SUCCESSFUL MYCOHERBICIDES

CF McRAE1,2 and BA AULD1,2

1CRC for Weed Management Systems, Australia;
2Orange Agricultural Institute, Forest Road, Orange, NSW 2800, Australia

Background and objectives
The mycoherbicide approach to biological control of weeds has been seriously researched for more than 20 years [1]. The literature is littered with papers investigating the merits of using a multitude of fungi to control many different weed species, yet to date, precious few mycoherbicides have been commercially released. There are many reasons to explain why mycoherbicides have remained largely experimental [2], but the most common and perhaps the over-riding constraint to the development of mycoherbicides is the dew period requirement of fungi. Frequently, moisture is not available or not available for the required dew period after field application of the fungus. This is particularly true in dryland cropping and grazing areas of Australia. Consequently, potential mycoherbicides fail to cause disease and thus fail to control weeds.

The real challenge, then, in mycoherbicide research and development is to overcome the dew period constraint. Formulation of the mycoherbicide can address this problem (and other handling, storage and application problems). With appropriate formulation it may be possible to deliver the fungus to the weed surface with sufficient water and to retain it for sufficient time to satisfy the dew period requirement of the fungus. The type of formulation varies with the method of application. For a soil-applied mycoherbicide, a granular formulation is most suitable, while a liquid-based formulation is the appropriate means of applying foliar mycoherbicides. In 1997, a project began at the NSW Agriculture's Agricultural Institute, Orange, to devise novel formulation systems for the delivery of pathogens as foliar mycoherbicides. The focus of the research is to find ways of reducing evaporation of the water sprayed onto plants at the time of inoculation and/or to find ways to retain that water on the plant surface for the dew period of the fungus under consideration. The model system for research is Colletotrichum orbiculare as the control agent of Xanthium spinosum. The techniques used by industries that deal in water-based products are under review for possible solutions to the problem. Further, the long-chain alcohols octadecanol and hexadecanol are being examined, because they have been shown to reduce evaporation of water under controlled conditions and have been considered as aids to reduce evaporation from farm dams in Australia [3].

Results and discussion
A toxicity register of surfactants and other adjuvants that could possibly be incorporated in a mycoherbicide formulation has been established. The possible formulants are currently being tested against the germination of C. orbiculare spores as well as the spores of other potential mycoherbicide agents under consideration at Orange: Drechslera avenacea, Rhynchosporium alismatis and Alternaria zinniae.

Both octadecanol and hexadecanol were trialled as aids to reduce evaporation from farm dams in Australia, but were finally abandoned mainly because of the large surface area of water and often windy surface conditions. In miniature these problems are less acute, and preliminary droplet studies have shown that crystal spread monolayers of both compounds slowed the evaporation of droplets containing C. orbiculare and Tween 80. Hexadecanol is being considered in plant trials. Mixtures of polymeric gels also show promise as water-retaining substances, particularly mixtures incorporating xanthan gum.

References
1. Templeton GE, Smith JR Jr, 1977. In Horsfall JG, Cowling EB, eds, Plant Disease, Vol 1. Academic Press, pp. 167-176.
2. Auld BA, Morin L, 1995. Weed Technology 9, 638-652.
3. Barnes GT, 1986. Advances in Colloid and Interface Science 25, 89-200.