The control of weeds through fungi; principles and prospects

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Abstract
The biocontrol of weeds by the introduction of exotic fungi (the classical method) has proved successful, most notably in the control of skeleton weed (Chondrilla juncea) in the wheat belt of Australia by the introduced rust, Puccinia chondrillina. There is now increasing interest in an alternative, inundative, method of weed control in which the target is controlled by a massive inoculation with a host‐specific indigenous fungal pathogen. It is proposed that through an increased understanding of host ‐ parasite interactions, in particular the enzymes and toxins that fungi use to disrupt the structure and function of weeds, screening of naturally occurring pathogens can be made more efficient. It should be possible in the immediate future to improve the efficacy of many host‐specific pathogens that currently cause little injury. In the longer term, changes in host specificity may be achieved through the techniques of genetic engineering. Meanwhile, improved formulation and methods of application can improve the range and reliability of mycoherbicides. The use of integrated control methods in which fungi are used in pairs, or in combination with either reduced rates of chemical herbicides or with arthropods, also offers hope for development. Studies of competition between plants of different species show that stunting of the weed, rather than complete kill, may be sufficient to protect crop growth. Two mycoherbicides have been successfully used in the field for almost ten years; in the next ten years we may expect to see a rapid increase in their number.