Introduction

Plant pathology is an extremely large field of study. LINK The goal of the subject is to understand the field well enough to reduce the damage caused by (mostly fungal) pathogens on economically important plants and plant products. The subject of plant pathology can be turned around, by applying the principles of plant pathology to increase the damage being done to economically important weeds.

The basis of the biological control of weeds relies on pathogens attacking the weedy host. The aim of using disease is to poison, or reduce rate of growth, fecundity and general vigour of the host to thereby reduce the weeds competitive position in the crop. In perennials, longevity is also likely to be reduced. Over time, the population of the weed is reduced to a size where it no longer has an economic impact. The pathogen does not eliminate the weed, as that would also eliminate the pathogen.

In fact, the use of fungi to reduce weed populations is extremely difficult. Weedy plants are usually aggressive competitors, growing away from their natural control agents, in an environment that suits their requirements. It is within this severely different environment that we attempt biocontrol of weeds using fungi.

Introduction of Natural Pathogens or Classical Biocontrol

Early attempts to control weeds took pathogens of the plant from the native habitat of both and introduced the pathogen to the isolated weed population in the new environment. The aim was to establish the pathogen on the weed in the new environment. The approach is called the Classical Strategy. In this approach, the pathogen is released, and the population of pathogens slowly increases. The result, if successful, is a subsequent reduction in the population of the weed.

In only a few instances has the classical approach been successful. The most useful fungi have been rusts. This is due to their capacity to form specific associations with their host, they are capable of air-borne dissemination and they can cause epidemics in ideal conditions. Further, many rust fungi tolerate broad environmental variation.

In Australia, the rust fungus Puccinia chondrillina has been successfully established on Skeleton weed (Chondrilla juncea), formerly a major weed of wheat throughout the eastern states. The weed has been reduced to tolerable densities. Interestingly, the initial development of control has been associated with a change in the genetic structure of the remaining plant population, with weed genotypes tolerant of the fungus surviving.

In Australia, release of rust fungi for control of blackberries, Parthenium grass, Mimosa pigra and Rubber vine (Cryptostegia) has also been moderately successful.

Use of natural pathogen populations to reduce populations of weeds has a number of problems, including:

Problems

1. The pathogen must be highly specific and the host highly susceptible.
2. The pathogen must be sustained at levels that enable control of weeds should conditions change to favour the weeds.
3. The pathogen must be able to mutate or change genetically to infect new races of weed.
4. Other control measures should be available (chemicals, insects, mechanical) should the usual variation in environment result in an explosion in the size of the weed population and suppression of the pathogen.
5. Lack of a commercial incentive for organizations to find new biocontrol agents.

This last problem, in particular, has led to the use of other approaches to weed control.

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Inundative Application of Pathogens

In recent decades, the search for fungi that can control weeds when applied in an inundative manner (fungal products are also known as Bioherbicides) has led to several commercially attractive products.

Effective bioherbicides must have:

1. Specificity to the weed and no other plant.
2. Virulence and pathogenicity to kill the weed when it is among its crop companion.
3. Reproduction in artificial conditions, forming stable and long-lived reproductive units.
4. Capacity to be delivered to control the weed.

Note that this process aims to kill the weed, much in the way a chemical herbicide functions. The pathogen does not survive in the crop after application. The control agent is just another commercial product.

In Australia, the fungus Colletotrichum orbiculare has been developed to control Bathurst bur (Xanthium spinosum). The weed forms spiny seed casings that can damage the feet and contaminate wool of sheep. The pathogen was first discovered in the early 1980s. The development of the bioherbicide took over 10 years. One major stumbling block was the close relationship between the weed and an important crop species, sunflower. However, specific races were found, and subsequently used to establish the effectiveness of control.

Spores of the fungus are suspended in vegetable oil, sprayed on young seedlings and considerable damage develops with 2 weeks.

Constraints to greater use of mycoherbicides include:

Biological – genetic variation in host, host resistance to infection, interaction with other microbes on the phylloplane, host range, genetic stability of the fungus.

Environmental – germination requirements, especially temperature and moisture, spore formation and survival.

Commercial – small returns.

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Conclusion

The future development of biocontrol of weeds requires an increase in our fundamental understanding of potential control agents and their interaction with hosts. While the future of biocontrol appears greater in currently underdeveloped countries, social pressures to reduce use of artificial chemicals because of their unintended effects, and costs of production, will increase pressure to seek alternative approaches to chemicals in developed countries.

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References

Brown J.F. & Ogle H.J. (1996) Plant parasitic fungi. Fungi of Australia. Vol 1B, pp 65 – 93.

Butt TM, Jackson CW & Magan N 2001 Fungi as Biocontrol Agents. CABI.

Hohn TM (1997) Fungal Phytotoxins: Biosynthesis and Activity. In: The Mycota V: 129 - 144. Eds GC Carroll & P. Tudzynski, Springer.

Robson GD, van West P & Gadd GM (eds) 2007. Exploitation of Fungi. CUP

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