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 or vegetation system. In perennial weeds, longevity is also likely to be reduced. Over time, the population of the weed is reduced by the control agent 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.
Early attempts to control weeds took pathogens and insect pests of the weed from the native habitat of both and introduced them to the isolated weed population in the new environment. The aim was to establish the pathogen and pest 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. See below for the note on control of Lantana.
Use of natural pathogen populations to reduce populations of weeds has a number of requirements, including:
This last problem, in particular, has led to the use of other approaches to weed control.
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:
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.
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.
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
Lantana camara L. is a native of central America and has established weedy infestations worldwide, largely through garden escapes. In Australia, the bush invades open woodland and is a particular problem in subtropical and tropical habitats. Part of the problem is associated with the enormous number of biotypes established: some 26 cultivars of Lantana may be found nationwide. On the north coast of NSW, the pink flowering form (pictured above) is the most common type found.
Lantana invades roadsides, forests and open areas where the dense cover of the weed smothers shrubs and trees, and the shade prevents emergence of seedlings. Current attempts to control the weed rely mostly on expensive chemical and management processes.
A series of biocontrol agents have been released to control the weed. Up to 1995, the 26 insects released have had limited effect on Lantana largely because the climate is unfavourable for the insects and the host drops its leaves containing insects in response to dry conditions. Remnants of these releases may still be found: a tunnelling insect is shown. Fungi are now being assessed.
The rust Propodium tuberculatum (Spreg.) Arthur was released in 2001 to control the pink and the red-edged pink flowered forms. The fungus is autoecious and microcyclic. While teliospores are found in the field and may aide overwintering, the fungus spreads via urediniospores. The fungus is adapted to subtropical conditions, and during summer may complete its life cycle within 3 weeks. Germination takes place between 10 and 25C, and 9 – 15 hours dew is necessary for colonisation. Heavy infestations of the fungus lead to premature loss of leaf.
A second rust fungus Puccinia lantanae is hosted by a wider range of cultivars, not including the pink form. It is better adapted to tropical conditions and may be more useful for the control of weed infestations in Qld. The fungus has not yet been released.
Only time will tell whether the pink flowering form of Lantana will succomb to biocontrol agents. On the North Coast of NSW the weed is now attacked by several insects and rust. Thier combined effects may be sufficient to reduce the growth and competitiveness of thw eed to the point where native plants will start to flourish again. Clearly, more work is required to slow the growth of other forms of the weed and these may spread to the North Coast once the pink flowering form is reduced.