Plant Pathogenic Rust Fungi

Future Research

Genetic Modification could be used in a number of useful ways to combat rust attack. There is evidence (Carlota Vaz Patto et al, 2001) that leaf morphology can greatly affect the incidence of rust fungi. If the correct genes could be isolated for resistance and a commercial plant could be engineered for variations in rust-resistant leaf morphology this could be a very effective way of overcoming the rust problem in many crops.

The various plant protection methods suggested all have their uses and drawbacks. Insect exclusion could be very effective, but generally it would have to be used in conjunction with other forms of protection in order to be a complete solution. This is a problem because every treatment method utilised greatly increases the cost of crop production. With sometimes vast areas of land having to be cleared of alternate host species this has the potential to be both impractical and prohibitively expensive as a crop protection method.

One of the more interesting methods of plant protection included in this study is the exclusion of insect reproductive gamete vectors. Although the suggested method of caging individual plants to completely prevent insect attack would be very expensive to carry out on a large scale farm, there is another option that is yet to be studied. If specific insects are found to be propagating the rust present on a commercial crop, a narrow spectrum insecticide directed only at the insect concerned. This would be a far more cost-effective as only two chemicals would have to be applied, the narrow-spectrum insecticide and a broad spectrum fungicide. It would also be very easy to implement once the insect vector was known and a suitable insecticide had been developed. Another huge benefit of this kind of treatment would be that it would slow down the rate of mutation of the rust fungus. Mutations in the fungal genotype can sometimes make rust fungi resistant to fungicides and there is a constant race to develop new fungicides to keep up with fungal mutations. If sexual reproduction is reduced by cutting out insect gamete vectors then the potential for gene exchange amongst rust fungi is greatly reduces, as the fungi would be forced to resort to asexual reproduction. Asexual reproduction would lead to a greater proportion of cloned fungal cells as opposed to sexually variated cells. This lack of variation could reduce the virulence of the fungus. This thinking can be backed up by referencing Groth and Roelfs (1982). Their experiment on type 1 and type 2 rusts showed that the asexually reproducing type 2 rusts were less hardy and did not produce as even a coverage of the plate. The sexually reproducing type 1 rusts were more resistant to environmental fluctiations and were therefore able to produce a more uniform spread. This showed improved hardiness and better survival rates. Therefore in order to reduce the hardiness and therefore virulence of rusts the sexual reproduction has to be somehow hindered. In the case of rusts where an insect vector is involved with the reproductive cycle, removal of the insect vector would do just that.

The production of chemical agents to inhibit proper appressoria differentiation (or to reduce the chances of the germ tube differentiating into an appressorium at a suitable location on the leaf for entry to the plant to be gained) is an important way of making fungicides more effective. There is little known about what the chemical causes of germ tube differentiation into appressoria, so more research into this area could have great benefits for novel fungicide. Selectively breeding for new plant morphologies that are resistant to rust fungi is a very safe and potentially useful method of conferring resistance to rust in new varieties of crops. Different crop types with differing morphological attributes could be produced to be resistant to crops in very different environmental conditions, allowing the farmer or grower to select the specific cultivar of crop that would be the most efficient against rust disease given the nature of the local environmental conditions and how susceptible the crop will be for rust infection. This could also be achieved more quickly using genetic modification technology, although the risks involved with this method of novel crop production are higher, and following development careful farm-scale pilot studies would have to be implemented in order to test the potential for cross-pollination with native species. This would also not be a viable solution for the many countries that oppose GM crop production and use. Leaf ridge height is a very important characteristic of the crop in relation to fungal resistance. Most rusts can sense the surface topography of the leaf and use this to navigate the leaf’s surface to find a suitable entry/infection point. It has been found that some accessions of plant species are more resistant to fungal attack due to a variation from the norm in their leaf ridge heights. A concentrated selection programme or genetic modification effort to produce plants with unusually low or unusually high leaf ridges could greatly increase resistance to attack, by blocking the normal development of appressoria.

Isolation of the fungal gene that causes self-inhibition could lead to new ways of resisting fungal attack in susceptible crops. The observation that the plant may be producing a chemical in response to the self inhibition compound in the fungus is interesting as it could lead to the possibility of introducing the gene for encoding the fungal compound into plants. If a plant can produce the fungal inhibitory chemical it would reduce the rate of fungal attack. It would behave as if the plant had already been infected with a rust, because the plant would be producing the raised fungal self-inhibitory factor before such an infection even occurred. In testing it was found that 73% of healthy leaves were susceptible to new rust infections, whereas only 13% of previously infected leaves were able to be infected. This 60% reduction in fungal incidence is very significant, and if this rate of resistance could be induced in the plant without the need for initial fungal infection it would be very beneficial for crop production in the future.

Biological control of weeds using rust fungi is an important commercial use, because of its specificity. Each rust fungus is highly specific to usually one species, so the potential for unintentional targeting of non-pest plants is greatly reduced. However care should be taken when introducing these organisms into new populations, as they have the potential to spread rapidly under the correct conditions. Also if the biological control spread too quickly or too far too suddenly, the adverse affect on wildlife from the loss of a considerable amount of ‘weed’ vegetation could be great. It is important to remember that many organisms use pest plants as habitats, so the use of rust fungi as a biocontrol could greatly reduce biodiversity.

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