Fungi used as a Fungicide

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Coniothyrium minitans as an example of mycoparasite
Coniothyrium minitans as an example of mycoparasite

Fungi have the ability to undergo autolysis when then environment they are growing in becomes depleted from the nutrients they need[1]. They are required to digest their own cell wall in order to reach the nutrient contained inside the hyphae. Thus, most if not all fungi are capable of synthesizing lytic enzyme such as chitinase and glucan-1,3-B-glucosidase [2] that will allow this destructive behavior to occurs. Mycofungicides are making use of this ability to destroy nuisible fungal pests using different strategies. Some of the most used species are Trichoderma and Coniothyrium minitans [2]


[edit] Processes

Coiling of Trichoderma spp. around the hyphae of Rhizoctonia solani
Coiling of Trichoderma spp. around the hyphae of Rhizoctonia solani

For a mycofungicide to be useful, the antagonistic fungi needs to be able to parasite an host. Mycoparasitism occurs widely in nature [3] and the two possible interactions that are in use for mycofungicides are the necrotroph and biotroph ones. The attack of a parasitic fungi on its host happens in four stages [2][4].

  1. Chemotropism. The parasitic specie is attracted to its potential host by the detection and reaction to a chemical released by that host. As in many form of taxis, the parasite follows the increase in concentration, the gradient, as it gets closer to the host [4]. This gradient might be made of amino acids or sugar molecules[4].
  2. Recognition. The recognition procedures are triggered by different gene-regulated mechanisms[1]. It is a short period that will determine if the parasite attach to the host, which will decide if the invasion happens or not. In Trichoderma spp. for example, the lectin carbohydrates are known to play a role in this critical stage of adhesion [4].
  3. Attachment and coiling. The parasitic fungi grows an appresoria around or along the host to prepare its invasion[4]. The appressorium structures will provide the turgour pressure needed to penetrate or invaginate the host cells during the penetration stage.
  4. Penetration and digestion. The antagonist fungi uses a variety of lytic enzymes to degrade the cell wall of the host and penetrate it. For example, Tricoderma spp. uses four family of enzymes: B-glucanases, cellulases, chitinases and proteinases to accomplish this step[4].

Once the mycoparasite has infected the host, one of the two main type of relationship will occur: necrotrophy or biotrophy.

[edit] Necrotroph

Necrotroph fungi will kill the host it is invading. The parasite typically starts to feed from the nutrients contained in the hyphae of the host. This will reach a point where the host cannot survive and will die from this relation. This can occur in two different way. The first one is by simple contact where the parasite releases enzymes without fully penetrating the hosts [1]. The second one is said to be invasive and involve a full penetration of the host by the parasite[1]. In this case, the death of the host occurs quicker and involves the destruction of the host hyphae[1].

[edit] Biotroph

Biotrophic relations are more complicated and are not as clear as necrotrophic ones. They require the host's cytoplasm to stay healthy althought it has a very restricted food source[1]. For a biotrophic mycofungicide to be useful, it required a very dominant relation of the parasite on its hosts[1]. The host is almost fully denied access to main nutrient such as nitrogen and phosphorus and its vegetative growth or ability to build reproductive structure is inhibited [5]. This will cause, in return, the prevention of the spread of the disease. There are three type of biotroph: intracellular, haustorial and fusion [1]. Intracellular involve a total penetration of the parasite in the host and require lower fungi parasites like oomycete. The haustorial biotrophs develop hyphae called haustoria that do not fully invade the hosts cells. Finally the fusion biotroph will accomplish anastamosis with the hosts which will allows a share of cytoplasm between the two organisms [1].

[edit] Specific example

example of Botrytis cinerea infection on strawberries, a disease that Trichoderma can fight
example of Botrytis cinerea infection on strawberries, a disease that Trichoderma can fight
[edit] Trichoderma spp.

Trichoderma spp. is the most commonly used fungal biocontrol agent [6]. The fungal disease Botrytis cinerea, also known as grey mould, affects a large number of crops in the world such as tomatoes and grapes. It attacks its host by degrading its cell wall, causing their deaths [6]. Trichoderma harzianum is one of the pathogen of that fungi and follows very closely the stages of infection described above. T. harzianum successfully inhibits the germination of B. cinerea by competing for important nutrients such as iron and by producing destructive protease enzymes [5].

[edit] Preparation and Applications of Mycofungicides

bags of Trichoderma spp ready to be applied on a field as fungicide
bags of Trichoderma spp ready to be applied on a field as fungicide

Mycofungicide preparations involves the production of structure that will be ready do infect an hosts, either spores or mycellium depending on the species. Those structures can be produced using liquid fermentation or solid state fermentation[1].

The application of such fungicide will not be described in detail since it is very similar to the one of any pesticide. Because the pesticide dealt with is a living organisms, the mycofungicide cannot be store for a long period of time [1]. The cycle of infestation takes few days which requires growers to apply the biofungicide in a more preventive way than with traditional chemical pesticides [2]. A new and very interesting method to spray spores have been tried and showed good results on strawberry crops. Those bees were forced into a structure containing the spores of Gliocladium roseum that were then carried across the field [5]. G. roseum is another fungi that have shown ability to prevent the spread of Bothrytis cinerea [5].

[edit] Notes and References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Moore D., Robson G.D., Trinci A.P.J. (2011). 21st Century Guide to Fungi. Cambridge University Press:New York.
  2. 2.0 2.1 2.2 2.3 Boland, G. J., & Kuykendall, L. (1998). Plant-microbe interactions and biological control / edited by Greg J. Boland, L. David Kuykendall. New York : Marcel Dekker, c1998.
  3. Jeffries P. (1995). Biology and Ecology of Mycoparasitism. Canadian Journal of Botany. (73):S1284-S1290
  4. 4.0 4.1 4.2 4.3 4.4 4.5 Steyaert J.M., Ridgway H.J., Elad Y., Stewart A.(2003). Genetic Basis of Mycoparasitism: A mechanism of Biological Control by Species of Trichoderma. New Zealand Journal of Crop and Horticultural Science. (31):281-291.
  5. 5.0 5.1 5.2 5.3 Elad Y., Freeman S.(2002). The Mycota: Agricultural Applications. Esser K, Bennett J.W. & Kempken F.(Eds.), Biological Control of Fungal Plant Pathogens(pp.93-104 ). Berlin: Springer-Verlag.
  6. 6.0 6.1 Cheng C., Yang C., Peng K. (2012). Antagonism of Trichoderma harzianum ETS 323 on Botrytis cinerea Mycelium in Culture Conditions. Phytophatology 102(11):1054-1063.
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