Fungi used as a Herbicide

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[edit] Introduction

This image shows different species The presence of weeds can result in the loss of up to half of a grower’s expected crop yield. The lost yield results in economic losses, which is a problem in the agriculture industry. Fungi can be used as an herbicide to gain control of weeds and solve this issue. Fungal herbicides have been legal in Canada since 1973 [1]. In order to use a fungus for this purpose, a mycoherbicide is created with the fungi as its active ingredient. This mycoherbicide must be able to inhibit weed growth without harming the crop or the living things surrounding it. A benefit is that the fungi used are host specific and will reside on the soil or weed host, while a commercial chemical herbicide would need to be reapplied to the crop. Certain mycoherbicides work better on certain strains of weeds because of the resistance of the plant in combatant with the virulence factors of the pathogen. There are currently 37 mycoherbicides and 8 techniques of applying them [1]. The fungi species most commonly used as herbicides in North America are Collectrichum gloeosporioides (Collego®) and Phytophthora palmivoraa (De Vine®)[1]. There are different ways for the pathogen to infect the host; two main ways that will be covered are spore germination causing infection as well as phytotoxins[1].

[edit] Virulence Factors

Virulence factors are characteristics of organisms that allow organisms to be more or less infectious. These factors can include spectrum of host, dispersal of spores, adhesive abilities, and other evolutionary advances that give it an edge on infecting hosts. In terms of organisms, the higher the virulence factors then the more likely the organism will infect more hosts and be more effective at doing so. This is especially true for the class of fungi called Rusts [2]. The reasons that Rusts have a high virulence factor is because of the five different types of spores present throughout its life cycle, the long distance dispersal of those spores, and the adhesive abilities and the broad species of hosts[2]. This allows Rusts to be very virulent as a mycoherbicide because there are many plants that are susceptible to infection from Rusts. Some common rusts that seem to have come up within research are Puccinia chondrillina, Puccinia carduorum , Uromycladium tepperianum and Puccinia romagnoliana . Virulence factors can be considered two of the sides of the disease triangle. The two sides were pathogen and host infected.

[edit] Process

Mycoherbicides release phytopathogens once they are sprayed onto the weed to suppress growth. The phytopathogens release phytotoxins that can kill the weed plants in up to five weeks [1]. The pathogens released have the ability to produce stable and temperature tolerant spores for reproduction. Fungal herbicides are easily made once the anti-herb properties have been testing as well as the living conditions for that fungus. Once the mycoherbicide is applied, the spores will germinate and penetrate the herb tissue and the phytotoxins will start to work. However, there are reasons fungal herbicides may not be used in place of a harmful chemical. In North America, fungal herbicides have to be registered and approved by the Environmental Protection Agency (EPA), which can take years [3]. The anti-herb capacity of fungi is generally less than that of a chemical herbicide. Fungal herbicides are not commonly used because their success rate depends on the environmental conditions and because they are so host specific.

Other ways that a mycoherbicide can infect its host are through physical infection via the germination of a spore. This process happens when a spore lands on a leaf of a plant that it can infect. A germ tube is used to infect the upper layers of the leaf, and then an appressoria is created. This appressoria is the place of infection where the spore then can stay adhered to the leaf. The infection then starts from there, and goes outwards from there [2]. Other ways of infection is that the fungi covers the stoma with the appressoria, leaving the stoma lodged open for more infections to become present [4]. When examining the intricate mechanism of infection, it is evident that this method is extremely complicated and from the perspective of the pathogen, requiring many moving parts.

[edit] Phytotoxins

Phytotoxins are low molecular weight secondary metabolites that cause plant disease. In this case, the plant being infected is a weed plant. Phytotoxins may also be responsible for visual symptoms of disease such as wilting or necrosis. A common phytotoxin found in the fungi species Penicillium is vulculic acid [3]. Vulculic acid works against weeds by increasing leaf membrane permeability, inducing lipid peroxidation and damaging the cell membrane [3]. Phytotoxins have a limited stable state because of their short half-life and sensitivity to the environment. Phytotoxins are difficult to produce for commercial sale because they are produced in very small quantities by fungi and difficult to isolate [3].

[edit] Plant Resistance

Plants can resist fungi through many ways, based upon the qualities that the plants have. This is related to one of the sides of the disease triangle, the host side. If the host has the ability to reject or break down the invading process than an infection will not persist. The genome of the plant, allows for only a small amount of pathogens to infect them, therefore infection has to be particular leaving herbicides to be a little less complicated. There are three ways that plants inhibit the infection of pathogens [2]:

  1. The pathogen is completely out of the host range, and it has no affect on the plants because of the two different genomes
  2. The plant has specific resistance genes to that of the pathogen
  3. The plant can tolerate or escape the infection itself

Other ways that plants can protect themselves from infection is through their own use of protective methods. Examples of one of these methods include the use of toxic chemicals or antifungal chemicals that break down infected cells, riding the plant of the pathogen [2].

[edit] Arrowhead

Arrowhead Weed
Arrowhead Weed

A specific weed strain commonly known as arrowhead (Sagitaria trifolia) is the cause of the largest rice plantation problem in Iran [5]. Arrowhead is notably resistant to chemical herbicides. Collego® is used in the USA as the solution to arrowhead [5]. The fungi Alternaria pellucida was tested and proved that it could successfully be used as a mycoherbicide against arrowhead in Iran., effectively preventing early growth of the weed. The fungi species was tested for its anti-herb properties on three indigenous cultivars of rice, two bred cultivars and lastly on an arrowhead infested cultivar, all cultivated separately in a greenhouse [5]. Alternaria pellucida significantly altered the height of the arrowhead weed and was concluded to be a mycoherbicide able to control the arrowhead weed [5].

[edit] Puccinia romagrolia Marie & Sacc.

This species of Puccinia is the bioherbicide that infects and kills the species Cyperus rotundus, which is the species that creates Purple nutsedge. This weed is found everywhere in agricultural sites, however a main concern is its impact on the growth and production of the mulberry plant. The mulberry plant is used for many reasons, one such example being the leaves of the mulberry plant are fed to silkworms which create other items of value in today's society [4]. Purple nutsedge is effective in inhibiting the growth of the mulberry plant because the mulberry plant has shallow roots, as well as purple nutsedge is able to infect deep into the ground and create tubers [4]. P. romagnoliana when inoculated on the purple nutsedge it caused necrotic areas on the specimen. When looking at the process by which this specimen is infected, it is evident that the necrotic area is created by spores that infect the leaves through a germ tube[4]. The germ tube is expelled because of the change in chemicals on the leaves, then an appressoria is developed. The appressoria then spreads over the stoma, which then forces the stoma open, causing more infection of the plant [4]. When looking at the overall experiment, the intended weed was completely infected and destroyed, however the mulberry plant was not touched, proving to be an effective bioherbicide.

[edit] Notes and References

  1. 1.0 1.1 1.2 1.3 1.4 Misra, H.P. Weed Management Through Fungal Herbicides. (2005) Orissa Review. 53-56
  2. 2.0 2.1 2.2 2.3 2.4 Barton,J. How good are we at prediction the field host-range of fungal pathogens used for classical biological control of weeds? (2004) Biological control. 99-122
  3. 3.0 3.1 3.2 3.3 Duan, G.F., Yu, L.Q., Zhang, J.P. and Zhou, Y.J. Fungal phytotoxins for weed management. (2011) International Allelopathy Foundation. 27(1): 1-14
  4. 4.0 4.1 4.2 4.3 4.4 Gupta, V.,Kumar, V.,Mishra,R.,Thiagarajan, V.,Datta,R.Puccinia romagnoliana Marie & Sacc.- a potential bioherbicide agent for biocontrol of purple nutsedge (Cyperus rotundus L.) in Mulberry (2002) J Phytopathy(150) 263-270.
  5. 5.0 5.1 5.2 5.3 Javadzadeh, Armin and Motlagh, M. Study of Alternaria pellucida as a promising mycoherbicide for controlling Arrowhead (Sagitaria trifolia) in paddy fields (2010) POJ 3(6):172-176
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