MICROBIAL SPECIES USED FOR THE BIOLOGIC CONTROL OF PHYTOPATHOGENS

Microbial species used for the biologic control of phitopathogens

Introduction

Plant pest and diseases has increased dramatically in recent years. Each year different types of disease include bacterial, fungal and viral pathogen cause huge loss in agricultural production. Due to the agricultural intensification, climate change, trade and globalization the resilience in production system has been reduced so, plants crops are more able to get infected and losses are higher. The plant pathogens are an important threat to food production and caused some high impact crop epidemics in the past. For example: the Irish potato famine 1946-48, the great Bengal famine 1943, the southern corn leaf blight 1970, the U.S. citrus greening epidemic 2005… 

Nowadays a wide range of chemical pesticides and fertilizers are available to cut down the infection and increase the productivity in plant crops. However, the conventional use of this chemicals is not safe for the human health and as well for environment considering that affects adversely the natural microflora, texture and productivity of soil. Also, other measures have been used to manage the plant disease like creation of resistant varieties but does not exist a resistance against all the diseases. Therefore, the demand of bio-control agent is increasing rapidly, as natural method doesn’t require the use of chemicals so, pose no threat to human health, crop production or to the beneficial organism of the soil. Also, are economically efficient and sustainable.

As shown in figure 1, 16% of the loss in global food production is caused by microbial diseases, but only 20% of it it’s not caused by fungi organisms. 

Figure 1. Loss of global production causes

How does the pathogen attack the plant?

Illustration 1. Examples of pathogen damage in plants

Microorganisms can be beneficial to plants when they are associated, but they can also be harmful. They cause several diseases in plants that lead to great losses of global food production. To attack the plant, microorganisms need to get inside their host to infect it. To accomplish this, microorganisms have different mechanisms. Once inside the plant, microorganisms have to neutralize defence reactions from the plant. 

Plant pathogens have develop many adaptations to let them invade the plant, overcoming defence mechanisms from it so they can colonize plant tissues and inhibit plant growth and reproduction. This mechanisms are:

Chemical weapons

In general, chemical weapons such as enzymes or toxins disintegrate the structural components of host cells or affect components of its membranes and the protoplast interfering with its functioning Systems or with the permeability of its membrane and with its functions. Other chemical weapons such as growth regulators have a hormonal effect on the cells and modify their ability to divide and enlarge.

Mechanical forces

Mechanical forces are less used than chemical weapons by bacteria, they are used more by fungi and virus, although last ones are introduced directly through plant cells by insects so it’s not a mechanical force as such. Fungi establish contact with the plant surface developing bulbs-like structures called appressorium, where hypha penetrates the plant through the cuticle and cell wall.

Illustration 2. Apressorium of a fungus invading a plant

Biocontrol agents and how do they act in front of the pathogen

Hyperparasitism 

-          The biocontrol agent attacks directly the pathogen and kills it or it kills its propagules.

• Obligate bacterial pathogens
• Hypoviruses
• Facultative parasites
• Predators 

Detoxification of Virulence Factors

Some biological agents produce proteins that connect with pathogen toxin and decrease the virulence potential of the pathogenic toxin.

Induction of Systemic Resistance

Exist two types of plant resistance the systemic acquire resistance (SAR) induced by pathogens and the induced systemic resistance (ISR) induced by nonpathogenic bacteria. After a pathogen infection a compound called salicylic acid is produced, this compound cause the expression of proteins related with the pathogenesis that cause the lysis of invading cells. Biocontrol agents increase the accumulation of this proteins causing the death of the pathogenic cells.

Antibiotics

Some microorganisms can produce antibiotics that can poison or kill other microorganisms in low concentrations. But, to be effective the antibiotics must be produced near the pathogen.

Lytic enzymes 

Some microorganisms produce extracellular hydrolytic enzymes that attack chitin and 3-glucan, that are the major constituents of the fungal cell walls. This degradation causes the death of the pathogen.

Iron Chelating Siderophores

Iron is an essential growth element for all the organisms so, this creates aggressive competition. Biocontrol agents can decrease the availability of different substance of the soil with the purpose to limit the growth of pathogens. Biocontrol agents produces low-molecular-weight compounds called siderophores that have high affinities for iron in comparation to fungal siderophores. Therefore, the biocontrol agents get the iron from the other microorganisms preventing their growth. Pseudomonas fluorescens is one of the bacteria that produces this siderophores.

Figure 2. Biocontrol agent ways to attack the pathogen

Examples of biocontrol agents in cereals

	Disease	Pathogen	Possible biocontrol agents	Mechanism of action of biocontrol agents
Rice	Blast	Pyricularia oryzae	P. fluorescens Trichoderma spp.	Iron Chelating Siderophores Lytic activity
	Bunt	Neovossia indica	T. harzianum, T. viride, T. virens, T. deliquescens	Lytic activity
	Bacterial leaf blight	Xanthomonas oryzae	Bacillus spp	Antibiotic activity
	Root rot	S. rolfsii	T. harzianum
	Take-all	F. oxysporum	T. harzianum	Lytic activity
Maize	Charcoal rot	Macrophomina phaseolina	Trichoderma spp.	Lytic activity
	Blight	R. solani	Trichoderma spp.	Lytic activity
Sorghum	Charcoal rot	M. phaseolina	A. niger AN27

Examples of biocontrol agents in oil seed crops

	Disease	Pathogen	Possible biocontrol agents
Mustard	Damping off	Pythium aphanidermatum	T. harzianum, T. viride	Antibiotic activity, Lytic activity
Seasamum	Blight	Phytophthora sp.	T. harzianum, T. viride	Lytic activity
	Wilt	F. oxysporum f. sp. sesami	A. niger AN27
	Root rot	M. phasolina	Trichoderma sp., Gliocladium sp., B. subtilis	Antibiotic activity, Lytic activity
Sunflower	Blight	Alternaria helianthii	T. virens
	Root/collar rot	S. rolfsii, R. solani	T. harzianum, T. hamatum	Antibiotic activity, Lytic activity

Examples of biocontrol agents in vegetables

	Disease	Pathogen	Possible biocontrol agents
Cauliflower	Damping off	Rhizoctonia solani, P. aphanidermatum	T. harzianum A. niger AN27	Antibiotic activity, Lytic activity
	Stalk rot	S. sclerotiorum	A. niger AN27
Cucumber	Seedling diseases	Phytophthora or Pythium sp	T. harzianum	Antibiotic activity
Potato	Black-scurf	R. solani	T. viride, T. viride, B. subtilis	Antibiotic activity
	Charcoal rot	M. phaseolina	A. niger
	Late blight	P. infestans	Trichoderma sp.	Lytic activity
Tomato	Damping off and wilt	F. oxysporum, B. cinerea	T. harzianum P. fluorescens	Iron Chelating Siderophores
	Root Knot-Meloidogyne	M. javanica	T. harzianum

Examples of biocontrol agents in pulses

	Disease	Pathogen	Possible biocontrol agents
Chickpea	Wilt	F. oxysporum	T. Viride, t. Harzianum, t. Virens, b. Subtilis A. Niger an27	Lytic activity
	Root rot	Rhizoctonia solani/ M. phaseolina	T. Viride, t. Harzianum	Antibiotic activity
	Collar rot	Sclerotium rolfsii	T. Viride, t. Harzianum, p. Fluorescens	Iron Chelating Siderophores, Lytic activity
Cowpea	Wilt	F. oxysporum f. sp. ciceris	T. Viride
	Charcoal rot and wilt	M. phaseolina, F. oxysporum f. sp. tracheiphilum	T. Viride, t. Harzianum, t. Koningii, T. Pseudokoningii	Lytic activity, Antibiotic activity
Soybean	Dry root rot	M. phaseolina	T. Viride, t. Harzianum

Why to use microbial species for biologic control of phytopathogens?

Biological control is nothing but ecological management of community of organisms. It involves using disease-suppressive microorganisms to improve plant health. Disease suppression by use of biological agent is the sustained manifestation of interactions among the plant (host), the pathogen, the biocontrol agent (antagonist), the microbial community on and around the plant and the physical environment. Some of the advantages of using this biocontrol process are:

• They are cheaper, efficient and economically
• They give protection to the crop. 
• They are highly efficacious against explicit plant diseases. 
• Its application is safer to the atmosphere and the person who applies them. 
• They don’t affect to the plant toxicity. 
• They can be expanded easily in the soil and leave no residual difficulties. 
• Biocontrol agents intensify the root and plant growth by way of helping the beneficial soil microflora. It helps also in the volatilization and sequestration of certain inorganic nutrients. 
• They are very easy to manage and apply to the target. 
• They can be combined with biofertilizers. 
• They are easy to manufacture.

Conclusions

Biocontrol agents are used nowadays as an alternative pesticide because they have the same effect as usual pesticide but the damage they cause to the environtment is less. They are also cheaper, more efficient and easier to manufacture. 

The benefits of using biocontrol agents as natural pesticides also includes that they aren't toxic for the plant, they don't accumulate inside it, they just eliminate the pathogen from the plant. 

Bibiliography

Shaowu Meng, Trudy Torto-Alalibo, Marcus C Chibucos, Brett M Tyler and Ralph A Dean (2009) BMC Microbiology20099 (Suppl 1) https://doi.org/10.1186/1471-2180-9-S1-S7

(PDF) Biological Control of Plant Diseases. Available from: https://www.researchgate.net/publication/233119112_Biological_Control_of_Plant_Diseases [accessed Dec 09 2018].

Moore, D., Robson D. G., Trinci, (2018). 21st Century Guidebook to Fungi. Recuperat de http://www.davidmoore.org.uk/21st_Century_Guidebook_to_Fungi_PLATINUM/Ch14_12.htm

Brzezinska, M., Jankiewicz, U., Burkowska, A., Walcsak, M. (2013) Chitimolytic Microorganisms and Their Possible Application in Environmental Proterction. 10.1007 / s00284-013-0440-4