**2.3 Biological weed control**

Biological control is the intentional use of biological agents (living organisms) to control plant pathogens or weeds in fields [52, 53]. The application of herbicides for sustaining agricultural production has created so many problems such as contamination of groundwater, destruction to the nontarget species, and induction of resistance against herbicides in a number of weed species [45], and other control methods become even more unsuitable where the land value is small and unaccessible with widespread weed infestations. This situation paved the way of researchers to move toward biological control as an alternative option in weed management. The chemical herbicides can persist in soil for longer periods of time, have limitations for crop rotation, and cause damage to the nontarget organisms [54]. Microbial herbicides on the other hand are more selective and affect only the target species [19]. The other advantage of using microbial agents is the reduced chance of induction of resistance in the target species [20].

application of mass-produced fungal spores or bacterial cultures in higher concentrations with the objective to eradicate invasive weeds in a managed area [57]. The inundative biocontrol is more related to the agricultural needs and turf management because its implementation is similar to the conventional herbicides as liquid sprays and solid granules [58, 59]. A number of microbial herbicide formulations based on

*Application Potentials of Plant Growth Promoting Rhizobacteria and Fungi as an Alternative…*

Rhizosphere is the region of the soil surrounded by plant roots and often extended from the surface of roots [94]. This constituency of the soil is much wealthier in bacteria than the contiguous bulk soil [95]. The plant growth-

promoting rhizobacteria are the soil bacteria that reside in the rhizosphere and are involved in the stimulation of plant growth through direct and indirect methods [96]. Agricultural production currently relies on the judicious use of synthetic fertilizers [97, 98] that have shown negative environmental impacts due to overuse of these chemical fertilizers [99]. Therefore, the use of PGPR inoculants can be considered as an environmentally sound alternative approach for the sustainable management, decreasing the use of synthetic fertilizers [100–102]. Within the context of PGPR research and their modes of actions, there has been an increasing trend in literature to search for the best PGPR candidate in order to commercialize as bio-fertilizer. Plant growth-promoting rhizobacteria are equipped with a plenty of mechanisms that can result in the promotion of plant growth. For instance, Parmar and Dadarwal [103] suggested the involvement of *fluorescent pseudomonads* to promote nodulation process and increased nitrogen fixation in chickpea [104], in another study, confirmed the ability of *Azospirillum* sp. inoculation on some significant agricultural crops in terms of increased dry weights of the root and shoot. Similarly, [105], who suggested that the foliar application of rhizobacteria in apricot and mulberry causes an increase in total surface area and chlorophyll contents as compared to uninoculated control [106], documented the growth response in wheat after the inoculation with rhizobacteria and revealed that the growth and development of wheat largely depends on the nature of PGPR and environmental factors. Spaepen et al. [107] reported that various genera of rhizobacteria use tryptophan as a precursor to produce IAA by different pathways. However, the plant pathogenic bacteria only use the indole acetamide pathway to synthesize IAA that causes tumor formation in plants. Swain et al. [108] suggested that cultures of *Bacillus subtilis* when applied on *Dioscorea rotundata* increased the root/stem ratio and

bacteria and fungi have been registered worldwide (**Table 2**).

number of sprouts as compared to the uninoculated control.

the rhizosphere [111, 112].

**253**

A recent study by Minorsky [109] reported the excellent colonization ability of a PGPR isolate *Pseudomonas fluorescens* (B16) in tomato roots. The positive effects were increased plant height, enhanced flowering, and increased fruit weight. Castro et al. [110] proposed that PGPR stimulates growth and development of crops both by direct and indirect methods. The direct methods of growth promotion may include biological nitrogen fixation, solubilization of mineral phosphorus and iron, production of phytohormones, and synthesis of enzymes and

As for the higher uptake of nutrients that is concerned through application of bacterial inoculants, Qin et al. [113] reported the ability of rhizobacteria to dissolve fixed phosphate is related to the rhizosphere acidification. The rhizobium inoculation in soybean plants causes increased availability of phosphorus as compared to

siderophores. Indirect growth promotion occurs through the production of antibiotics and fungal-degrading enzymes and competition for niche exclusion in

**3. PGPR and stimulation of plant growth**

*DOI: http://dx.doi.org/10.5772/intechopen.86339*

Primarily there are two fields of application within the context of biological weed control, viz., the classical and augmentative or inundative. Classical biological control is the introduction and subsequent discharge of a natural enemy of a pest predator with the objective to reduce its virulence without becoming a pest itself [55]. This method is suitable for the control of perennial weeds that grow over a range of large areas such as in the forests, rangelands, along waterways, and roadsides and where reduction in weed competitiveness is required [56]. Several agents might be used in this strategy such as insects, fungi, mites, and different herbivores. The inundative biological control also called as bioherbicide approach is the


#### **Table 2.**

*Successful microbial herbicides (registered) worldwide.*

*Application Potentials of Plant Growth Promoting Rhizobacteria and Fungi as an Alternative… DOI: http://dx.doi.org/10.5772/intechopen.86339*

application of mass-produced fungal spores or bacterial cultures in higher concentrations with the objective to eradicate invasive weeds in a managed area [57]. The inundative biocontrol is more related to the agricultural needs and turf management because its implementation is similar to the conventional herbicides as liquid sprays and solid granules [58, 59]. A number of microbial herbicide formulations based on bacteria and fungi have been registered worldwide (**Table 2**).

### **3. PGPR and stimulation of plant growth**

**2.3 Biological weed control**

*Sustainable Crop Production*

Biological control is the intentional use of biological agents (living organisms) to control plant pathogens or weeds in fields [52, 53]. The application of herbicides for sustaining agricultural production has created so many problems such as contamination of groundwater, destruction to the nontarget species, and induction of resistance against herbicides in a number of weed species [45], and other control methods become even more unsuitable where the land value is small and

unaccessible with widespread weed infestations. This situation paved the way of researchers to move toward biological control as an alternative option in weed management. The chemical herbicides can persist in soil for longer periods of time, have limitations for crop rotation, and cause damage to the nontarget organisms [54]. Microbial herbicides on the other hand are more selective and affect only the target species [19]. The other advantage of using microbial agents is the reduced

Primarily there are two fields of application within the context of biological weed control, viz., the classical and augmentative or inundative. Classical biological control is the introduction and subsequent discharge of a natural enemy of a pest predator with the objective to reduce its virulence without becoming a pest itself [55]. This method is suitable for the control of perennial weeds that grow over a range of large areas such as in the forests, rangelands, along waterways, and roadsides and where reduction in weed competitiveness is required [56]. Several agents might be used in this strategy such as insects, fungi, mites, and different herbivores.

The inundative biological control also called as bioherbicide approach is the

Casst *Alternaria cassia* Sicklepod, coffee senna [61] Biochon *Chondrostereum purpureum* Woody weeds [62]

*Phoma Phoma macrostoma* Broadleaf weeds [18, 63] Devine *Phytophthora palmivora* Strangle vine [64]

Hakatak *Colletotrichum acutatum Hakea sericea* [17] Myco-tech *Chondrostereum purpureum* Deciduous tree species [66] Smolder *Alternaria destruens* Dodder [67] Dr. Biosedge *Puccinia canaliculata* Yellow nutsedge [68] Lubao *Colletotrichum gloeosporioides* f. sp. Dodder [61] Woad warrior *Puccinia thlaspeos* Dyer's woad [69] Chontrol *Chondrostereum purpureum* Alders and other hard

Sarritor *Sclerotinia minor* Dandelion [70]

*Xanthomonas campestris* pv. Annual bluegrass [65]

woods

**Trade name Microbe(s) involved Target weed(s) Representative/initial**

Round-leaved mallow [60]

Northern joint vetch [61]

**report reference**

[66]

chance of induction of resistance in the target species [20].

BioMal *Colletotrichum gloeosporioides* f. sp.

Collego *Colletotrichum gloeosporioides* f. sp. *aeschynomene*

*Successful microbial herbicides (registered) worldwide.*

*nalvae*

Camperico poae

**Table 2.**

**252**

Rhizosphere is the region of the soil surrounded by plant roots and often extended from the surface of roots [94]. This constituency of the soil is much wealthier in bacteria than the contiguous bulk soil [95]. The plant growthpromoting rhizobacteria are the soil bacteria that reside in the rhizosphere and are involved in the stimulation of plant growth through direct and indirect methods [96]. Agricultural production currently relies on the judicious use of synthetic fertilizers [97, 98] that have shown negative environmental impacts due to overuse of these chemical fertilizers [99]. Therefore, the use of PGPR inoculants can be considered as an environmentally sound alternative approach for the sustainable management, decreasing the use of synthetic fertilizers [100–102]. Within the context of PGPR research and their modes of actions, there has been an increasing trend in literature to search for the best PGPR candidate in order to commercialize as bio-fertilizer. Plant growth-promoting rhizobacteria are equipped with a plenty of mechanisms that can result in the promotion of plant growth. For instance, Parmar and Dadarwal [103] suggested the involvement of *fluorescent pseudomonads* to promote nodulation process and increased nitrogen fixation in chickpea [104], in another study, confirmed the ability of *Azospirillum* sp. inoculation on some significant agricultural crops in terms of increased dry weights of the root and shoot. Similarly, [105], who suggested that the foliar application of rhizobacteria in apricot and mulberry causes an increase in total surface area and chlorophyll contents as compared to uninoculated control [106], documented the growth response in wheat after the inoculation with rhizobacteria and revealed that the growth and development of wheat largely depends on the nature of PGPR and environmental factors.

Spaepen et al. [107] reported that various genera of rhizobacteria use tryptophan as a precursor to produce IAA by different pathways. However, the plant pathogenic bacteria only use the indole acetamide pathway to synthesize IAA that causes tumor formation in plants. Swain et al. [108] suggested that cultures of *Bacillus subtilis* when applied on *Dioscorea rotundata* increased the root/stem ratio and number of sprouts as compared to the uninoculated control.

A recent study by Minorsky [109] reported the excellent colonization ability of a PGPR isolate *Pseudomonas fluorescens* (B16) in tomato roots. The positive effects were increased plant height, enhanced flowering, and increased fruit weight. Castro et al. [110] proposed that PGPR stimulates growth and development of crops both by direct and indirect methods. The direct methods of growth promotion may include biological nitrogen fixation, solubilization of mineral phosphorus and iron, production of phytohormones, and synthesis of enzymes and siderophores. Indirect growth promotion occurs through the production of antibiotics and fungal-degrading enzymes and competition for niche exclusion in the rhizosphere [111, 112].

As for the higher uptake of nutrients that is concerned through application of bacterial inoculants, Qin et al. [113] reported the ability of rhizobacteria to dissolve fixed phosphate is related to the rhizosphere acidification. The rhizobium inoculation in soybean plants causes increased availability of phosphorus as compared to

non-inoculated plants, hence positively influencing plant growth. Ambrosini et al. [114] suggested that sunflower-associated *Burkholderia* strains were found to be solubilizing Ca3(PO4)2, hence availing phosphorus for plant use. The management of soil, plant, and environmental interactions evidenced by boosted crop yields is gaining much attention globally. Moreover, agricultural inoculants (cultures) contain plant beneficial bacteria that help plants to meet the demands for nutrients.
