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

**Microbe(s)**

**256**

*Pseudomonas*

*Fusarium tricinctum*

*Trichoderma*

*Colletotrichum*

*malvae*

*Fusarium solani* f. sp.

*Nectria ditissima* Multiple isolates were screened

belonging to

*Xanthomonas* spp. *Sclerotinia sclerotiorum*

*Pseudomonas*

*Pseudomonas*

Collection of multiple

*Pseudomonas*

 *syringae* st. 1 and st. 2

rhizobacteria

 Leafy spurge

*Polypogon*  *Convolvulus*

*Phalaris paradoxa*

 *arvensis*, and

*monspeliensis*,

Laboratory

ND

> and field

Axenic

 Phytotoxin

 synthesis

 *fluorescens* and *P. putida*

 *Striga hermonthica* (Del.)

Benth.

 *putida*

Garden asparagus

Pot Pot

ND

Succinic acid and lactic acid production

Dandelion

Field

 Necrosis and

discoloration

80.7% reduction in number of dandelion

[86]

[87]

[88]

 [89] [90]

plants and overall weight reductions

 ND Significant reduction of weeds and

improved biomass of maize

30% reduction in leafy spurge growth Reduction in biomass up to 47.5%, 22.8%,

and 51.3%. Inhibited 40%, 32.6%, and

46.4% of biomass over control in field

conditions

*Pseudomonas* spp. and

Texas gourd

Red alder Jointed goat grass

Field

Field

Axenic and

ND

field

 Infection

 ND

*gloeosporioides*

 f. sp.

Round-leaved

 mallow

 Greenhouse

 ND

 *virens*

Several weeds

Field

 Rhizosphere

production

viridiol

 of herbicidal compound

 competence

 and

 *syringae* pv.

*phaseolicola*

Kudzu Dodder

 **involved**

**Target weed(s)**

 **Growth**

**Mechanism(s)**

**Observed** 

**effects/comments**

 **References**

[80]

[70]

*Sustainable Crop Production*

**condition(s)**

Greenhouse

Field

 ND

 ND

ND Effectively controlled dodder at

preemergence

application

Reduced emergence and seedling growth of

[81]

different weeds up to a significant extent

Significant biomass reduction, reduced

[82]

fresh and dry weight, and inhibited root

growth

Greater than 78% mortality, reduced vigor [83]

ND Inhibition of weeds by 71% in growth chamber and by 20–74% in different field

conditions

[84]

[85]

 and

postemergence

revealed the production of photobleaching of macrocidins [138] that do not have any inhibitory effects on monocot plants [18]. Despite this macrocidins an anthraquinone pigment in *P. macrostoma* has shown prominent herbicidal effects on some

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

*chenopodicola* that is studied widely for its potential against common lamb's quarter [62]. The mechanism behind its virulence against lamb's quarter is the production of diterpene and chenopodolin, a phytotoxic compound isolated from this species [62]. Two species within the genus *Sclerotinia* have been investigated for their herbicidal activity. It is evidenced by the work of Abu-Dieyeh and Watson [140] that *Sclerotinia minor* effectively controlled dandelions in turf management systems. A closely related species of this genus *S. sclerotiorum* has also shown the potential against noxious weeds [141]*.* Production of oxalic acid has been found by these two

Apart from these three genera, there are other fungal candidates that are registered to control weeds in forest lands and ecosystem managements [143]. A worth mentioning bioherbicide is De Vine containing a fungus *Phytophthora palmivora* [144]. This formulation was registered in 1981 and again in 2006 with the EPA [144]. The mycoherbicide "EcoClear" contains *Chondrostereum purpureum*, a pathogenic fungus which should be applied after the injury to the weeds' branches to

Soil-borne fungi also serve as an important tool in weed management. Their direct application in the soil causes decay of the seeds or emerging seedlings [146]. *Trichoderma virens* is one example that reduces weed populations in horticultural

Khattak et al. [147] tested two fungi *Aspergillus* and *Penicillium* for their herbicidal activity against two separate weeds *Silybum marianum* L. and *Lemna minor*. Results showed excellent weed-suppressive characters in the extracts of these fungi.

Biological control of weeds using bacteria and fungi should be the prime priority

The future advancement in biological agents for weed control should be based on advancements in microbial genetics (metagenomics), microbe-plant interactions, and microbial community-level analyses. Further investigations need to be discovered in the future in order to make biological weed control more pragmatic and instrumental. In this context, additional microbe-host relationships containing a match of biological agent and its potential host at greater susceptibility of virulence should be further explored. Since the 1960s a number of formulations have been registered in the world. Formulations that can ensure greater shelf lives, efficacy, and survival of microbial agents should be investigated in the future. Investigations on microbial community structure and function can advance microbial weed control. Traditional methods of microbial community structure solely rely on phenotypic characters; molecular-level characterization should be explored in

for mitigating the negative impressions posed by conventionally adopted weed control methods in order to ensure environmental safety and human health. These biological control agents should be adopted in areas with higher and multiple weed infestations; areas of low value land, where weeds have gotten resistance against herbicides; and areas with lack of labor and where the recommended cultural practices cannot be carried out, for example, restrictions posed by topography and narrow rowed crop cultivations. However, in special cases the combination of biological control agents with other methods could also be a promising approach as

weeds in Central India [139]. The third species under this genus is *Phoma*

species that cause virulence on the host plant [142].

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

retard resprouting [145].

**6. Conclusion and future strategy**

an alternative to conventional methods.

crops [81].

**259**

#### **Figure 2.**

*Possible mechanisms of plant growth-promoting rhizobacteria and fungi involved in herbicidal activity. IAA refers to indole-3 acetic acid, and ALA refers to aminolevulinic acid.*

Banowetz et al. [118] tested the germination inhibition activity in various monocot and dicot plants by the application of *P. fluorescens* (strain WH6). The germination inhibition activity was attributed due to the production of a compound called as Germination-Arrest Factor (GAF). Patil [129] screened 15 strains of deleterious rhizospheric bacteria isolated from rhizosphere of different weeds. Among these strains five isolates caused a significant reduction in root and shoot growth of weeds while showing no harmful effects on crop plants. Boyette and Hoagland [130] suggested that *X. campestris* (strain LVA-987) have shown strong growth suppressive effects against horseweed (*Conyza canadensis*). Some of the key herbicidal mechanisms shown by bacteria and fungi are shown in **Figure 2**.

### **5. Fungi (mycoherbicides) in biological weed control**

A list of fungal biological weed control agents is given in **Table 3**. Within the scientific context, three genera of fungi have received worldwide attention to be used in biological weed control. In addition to the abovementioned BioMal and Collego, different other species of genus *Colletotrichum* have been researched extensively. Additionally, *C. truncatum* have been reported to control sesbania (*Sesbania exaltata*) [131] and *C. orbiculare* that has been found to control spiny cocklebur (*Xanthium spinosum*) [63, 132]. It is evident from the literature that these two *Colletotrichum* species produce indole acetic acid [133] which is a phytohormone and derivatives of which show herbicidal activity [134].

Within the genus *Phoma*, three species have a potential against weed control. *P. herbarum* is a fungus that is isolated from lesions of dandelion leaf that have shown control effects of dandelion [135]. *P. macrostoma* has also been studied for weed control due to its inhibitory effects on the dicot plants [18, 136, 137]. *P. macrostoma* strain (94-44B) has been found to control turf associated with broad-leaved weeds in Canada. Mass spectrometric analysis of *P. macrostoma*

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

revealed the production of photobleaching of macrocidins [138] that do not have any inhibitory effects on monocot plants [18]. Despite this macrocidins an anthraquinone pigment in *P. macrostoma* has shown prominent herbicidal effects on some weeds in Central India [139]. The third species under this genus is *Phoma chenopodicola* that is studied widely for its potential against common lamb's quarter [62]. The mechanism behind its virulence against lamb's quarter is the production of diterpene and chenopodolin, a phytotoxic compound isolated from this species [62].

Two species within the genus *Sclerotinia* have been investigated for their herbicidal activity. It is evidenced by the work of Abu-Dieyeh and Watson [140] that *Sclerotinia minor* effectively controlled dandelions in turf management systems. A closely related species of this genus *S. sclerotiorum* has also shown the potential against noxious weeds [141]*.* Production of oxalic acid has been found by these two species that cause virulence on the host plant [142].

Apart from these three genera, there are other fungal candidates that are registered to control weeds in forest lands and ecosystem managements [143]. A worth mentioning bioherbicide is De Vine containing a fungus *Phytophthora palmivora* [144]. This formulation was registered in 1981 and again in 2006 with the EPA [144].

The mycoherbicide "EcoClear" contains *Chondrostereum purpureum*, a pathogenic fungus which should be applied after the injury to the weeds' branches to retard resprouting [145].

Soil-borne fungi also serve as an important tool in weed management. Their direct application in the soil causes decay of the seeds or emerging seedlings [146]. *Trichoderma virens* is one example that reduces weed populations in horticultural crops [81].

Khattak et al. [147] tested two fungi *Aspergillus* and *Penicillium* for their herbicidal activity against two separate weeds *Silybum marianum* L. and *Lemna minor*. Results showed excellent weed-suppressive characters in the extracts of these fungi.

#### **6. Conclusion and future strategy**

Biological control of weeds using bacteria and fungi should be the prime priority for mitigating the negative impressions posed by conventionally adopted weed control methods in order to ensure environmental safety and human health. These biological control agents should be adopted in areas with higher and multiple weed infestations; areas of low value land, where weeds have gotten resistance against herbicides; and areas with lack of labor and where the recommended cultural practices cannot be carried out, for example, restrictions posed by topography and narrow rowed crop cultivations. However, in special cases the combination of biological control agents with other methods could also be a promising approach as an alternative to conventional methods.

The future advancement in biological agents for weed control should be based on advancements in microbial genetics (metagenomics), microbe-plant interactions, and microbial community-level analyses. Further investigations need to be discovered in the future in order to make biological weed control more pragmatic and instrumental. In this context, additional microbe-host relationships containing a match of biological agent and its potential host at greater susceptibility of virulence should be further explored. Since the 1960s a number of formulations have been registered in the world. Formulations that can ensure greater shelf lives, efficacy, and survival of microbial agents should be investigated in the future. Investigations on microbial community structure and function can advance microbial weed control. Traditional methods of microbial community structure solely rely on phenotypic characters; molecular-level characterization should be explored in

Banowetz et al. [118] tested the germination inhibition activity in various monocot and dicot plants by the application of *P. fluorescens* (strain WH6). The germination inhibition activity was attributed due to the production of a compound called as Germination-Arrest Factor (GAF). Patil [129] screened 15 strains of deleterious rhizospheric bacteria isolated from rhizosphere of different weeds. Among these strains five isolates caused a significant reduction in root and shoot growth of weeds while showing no harmful effects on crop plants. Boyette and Hoagland [130] suggested that *X. campestris* (strain LVA-987) have shown strong growth suppressive effects against horseweed (*Conyza canadensis*). Some of the key herbi-

*Possible mechanisms of plant growth-promoting rhizobacteria and fungi involved in herbicidal activity. IAA*

cidal mechanisms shown by bacteria and fungi are shown in **Figure 2**.

A list of fungal biological weed control agents is given in **Table 3**. Within the scientific context, three genera of fungi have received worldwide attention to be used in biological weed control. In addition to the abovementioned BioMal and Collego, different other species of genus *Colletotrichum* have been researched extensively. Additionally, *C. truncatum* have been reported to control sesbania (*Sesbania exaltata*) [131] and *C. orbiculare* that has been found to control spiny cocklebur (*Xanthium spinosum*) [63, 132]. It is evident from the literature that these two *Colletotrichum* species produce indole acetic acid [133] which is a phytohor-

Within the genus *Phoma*, three species have a potential against weed control. *P. herbarum* is a fungus that is isolated from lesions of dandelion leaf that have shown control effects of dandelion [135]. *P. macrostoma* has also been studied for weed control due to its inhibitory effects on the dicot plants [18, 136, 137]. *P. macrostoma* strain (94-44B) has been found to control turf associated with broad-leaved weeds in Canada. Mass spectrometric analysis of *P. macrostoma*

**5. Fungi (mycoherbicides) in biological weed control**

*refers to indole-3 acetic acid, and ALA refers to aminolevulinic acid.*

**Figure 2.**

*Sustainable Crop Production*

**258**

mone and derivatives of which show herbicidal activity [134].

the future. In a nutshell, fatty acid profiling should be the initial step in targeted weed control. Nucleic acid tools, array pyrosequencing, metagenomics, construction of molecular probes, selection of hyper virulence, genomic studies, and hostmicrobe interactions should be investigated for the development of innovative weed control methods, reducing reliance on herbicide usage.

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