Leguminous Crop Diseases Management and Protection Mechanisms

Chickpea, pigeon pea, pea, and lentil are attacked by several diseases, some of which cause considerable crop damage. Chickpea is mainly affected by wilt (Fusarium oxysporum f.sp. ciceris), blight (Mycosphaerella pinodes), and rust (Uromyces ciceris). Ascochyta blight, a disease caused by a complex of Ascochyta pinodes, Ascochyta pinodella, Ascochyta pisi, and/or Phoma koolunga, is a critical problem in many field peas (Pisum sativum L.) growing regions, and it results in significant losses in grain yield. Ranjbar Sistani et al. (2017) reported that Rhizobium plays an effective role in resistance induction and yield enhancement strategy of seeds production. Many species of rhizobia promote plant growth and also inhibit the growth of certain pathogenic fungi and bacteria (Tables 4.2 and 4.3). The mechanisms of biocontrol by rhizobia include competition for nutrients (Arora et al. 2001), production of antibiotics (Bardin et al. 2004, Chandra and Pareek 2002), production of enzymes to degrade cell walls (Ozkoc and Deliveli 2001), and production of siderophores (Carson et al. 2000. Deshwal et al. 2003) (Tables 4.2 and 4.3).

TABLE 4.2

List of Bacterial Diseases with Their Biocontrol Agents

TABLE 4.3

List of Fungal Diseases with Their Biocontrol Agents

Liu et al. (2016) isolated three biocontrol strains of Bacillus sp. and one of Pantoea agglomerans from pea-related niches, which were found to reduce the severity of disease under greenhouse and field conditions. Sreevidya et al. (2015) reported Penicillium citrinum for their antagonistic potential against Botrytis cinerea causing Botrytis gray mold disease in chickpea. B. cinerea is seed-borne because of poor germination and soft rot of the lower stem in chickpea. Burgess et al. (1997) evaluated G. roseum for control of B. cinerea on chickpea seed to assess its compatibility with rhizobium inoculant and to study the mechanism of action of biocontrol. Chakraborty and Purkayastha (1984) reported that some rhizobitoxine-producing strains of Bradyrhizobium japonicum protected soybeans from the infection of Macrophomina phaseolina, the charcoal rot fungus of leguminous crops.

Fusarium wilt caused by F. oxysporum is one of the most important diseases in chickpea (Cicer arietinum L.) throughout the world. It is one of the main yield-limiting factors in all chickpea-growing areas of the world (Dubey et al. 2007). General symptoms of Fusarium wilt is a serious threat in chickpea comprise drooping, yellowing, drying of the leaves and discoloration of vascular system. Upon infection, the pathogen enters the roots of the susceptible host, establishes in the vascular system of plants, followed by toxin(s) production that kills plants by blocking xylem vessels and restricts water transport (Anjaiah et al. 2003. Gopalakrishnan et al. 2005). Mung bean or green gram, Vigna radiata (L.) Wilczek, an important pulse crop, is an excellent source of low-cost and high-quality protein (Taylor et al. 2005). Since mung bean roots through symbiosis with nitrogen-fixing rhizobia fix the atmospheric N, this crop is important both economically as well as nutritionally (Yaqub et al. 2010). The low productivity and poor quality of mung bean can be attributed to several biotic constraints of which diseases caused by fungi are of great importance (Khan et al. 2001). Of these, root rot caused by the soil and seed-borne fungus M. phaseolina (Tassi) Goid is a major limiting factor in the mung bean production (Raguchander et al. 1993). It is a serious disease of many crops, inflicting up to 100% yield losses in mung bean under dry and hot conditions. Lentil (Lens culinaris) is one of the most significant pulse crops among the grain legumes. There are various biotic and abiotic stresses in lentil crop production including limiting factors such as poor crop management and diseases. Lentil crops are affected by a wide range of pathogens, among them fungal diseases being the most significant. Blight caused by Stemphylium botryosum is becoming a serious threat to lentil production. Subedi et al. (2015) reported T. viride and T. harzianum as the best antagonist that suppressed the growth of S. botryosum (Table 4.3). According to Xue (2003), Clonostachys rosea strain ACM941, a mycoparasite, is an effective bioagent in controlling pea root rot complex caused by A. pinodes and Rhizoctonia solani.

Rhizobial strains in soil compete for nutrients by inhibiting the pathogens. Colonization behavior of P. fluorescens and S. meliloti in alfalfa rhizosphere were reported as effective biocontrol agents to suppress pathogens (Villacieros et al. 2003). The synthesis of metabolites such as HCN, phenazines, vis-coinamide pyrrolnitrin, and tensin by rhizobia were also reported as other mechanisms (Bhattacharyya and Jha 2012). HCN production by rhizobacteria is one of the potential and eco-friendly mechanisms for biological control in weeds (Kremer and Souissi 2001).

4.3.1 Siderophore

Iron in soil occurs in divalent or trivalent forms, depending on pH and the availability of other minerals (Bodek 1988). Because of the presence as insoluble hydroxides and oxyhydroxides, iron in soil becomes unavailable to both plants and microbes. This is made available by the synthesis of low molecular weight compounds called siderophores, which have a high affinity toward iron. Iron chelation by microbial siderophores from soil affect by pH, concentration, redox potential, stability constant, and receptor availability to exchange with phytosiderophores (Crowley 2006). Rhizobia produce high-affinity siderophores that inhibit the pathogens by limiting available iron (Arora et al. 2001). Microbes synthesize a variety of siderophores including catechols and hydroxamate. Many strains of actinobacteria have been reported as siderophore producers (Wang et al. 2014).

4.3.2 Lytic Enzymes

The majority of soil bacteria produce extracellular enzymes including lipases, glucanases, xylanases, proteinases, amylases, peptidases, chitinases, peroxidases, cellulases, pectinase, ligninases, hemicellulase, and keratinase. These enzymes are involved in biocontrol mechanisms against phytopathogens because of the chemical composition of their cell wall (Gupta et al. 1995. Fodil et al. 2011). R. leguminosarum bv. trifolii, R. leguminosarum bv. viciae, R. meliloti, R. trifolii, S. meliloti, and B. japonicum have been reported to produce antibiotics and cell wall-degrading enzymes that can reduce the growth of phytopathogens (Òzkoc and Deliveli 2001, Chandra and Pareek 2002. Bardin et al. 2004).

4.3.3 Antibiotics

Antibiotics are toxins produced by microbes at low concentrations that can kill other microorganisms. Antibiotic production on the plant surface or within plant tissues by microbes is critical for biocontrol mechanism to control disease. Bacillus subtilis and Bacillus amyloliquefaciens are well recognized for their biocontrol of fungal and bacterial diseases. The most important mechanism is the synthesis of a variety of antibiotics with diverse activities by Bacillus sp. (Stein 2005). Among these antimicrobial compounds, cyclic lipopeptides (LPs) of the fengycin, iturin, and bacillomycin families exhibit strong in vitro antifungal activities against a broad variety of fungal pathogens. The ability to produce a range of antibiotics by microbes that can differentially reduce pathogen growth and disease condition is important for sustainable agriculture (Pal and Gardener 2006).

4.3.4 Induced Systemic Resistance (ISR)

Induced systemic resistance and enhanced expression of plant defense-related genes by rhizobia is one of the important mechanisms against pathogens to minimize disease in pulse crops. ISR is a key mechanism by which some rhizobacteria prime the whole plant for improved defense against a wide range of pathogens. Bacteria such as Bacillus, Serratia, Pseudomonas, Streptomyces, and Stenotrophomonas and fungi such as Ampelomyces, Coniothyrium, and Trichoderma are well-known microbes with beneficial influence on plant health. This level of resistance in plants is characterized by the activation of latent defense mechanisms that are expressed upon pathogen attack not only at the site of infection but also systemically in all plant parts spatially separated from the inducer (Pieterse et al. 2014). Inoculation of appropriate rhizobial strain leads to elicitation of phenolic compounds, isoflavonoid phytoalexins, and activation of enzymes such as 1 -phenylalanine ammonia lyase (PAL), peroxidase (POX), chaicone synthase (CHS), and polyphenol oxidase (PPO) (Das et al. 2017). Various signaling pathways that activate during pathogen challenge and stimulate the plant defense response by rhizobia may be expected to affect plant-pathogens interactions. Azpilicueta et al. (2004) reported that the systemic stimulation of phytoalexins production in peanut crops inoculated with Bradyrhizobium sp. SEMIA6144 is the key process for protection against Aspergillus niger and Cladosporium cucumerinum (Azpilicueta et al. 2004). In nutshell, rhizobia promote the growth of plants either directly through N2 fixation, phosphate solubilization, and synthesis of phytohormones, or indirectly as a biocontrol agent by inhibiting the growth of pathogens by producing siderophores, lytic enzymes, and secondary metabolites. The potential effect of leguminous microbiomes due to their multifaceted beneficial traits is likely to play an important role in modern, highly intensive agricultural practices.