Potential antagonistic bacterial candidates for biological disease suppression of agro-horticultural crops
Authors: ¹Ajinath Dukare, ²Sangeeta Paul
¹Horticultural Crop Processing Division
ICAR-Central Institute of Post-Harvest Engineering & Technology
Abohar, Punjab, India-152116
²ICAR-Division of Microbiology, IARI, New Delhi, 110012

Summary

Antagonistic bacteria present a highly efficient, environmentally compatible and economical method for biological control of phytopathogenic diseases. Plant rhizosphere associated microorganisms often provide the first barrier against plant pathogens which attack the plant roots. Multiple traits of bacteria like secretion of antifungal metabolites (antibiotics, volatile compounds), competition for nutrients and space, mycoparasitism, production of cell wall lytic enzymes and /or induced systemic resistance are involved in biological disease suppression.

Potential bacterial agents for biological disease suppression

Several genera including Actinoplanes, Agrobacterium, Alcaligenes, Amorphosporangium, Arthobacter, Azotobacter, Bacillus, Cellulomonas, Enterobacter, Erwinia, Flavobacterium, Hafnia, Micromonospora, Pseudomonas, Pasteuria, Rhizobium and Bradyrhizobium, Serratia, Streptomyces and Xanthomonas have been identified as biocontrol agents. However, when compared to other bacteria, Bacillus and Pseudomonas spp. were widely used for disease suppression.

1. Pseudomonas

Plant growth promotion effect of Pseudomonas species is largely reviewed and documented in several reports. Plant growth promotion activities of Pseudomonas species for many crops such as potato, sugar beet, and radish have been reported. Mechanisms involved in disease suppression by rhizobacteria have been understood to a large extent by using strains of fluorescent pseudomonads for biological disease control. Various mechanisms such as antibiosis, competition, or parasitism have contributed in plant disease control by strains of Pseudomonas. Within the genus Pseudomonas, P. fluorescens ubiquitous rhizosphere inhabiting bacterial species, have been the most studied group as suitable and potential candidates for biological disease control. Suppression of Fusarium wilts of various plant pathogens and Clavibacter michiganenis subsp. michiganesis, causal agent of tomato bacterial canker have been successfully achieved using Fluorescent pseudomonads as biocontrol agent. Numerous studies have been undertaken regarding use of P. fluorescens as biocontrol agent to prevent and control the spread of various pathogens of several plants.

2. Bacillus and Paenibacillus

Besides being well known for plant growth promotion activity, Bacillus strains also play vital role in suppression of soil borne fungal disease and induction of systemic resistance to diseases in plants. These bacteria are capable of forming endospores, resting dominant spores that allow them to survive under unfavorable environmental conditions for extended periods of time. Several Bacillus species were documented as efficient biocontrol agents in greenhouse or field conditions. An increase in onion seedling emergence and yield was observed due to inhibition and suppression of incidence of onion white rot caused by Sclerotia cepivorum by antibiotic producing isolates ofBacillus subtilis Antibiotics producing members of Bacillus were reported as biocontrol agents for inhibition of several phytopathogenic fungi including R. solani and F. oxysporum f. sp. cicer. Bacillus strains have also known for strong induction of systemic resistance (ISR) against phytopathogens in host plants. It was observed that when plants were inoculated with PGPR mixture of Bacillus amyloliquefaciens strain IN937a and Bacillus pumilus strain IN937b had higher induction of systemic resistance against anthracnose of long cayenne pepper caused by Colletotrichum gloeosporioides, southern blight of tomato caused by S. rolfsii and mosaic disease of cucumber caused by cucumber mosaic virus.

3. Actinomycetes/actinobacteria

Actinomycetes are less studied group of bacteria regarding biological disease suppression. Plant growth promoting actinomycetes were screened for antibacterial or antifungal activity for suppression of plant disease. Plant growth promoting Streptomyces griseus have the biocontrol potential against R. solani in carrot. Effective biocontrol of some diseases including those caused by Alternaria solani,Clavibacter michiganensis subsp. michiganensis, Pseudomonas solanacearum, F. oxysporum and Verticillium alboatrum in tomato with use of three Streptomyces spp. has been reported by El-Abyad et al. (1993). Significant improvement in tomato seedling growth was observed when seed coating was carried out with antifungal metabolite producer, Streptomyces violaceusniger strain YCED9.

4. Other reported bacteria as biocontrol agents
   Beside Pseudomonas species, other bacteria belonging to gram negative group such as Aeromonas spp., Arthrobacter spp., Enterobacter spp., Lysobacter spp. and Serratia spp. have been used as biocontrol agents for suppression of phytopathogens. Lytic enzymes produced by Aeromonas caviae was involved in the biological control of soil-borne plant pathogens such as R. solani , F. oxysporum f. sp. vasinfectum and S. rolfsii. Similarly, Chitinolytic bacterium S. marcescens and Streptomyces viridodiasticus were applied for biological control of Sclerotinia minor . Three soil inhabiting Enterobacter agglomerans strains were found antagonistic to R. solani under in vitro growth conditions. These strains produced proteins with higher chitinolytic activity. About 64 to 86% of reduction in disease incidence was observed by application of these bacteria while mutants of one of the isolates, deficient in chitinolytic activity, were unable to protect cotton plants against the soil borne disease.

References:

1. El-Abyad, M.S, El-Sayed, M.A., El-Shanshoury, A.R. and El-Sabbagh, S.M. (1993) Towards the biological control of fungal and bacterial diseases of tomato using antagonism Streptomyces spp. Plant Soil. 149:185–195.

2. Kloepper, J. W, Ryu, C. M., and Zhang, S. (2004). Induce systemic resistance and promotion of plant growth by Bacillus spp. Phytopathology. 94:1259-1266.

3. Weller, D.M. (1988). Biological control of soil borne plant pathogens in the rhizosphere with bacteria. Annual Review of Phytopathology 26: 379-407.




About Author / Additional Info:
Working as scientist Horticultural Crop Processing Division, ICAR-CIPHET, Abohar, Punjab.