Effect of medicinal plant materials on efficacy of biocontrol agents used against soil borne plant pathogens
Authors: Zakaullah Khan, Bharat H. Gawade, A. Kandan and N. K. Gautam
ICAR-National Bureau of Plant Genetic Resources, Pusa Campus, New Delhi 110012

Control of pests and pathogens mainly relies on the use of chemicals, but the overuse of chemical pesticides lead to adverse impacts on the environment, humans and animal health and also induces a pathogenic resistance to pesticides. These issues and a means to develop eco-friendly and safer control methods have encouraged research efforts aimed at finding new and effective methods for controlling plant diseases. Emerging strategies for plant disease management involves integrated approach by applying antagonistic microorganisms alone or in combination with and/or alternating with pesticides/fungicides or natural materials. Biological control appears to be an environmentally safe and economically feasible strategy for plant protection and has great potential to promote sustainable agriculture. However, although numerous microorganisms have been considered potential agents for suppressing plant pathogens, only a few have been commercialized.

Trichoderma harzianum is a filamentous soil fungus that functions as a biocontrol agent for a wide range of economically important plant pathogens. Trichoderma spp. are worldwide in occurrence and easily isolated from soil, decaying wood and other forms of plant organic matter. Trichoderma produces a large range of secondary metabolites with inhibitory traits, including direct mycoparasitism as well as lysis and degradation of the cell wall by enzymes. Several strains of Trichoderma have been tested as an alternative to chemical fungicides. However, achieving full disease control in soil is difficult mainly because of the complexity of the plant-soil system, whereby the microbial inoculum is affected by biotic and abiotic factors and shows relatively lower efficacies compared to those of applying chemicals under various environmental conditions. Therefore, the integration of microorganisms and chemical pesticides and/or natural materials may be an effective method applicable in the field. Combined applications of a biocontrol agent and synthetic chemicals or plant materials often provide better plant protection than individual treatments. The combined application of Paeonia suffruticosa cortex (medicinal plant) and T. harzianum is more effective than either treatment alone for controlling Rhizoctonia damping-off (Lee et al., 2008). A combined application of metalaxyl with either T. harzianum or Pseudomonas cepacia resulted in lower disease incidences of red pepper phytophthora blight than by any single treatment (Kim et al., 1991). Two different biocontrol agents used in a mixture of Pichia guilliermondii and Bacillus mycoides are higher in control efficacy than that achieved by separate inoculation, and the variability of suppression was reduced.

The antimicrobial activity of plant materials and their extracts have been recognized for many years. In particular, the antimicrobial activities of plant materials and essential oils have formed the basis of preservation, pharmaceutical, alternative medicine and natural therapies. Natural plant materials are possible sources of environmentally friendly agrochemicals that are effective against pathogens. Several species of medicinal plant materials were tested for their antifungal activity against Rhizoctonia solani and T. harzianum to select plant species that can be used to improve the biocontrol efficacy of T. harzianum (Lee et al., 2008, Lee et al., 2011). Some species were effective against R. solani but were also antagonistic to T. harzianum. However, peony (Paeonia suffruticosa) root bark (PRB) and Cinnamomum loureirii stem bark (CSB) showed the strongest antifungal activity against R. solani, and were not antagonistic to T. harzianum. The purified antifungal compound separated from the CSB and PRB inhibited the growth of several plant pathogenic fungi, including R. solani, but not T. harzianum.

The mode of antifungal action against R. solani may involve the release of volatile antimicrobial substances, such as paeonol and cinnamaldehyde in PRB and CSB, respectively, which inhibit fungal mycelial growth. The fact that volatile constituents of Paeonia clusii were active against several microorganisms, including pathogenic fungi and exhibited the highest antifungal activity among the Paeonia taxa tested, may be attributed to the existence of paeonol, because P. clusii was found to contain a high percentage of paeonol. Recently, paeonol and benzoic acid were identified in Paeonia suffruticosa root bark that showed acaricidal activities against the copra mite Tyrophagus putrescentiae; these activities were much higher in closed containers than in open ones, indicating that the effects of these compounds were largely due to action in the vapor phase. Paeonia albiflora and P. moutan root bark display antifungal activity against Alternaria alternata. Medicinal plant soil amendments, including PRB and CSB, reduced root-knot nematode infection in tomato (Kim et al., 2003). The biocontrol efficacy of radish damping-off was increased greatly by the combined treatment of CSB and T. harzianum, and PRB and T. harzianum in various concentrations, compared to the use of either treatment alone. The combined treatment of CSB/PRB and T. harzianum enhanced the control efficacy more than the sum of single treatment alone. Both CSB and PRB did not affect T. harzianum population growth, as it was almost the same in rice-bran peat medium (culture) amended with various concentration of CSB and PRB powder as in non-amended medium. Thus the enhanced efficacy of T. harzianum in rice-bran peat medium amended with CSB and PRB powder, suggesting that PRB and CSB powder or extract may be used as a safe additive to T. harzianum to improve the control of the soil borne diseases.

References cited:

Kim, H. K., Kim, K.D. and Lee, H.J. (1991). Enhanced suppression of red-pepper Phytophthora blight by combined applications of antagonist and fungicide. Korean Journal of Plant Pathology 7: 221-225.

Lee, T.O., Khan, Z., Kim, S.G. and Kim, Y.H. (2008). Amendment with peony root bark improves the biocontrol efficacy of Trichoderma harzianum against Rhizoctonia solani. Journal of Microbiology and Biotechnology 18:1537-1543.

Lee, H.M., Khan, Z., Kim, S.G., Baek, N.I. and Kim,Y.H. (2011). Evaluation of biocontrol potential of some medicinal plant materials alone and in combination with Trichoderma harzianum against Rhizoctonia solani AG 2-1. The Plant Pathology Journal 27: 68-77.

About Author / Additional Info:
Senior Scientist (Nematology), Division of Plant Quarantine, Indian Council of Agricultural Research (ICAR)-National Bureau of Plant Genetic Resources (NBPGR), Pusa Campus, New Delhi-110012, INDIA.