IMPORTANT BIOCONTROL TRAITS OF MICROBIAL ANTAGONISTS OF LEAF ROT DISEASE OF COCONUT By

Leaf rot disease of coconut, which occurs superimposed on root (wilt) disease of coconut in about 65 % of the cases, is an important disease prevalent in Kerala State of India. Colletotrichum gloeosporioides and Exserohilum rostratum are the main fungal pathogens of the disease, which disfigure the leaves and reduce yield substantially. Several biocontrol agents have been identified which reduce the disease development by these fungal pathogens. Pseudomonas spp. exhibit a host of mechanisms, which act in concert to bring about biocontrol activity. Bacillus spp. work mainly through production of antibiotics and other non-volatile metabolites. Mycoparasitism was the main contributing factor for actinomycetes group. INTRODUCTION Micro-organisms are central to our understanding of biological control of plant diseases and their potential in reducing disease incidence and severity is well demonstrated in many crops worldwide, leading to their practical applications in agriculture. The surface of aerial plant parts provides a habitat for epiphytic micro-organisms, many of them are capable of influencing the growth of pathogens, thus, paving the way for biological control on foliar surfaces (Blakeman and Brodie, 1976; Blakeman and Fokkema, 1982). These saprophytic organisms play an important part in control of foliar pathogens thus reducing the incidence of foliar diseases on crops in the field. In recent times, bacterial antagonists have received considerable attention mainly because of their ability to reduce the inoculum’s density of the pathogen, which is effected through several means. An efficient bio-control agent may affect pathogens by a combination of mechanisms (Chet, 1987). Hence, to devise ways of enhancing the antagonistic effects of foliar micro-organisms against pathogens, it is necessary to understand the mechanisms responsible for biological control. The production of volatile and non-volatile secondary metabolites, siderophores, hydrogen cyanide (HCN), antibiotics, etc. have been implicated in the control of plant pathogens (McKeen et.al., 1986; Fravel, 1988; Droby et al., 1989; Chet et. al., 1990; Thomashow and Weller, 1990; Ferriera et.al., 1991). Mycoparasitism / hyperparasitism by microbial antagonists can also be distinguished, which involve hyphal interference, penetration of host mycelium and hyphal lysis (Amer et. al., 1997; Gupta et al., 2001). Leaf rot disease, which occurs in the Southernmost District (Kerala) of India, is an economically important disease of coconut plantation crop. The disease is caused mainly by fungi Colletotrichum gloeosporioides and Exserohilum rostratum, which produce lesions on innermost tender leaves. These lesions coalesce rotting a large leaf area, which falls off after drying. Each successively emerging leaf gets infected, thus, reducing the photosynthetic area to a great extent and the yield of the palm is adversely affected. Several microbial antagonists have been identified against the fungal pathogens of this disease. Out of this, seven bacterial / actinomycetal anta-gonists were selected for further studies. 1 Central Plantation Crops Research Institute, Regional Station, Kayangulam, Krishnapuran, India.


INTRODUCTION
Micro-organisms are central to our understanding of biological control of plant diseases and their potential in reducing disease incidence and severity is well demonstrated in many crops worldwide, leading to their practical applications in agriculture. The surface of aerial plant parts provides a habitat for epiphytic micro-organisms, many of them are capable of influencing the growth of pathogens, thus, paving the way for biological control on foliar surfaces (Blakeman and Brodie, 1976;Blakeman and Fokkema, 1982). These saprophytic organisms play an important part in control of foliar pathogens thus reducing the incidence of foliar diseases on crops in the field.
In recent times, bacterial antagonists have received considerable attention mainly because of their ability to reduce the inoculum's density of the pathogen, which is effected through several means. An efficient bio-control agent may affect pathogens by a combination of mechanisms (Chet, 1987). Hence, to devise ways of enhancing the antagonistic effects of foliar micro-organisms against pathogens, it is necessary to understand the mechanisms responsible for biological control.
Leaf rot disease, which occurs in the Southernmost District (Kerala) of India, is a n economically important disease of coconut plantation crop. The disease is caused mainly by fungi Colletotrichum gloeosporioides and Exserohilum rostratum, which produce lesions on innermost tender leaves. These lesions coalesce rotting a large leaf area, which falls off after drying. Each successively emerging leaf gets infected, thus, reducing the photosynthetic area to a great extent and the yield of the palm is adversely affected.
Several microbial antagonists have been identified against the fungal pathogens of this disease. Out of this, seven bacterial / actinomycetal anta-gonists were selected for further studies. This paper presents investigations into some of the mechanisms employed by these bio-control agents, which may explain the basis of their bio-control activity.

MATERIALS AND METHODS
The biological control agents of leaf rot disease of coconut used in the study were: Pseudomonas aeruginosa Pseudomonas sp.

RESULTS AND DISCUSSION
Naturally occurring resident micro-organisms on aerial plant surfaces have the ability to survive and grow in this extremely difficult habitat, and if these micro-organisms possess an effective antagonistic action against a plant pathogen, then they can become ideal bio-control agents. This guiding principle had formed the basis for the work on biological control of leaf rot, a foliar disease of coconut.
As a result, seven bio-control agents were identified as promising candidates for biological control of leaf rot disease of coconut (Gupta et.al., 2000). However, for determining the mode, form and optimum timing of application of antagonist inoculums, it is necessary to understand the mechanisms by which the antagonistic interactions between the phylloplane microflora and pathogens are brought about. Hence, the bio-control agents of leaf rot disease were screened for different traits which may be involved in the antagonistic reactions on foliar surfaces.
All the seven bio-control agents produced antibiotics (Table.1). Production of antifungal antibiotics by phylloplane bacteria -Pseudomonas spp. (Teliz-Ortiz and Burkholder, 1960), Bacillus (Swinburne et.al., 1975) and actinomycetes -Streptomyces spp. (Fawcett and Spencer, 1970) is a widely reported phenomenon and has been proposed to be a probable cause of pathogen inhibition (Johnson, 1931).
Siderophores have been implicated in the control of plant pathogens particularly by fluorescent pseudomonads (Thomashow and Weller, 1990). In our studies also, the two Pseudomonas species and one unidentified bacterium were found to produce siderophores, volatile metabolites and also hydrogen cyanide (details in Table.2). Cyanide production by certain fluorescent pseudomonades has been found to influence plant pathogens (Voisard et. al., 1989) and may contribute to biological control due to induction of plant resistance (O'Sullivan and O'Gara, 1992).
All seven bio-control agents were found to produce non-volatile secondary metabolites which were inhibitory towards fungal pathogens. Mycoparasitism which is the most vital mechanism of antagonism of fungal antagonists (Elad et. al., 1981;Howell, 1982) was evident in the case of actinomycetes, Thermomonospora mesophila and Streptomyces sp. Coiling around host hyphae was observed in both the antagonists (Table.3). Some kind of hyphal interference was also observed in case of pseudomonas which was probably due to the production of antifungal secondary metabolites.
Antibiosis-mediated hyphal perforations, lysis and fragmentation by Pseudomonas GRC 2 against fungal pathogens had earlier been reported by Gupta et al. (2001).
From the results presented here, it is evident that the antagonists of leaf rot disease especially Pseudomonas spp. possess a combination of bio-control traits, which can play an important role in plant disease control.

ACKNOWLEDGMENT
The author is grateful to the Head of Divisions of Crop Protection and Crop Production for their encouragement and useful suggestions.
--------Also produces blue-green, water soluble siderophores. Initial studies suggest that these possess both hydroxamate and phenolate groups and hence, fall under the category of pyoverdins.
Produces water soluble and diffusible siderophores.
Produces more than one type of antibiotics including pyocyanin.
Produces more than one type of antibiotic compounds.
Antibiotics (water soluble) are secreted into the growth medium.
Antibiotics appear to be the main mode of inhibitory action.
A mixture of antibiotic compounds are produced.
Produces HCN which may work through plant-induced resistance to certain pathogens.
Also produces hydrogen cyanide.
Produces both volatile and nonvolatile inhibitory secondary metabolites.
Produces volatile and nonvolatile metabolites during the stationary phase of the growth.
Non-volatile, antifungal in character, secondary metabolites are produced.
Non-volatile secondary metabolites are produced.
Both volatile and non-volatile metabolites are synthesized.
Some kind of antifungal secondary metabolitemediated hyphal interference is evident.
Hyphal interference in the zone of antibiotics elaboration and diffusion is seen.
Ability to use diverse substrates for its growth.
--Able to utilize an array of growth substrates. --