Field experiment was carried out to assess the efficacy of plant oils, plant extracts and antagonistic microorganisms against leaf blight disease of onion caused by Alternaria alternata. Two sprays of neem oil (3%) given on onion plants at the first appearance of the disease and the second on 15 days later recorded significantly the lowest percent disease index (22.22%) and besides increasing the yield. In plant extracts Acorus calamus rhizome extract 10% was significantly reduced the disease incidence (34.78%) followed by Mentha arvensis leaf extract.
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Onion (Allium cepa var., aggregatum) the queen of kitchen, considered as the poor mans staple spice, is flying out of the reach of even middle class families, grown for its bulb and also medicinal values. Like the crops, several diseases, among which leaf blight causes heavy losses in yield of onion. Leaf blight (Alternaria alternata) is a major problem in production of onion. Management of onion leaf blight disease by various strategies to be involved. The studies on the control of leaf blight caused by Alternaria sp. in onion and other crops have been carried out by various workers (Babu, 1994; Basin and Katircioglu, 1994; Sastrahidayal et al., 1995; Mohan, 1996; Karthikeyan et al., 2008) by chemicals. Studies were conducted on the control of the disease by using fungicides (Kannan and Subbaraja, 1999; Srivastava et al., 1999). Substantial use of chemical pesticides induces problems of health and environmental hazards in agricultural system. So, for human, plants and natural products of antimicrobial activity are best birational alternative today (Tiwari et al., 2007). Over the last two decades, intensive effort has been made to discover chemically useful antibacterial or antifungal drugs of plant origin (Valsaraj et al., 1997; Perumal Samy et al., 1999).
Reduction in the use of chemical pesticides in agriculture has been encouraged for a sustainable agriculture which necessitates the use of alternative strategies in combating the plant diseases. To overcome these problems use of plant products having more number of antimicrobial compound and biocontrol agents to be involved in disease management.
It has been estimated that 14-28% of higher plant species are used medicinally and that 74% of pharmacologically active plant derived components were discovered after following up on ethano medicinal use of the plants (Ncube et al., 2008). Recently, the acceptance of traditional medicine as an alternative form of health care and the development of microbial resistance to the available antibiotics has led authors to investigate the antimicrobial activity of medicinal plants (Lis-Balchin and Deans, 1996; Maoz and Neeman, 1998; Hammer et al., 1999).
More number of plant species has been reported to possess natural substances that are toxic to many fungi causing plant diseases (Amadioha, 2000: Kagale et al., 2004). Bioagents of late have been known to induce systemic resistance against several plant diseases (Jetiyanon, 2007; Choudary et al., 2009). Bioagents like Pseudomonas and Bacillus strains consistently provided systemic protection against multiple diseases in various crops (Jetiyanon, 2007). Several strains of Pseudomonas and Bacillus elicit significant reduction in the incidence or severity of various diseases on diversity of hosts by elicitation of ISR which has been demonstrated (Choudary et al., 2009). With this background, in the present study, plant products and antagonistic microorganisms were exploited for the effective management of fruit rot incidence in glass house and field conditions.
MATERIALS AND METHODS
Preparation of talc-based formulation: The talc-based formulations of the individual bacterial strains were prepared by the following method described by Vidhyasekaran et al. (1997). Briefly, a loopful of bacteria was inoculated into the Kings B broth (King et al., 1954) and incubated in a rotary shaker at 150 rpm for 48 h at room temperature (25+28°C). One kg of talc powder was taken in a sterilized metal tray and its pH was adjusted to neutral by adding CaCO3 at the rate of 15 g kg-1. Ten grams of Carboxy Methyl Cellulose (CMC) was added to 1 kg of talc and mixed well and the mixture was autoclaved for 30 min on each of two consecutive days. The 400 mL of 48 h grown bacterial suspension containing 96x108 cfu mL-1 was mixed with carrier cellulose mixture under aseptic conditions. For Trichoderma, 50 mL of molasses yeast (molasses 2%, yeast 0.3%) broth, autoclaved flasks was inoculated with 5 mm mycelial bits of T. viride and incubated at 28+2°C for 15 days (Singh and Majumdar, 2001). The spore concentration was adjusted to 1.56x108 cfu g-1. After drying (approximately 35% moisture content) overnight under sterile conditions, it was packed in a polypropylene bag, sealed and stored at room temperature (Jayarajan et al., 1999). At the time of application, the population of fungi in the formulation was 3.6x108 cfu g-1 of talc powder.
Preparation of plant extracts: The fresh plant materials (Acorus calamus Rhizome, Allium sativum bulb, Catharanthus roseus/Mentha arvensis and Prosopis juliflora leaf) were separately washed in fresh water and finally in sterile water. These were separately ground in sterile water at the rate of 1 mL g-1 of the plant material in a pestle and mortar. The extract was expressed by squeezing the macerate with sterilized cotton wool. It was strained through two layers of muslin cloth and finally through sterilized Whatman No.1 filter paper. The extract was then passed through seitz filter to free from bacterial contamination. This formed the standard plant extract solution (100%). This was further diluted to the required concentration with sterilized medium/distilled water (Shekhawat and Prasad, 1971).
Field experiment: The field experiment was conducted at Randomized Block Design (RBD) with 12 treatments and 3 replications during kharif season at onion growing areas in Paavurchathiram at Tirunelveli district of Tamil Nadu, to assess the efficacy of plant extracts/oils and antagonistic microorganisms against onion leaf blight disease caused by A. alternata. The Agrifound red onion variety was used as test crop. The plants were raised with a standard spacing of 45x10 cm and the plot size was 4x3 m. The treatments were as follows:
|•||Bacillus subtilis at 0.2%|
|•||Pseudomonas fluorescens at 0.2%|
|•||Trichoderma viride at 0.2%|
|•||Palmarosa oil at 0.05%|
|•||Palmarosa oil at 0.1%|
|•||Neem oil at 3.0%|
|•||Acorus calamus at 10.0%|
|•||Allium sativum at 10.0%|
|•||Catharanthus roseus / Mentha arvensis at 10.0%|
|•||Prosopis juliflora at 10.0%|
|•||Mancozeb at 0.2%|
Triton-E at 0.1% mixed with water used as a sticker/emulsifier for all the treatments. For control plots, triton-E mixed with water used as spray. The spraying was given on the initial appearance of the disease and another at 15 days later. The disease intensity was recorded at 15 days after the second spraying. The following score chart of 0-9 grade chart (TNAU, 1980) used for scoring the disease.
The percent disease index was calculated by using Mckinney (1923) formula:
where, y is the maximum category value in the score chart.
Statistical analysis: The data were statistically analyzed using the IRRISTAT version 92 developed by the International Rice Research Institute (IRRI) Biometrics unit, the Phillippines (Gomez and Gomez, 1984). Prior to statistical analysis of variance (ANOVA) the percentage values of the disease indices were arcsine transformed. Data were subjected to analysis of variance (ANOVA) at two significant levels (p<0.05 and p<0.01) and means were compared by Duncans Multiple Range Test (DMRT).
Twelve treatments including biocontrol agents, plant oils, plant extracts and chemicals were used. Each treatment by spraying and all the treatments were found to be superior over control.
|Table 1:||Efficacy of antagonistic microorganisms, plant oils and plant extracts against leaf blight disease of Onion (Foliar sprays)|
|Data in parentheses are arc sine transformed values, *Means followed by a common letter are not significantly different at 5% level by LSD, Mean of three replications|
The results revealed that two spraying of neem oil (3%) given on onion plants first at the appearance of the disease and the second spray at 15 days later recorded lowest Percent Disease Index (PDI) 22.22 as against control (63.83) and this treatment recorded yield of 7916.67 kg ha-1 as against 4458.30 kg ha-1 in the control. This was followed by Palmarosa oil (0.1%) and A. calamus rhizome extract (10%) ranked next by recording 25.89 PDI and 34.78 PDI, respectively. These treatments were next only to the fungicide, Mancozeb (0.2%) which recorded the least disease intensity (PDI-20.66) and also highest yield 8100 kg ha-1.
The yield was increased when two sprayings of neem oil 3.0% (77.77%) were given which was followed by palmarosa oil 0.1% (76.07%) and A. calamus rhizome extract 10% (74.58%). The yield was reduced in T. viride (23.19%) sprayed plots. However, two sprayings of Mancozeb (0.2%) were significantly found to be highly effective in increasing the yield (81.68%) (Table 1).
The present study was supported by (Sujatha Bai, 1992) who reported that neem oil 1.0% was used for the reduction of fruit rot disease incited by A. tenuis.
A similar result was supported by Karthikeyan et al. (2006). Neem oil reduced the leaf blight disease of onion incited by A. palandui in pot culture and field condition.
Neem (Azadiracta indica L.) is widely used and well known tree and seed extracts and oils are commonly used to control the insects and pathogens. A high content of Azadiractin, its active ingredient can be found both in the oil and in the extract (Mordue and Nisbet, 2002). Govindachari et al. (1998) also studied the antifungal activity of neem oil towards Drechslera oryzae, Fusarium oxysporum and Alternaria tenuis. Neem oil yields various acids, sulphur, etc. Meliantiol and azadiractin are obtained from seeds and decatylimbin also contains quecetin and sitosterol. The fungicidal spectrum of Azadirachta indica has been attributed to azadiractin which belongs to C25 terpenoides (Subramanian and Srinivasa Pai, 1953). In tomato crops, neem oil and extract have been used to control of white flies (Bemisia tabaci) (Kumar and Poehling, 2006), nematodes, fungi (Abbasi et al., 2005) and also P. infestans (Rani et al., 2006). The present investigation of various botanicals inhibiting the growth of A. alternata is in line with the earlier findings (Amaresh, 2000; Singh and Majumdar, 2001; Rao, 2006; Pramod Kumar, 2007).
Mohan et al. (2002) stated that palmarosa oil at 0.1% and 3.0% to control the onion leaf blight disease. Alice (1984) also reported that A. calamus rhizome extract (10%) significantly reduced the blight incited by A. brassicae in field. The highest disease intensity was recorded in case of T. viride sprayed plots (50.33) and therefore this was the least effective against the disease.
In summary, spraying of neem oil was the most effective plant products for control of onion leaf blight disease causing Alternaria alternata. However, the extract should be used on appropriate concentration that is non toxic to the host plant bulbs.
The authors gratefully acknowledge Tamil Nadu Agricultural University Coimbatore-3 for doing research work.
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