Browsing by Author "Somachandra, K. P."

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    Effect of the fungicide manganese ethylene-bis-dithiocarbamate on in vitro growth of Agrobacterium tumefaciens
    (4th International Research Symposium on Pure and Applied Sciences, Faculty of Science, University of Kelaniya, Sri Lanka, 2019) Somarathna, G. M. T. K.; Somachandra, K. P.; Attanayake, R. N.
    Agrobacterium is a soil borne, Gram negative, rod shaped, motile and aerobic bacterium causing tumors on a wide range of plant species. Out of many soil inhabitant Agrobacterium species, only the pathogenic strains of Agrobacterium tumefaciens can cause the crown gall disease. Although antibiotics and growth promoters have been recommended in other countries, no proper control measures have been developed and allowed in Sri Lanka for bacterial infections in plants. The objectives of this research were to identify the virulence strains of A. tumefaciens using DNA sequence data, to determine whether commonly used antibiotics have an inhibitory activity against A. tumefaciens isolated from soil and also to find a cheap control measure based on the fact that certain fungicides have antibacterial effects. For molecular characterization, PCR was carried out using Agrobacterium specific primers targeting virD2 gene. Antibiotic sensitivity was determined by disc diffusion method using Kirby-Bauer technique. Concentration series of 25, 30 and 35 μg/mL of kanamycin and chloramphenicol and 5, 10, 15 μg/mL of rifampicin were prepared. Fungicide assay was also conducted by disc diffusion method using the fungicides Mancozeb (Manganese ethylene-bis-dithiocarbamate) at 0.20%, 0.25%, 0.30% (w/v) and Carbendazim (Methyl benzimidazole-2-yl carbamate) at 0.025%, 0.05%, 0.10% (w/v) concentrations. Plates were incubated at 28 °C for 48 hours. The soil isolates were confirmed to be A. tumefaciens from the sequencing results of virD2 region. In antibiotic sensitivity test all isolates were unable to produce clear zones showing that they were resistant to the three different concentrations of all three antibiotics used. Carbendazim could not inhibit the growth of A. tumefaciens isolated from soil at all three concentrations tested. However, the fungicide Mancozeb was able to inhibit the growth of the pathogen in all tested concentrations. According to the results it was concluded that the local isolates of A. tumefaciens is resistant to the tested antibiotics and therefore, cannot be used as a control measure. While Carbendazim is not helpful in controlling the growth of A. tumefaciens, Mancozeb has a potential to mitigate the in vitro growth of the pathogen
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    Evaluation of fungicide resistance of Sclerotinia sclerotiorum causing cabbage white mold in Sri Lanka.
    (International Research Symposium on Pure and Applied Sciences, 2017 Faculty of Science, University of Kelaniya, Sri Lanka., 2017) Mahalingam, T.; Somachandra, K. P.; Rajapakse, C. S. K.; Attanayake, R. N.
    Incidence of severe white mold disease was extensively observed in upcountry commercial cabbage fields of Sri Lanka in early 2014. The pathogen was identified as Sclerotinia sclerotiorum, a nectrophic soil borne fungus that can infect more than 400 plant species worldwide. Chemical control is the most effective method available to date due to lack of complete resistant cabbage varieties. However, improper fungicide applications can lead to the development of fungicide resistant isolates. In Sri Lankan small scale upcountry vegetable production system, more than one crop species is grown per year and most of these crops serve as hosts for the pathogen S. sclerotiorum. Frequent fungicide and pesticide application is also a common practice. Therefore, it was hypothesized that resistant S. sclerotiorum isolates against commonly used fungicides are present in Sri Lankan pathogen population. Objectives of this study were to determine the variation in insensitivities of the pathogen against commonly used fungicides in upcountry vegetable production systems, to search for the presence of resistant isolates and to determine if there are signals of positive cross resistance for the fungicides in Sri Lanka. Pathogen population was tested against three selected fungicides, carbendazim and thiophanate methyl (benzimidazole fungicides) and tebuconazole (triazole group fungicide) using in vitro plate assay. Firstly, a discriminatory concentration, which showed the highest variance in percentage inhibition of mycelial growth of a few randomly selected S. sclerotiorum isolates were determined as 0.25 μg/mL for carbendazim and 0.5 μg/mL for both thiophanate methyl and tebuconazole. Later, all of the 34 isolates were assayed for their insensitivities using fungicide amended PDA at the above concentrations with three replicates per isolate and fungicide unamended PDA plates were used as the control. Colony diameters were measured after 36 hr of incubation and percentage inhibition was calculated. While only 12% of the isolates showed > 50% inhibition for thiophanate methyl, the majority of the isolates (63 %) showed > 50% inhibition for the fungicide, carbendazim. The highest variance observed in percent inhibition of the pathogen population for carbendazim indicated that the pathogen population has the potential to adjust to the challenging environments, in this case fungicide applications. Based on the in-vitro plate assay, thiophanate methyl was found to be the least effective fungicide than the other two. Twelve isolates exhibited <50% inhibition to both carbendazim and thiophanate methyl and nine isolates showed <50% inhibition to all three fungicides. Results indicated that there is a potential to develop cross resistance and hence farmers should be prudent in selecting the fungicide regime for cabbage cultivation. Molecular basis of the resistance is yet to be elucidated.