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Carbon Source dependent - anaerobic Soil Disinfestation (ASD) Mitigates the Sclerotial Germination of Sclerotinia Sclerotiorum
(Tropical Plant Pathology (2020), 2020) Mahalingam, T.; Rajapakse, C.S.K.; Somachandra, K.P.; Attanayake, R.N.
Though Sclerotinia sclerotiorum IS a well-studied plant pathogen that causes significant economic damage worldwide, sustain able and environmental friendly control methods are difficult to establish due to it wide host range, cosmopolitan distribution and production of recalcitrant structures that can survive in soil for a long time. The pathogen was found causing a severe disease incidence on cabbage in 2016 in Sri Lanka. It was hypothesized that a) isolates of the recent disease outbreak display cross resistance to commonly applied fungicides and b) carbon (C) source supplemented Anaerobic Soil Disinfestation (ASD) is effective in mitigating the germination of sclerotia. In vitro fungicide sensitivity assays showed large variation in mycelial growth inhibition indicating high adaptability of the population towards environmental fluctuations and management practices. Signatures of cross resistance were evident. ASD was carried out using cabbage (Brassica oleracea) and leek (Allium ampeloprasum) cull piles, durian (Durio zibethinus) peels and grass cuttings (Axonopus compressus) as C sources and determined the sclerotial viability. Cabbage and leek cull piles at rates of 60—100 mg/g soil completely inhibited sclerotial germination. Maintaining anaerobic condition along with C source amendments was found to be a critical step in mitigating the sclerotial germination. GC-MS analysis of the volatiles of cabbage leaves, leeks and durian further confirmed the presence of various bioactive compounds with potential antifungal activity. Therefore, in addition to elevated microbial activity in treatments, the volatiles of C sources may have helped mitigating sclerotial germination.
Phenotypic variation of cabbage white mold pathogen, Sclerotinia sclerotiorum in the upcountry commercial cabbage fields in Sri Lanka
(Journal of the National Science Foundation of Sri Lanka 2018 46 (2): 159 – 164, DOI: http://dx.doi.org/10.4038/jnsfsr.v46i2.8416, 2018) Mahalingam, T.; Guruge, B.M.A.; Somachandra., K.P.; Jayasekara, E.A.E.S.S.; Rajapakse, C.S.K.; Attanayake, R.N.
Sclerotinia sclerotiorum is a cosmopolitan, necrotrophic and soil borne plant pathogen, capable of infecting many economically important crops. Although it is a well-studied pathogen elsewhere, there is no research conducted in Sri Lanka. The objectives of the current research were to study the phenotypic variations among isolates within the pathogen population in the upcountry commercial cabbage fields in Sri Lanka and to determine their sensitivity to a commonly applied fungicide, Mancozeb. Forty-six isolates obtained from infected cabbage heads were identified as S. sclerotionun based on its unique morphological characteristics. Colony diameter was measured on potato dextrose agar (PDA) plates at 23 °C and mycelia' colour was observed after two-week incubation. All the isolates were grown in pH indicator media to determine their add producing abilities. Sensitivity of the isolates to Mancozeb was assayed in vitro. There was a significant difference in colony growth among the isolates and sensitivity to Mancozeb. Two isolates showed abnormal mycelia' growth characteristics; a slower growth rate on PDA and reduced pathogenicity on cabbage leaves. Two levels of mycelia' pigmentation were observed on PDA. All the isolates were acid producers, inferring that all of them were pathogenic. In summary, the upcountry S. sclerotiorton pathogen population was found to be diverse for selected phenotypic traits, especially for mycelia( growth, and fungicide sensitivities inferring that the pathogen population is capable of adapting to changing environmental conditions. Therefore, disease management will be challenging if an epidemic occurs, and it is recommended that a proper management system should be identified before further expansion of upcountry cabbage cultivation.
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.

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