Identification of Causative Pathogen of Flower Bud Wilt Disease in Dendrobium sp. and In-vitro Growth Inhibition by Medicinal Plant Extracts

Aims: The aim of this study was to isolate and identify the causal agent of wilting of flower buds of Dendrobium orchid in Sri Lanka and to search for environmental friendly control measures for flower bud wilt disease as a possible alternative to synthetic fungicides.
Study Design: The experiment was conducted using a completely randomized design.
Place and Duration of Study: Faculty of Agricultural Sciences, Sabaragamuwa University of Sri Lanka from June 2013 to May 2014.
Methodology: The causal fungus isolated from the diseases plant was identified based on their morphology, pathogenicity and ribosomal DNA spacer sequences. The media amended with methanol plant extracts of different concentrations (25%, 12.5%, 6.25%, 3.125%) of tested medicinal plants were inoculated with mycelia discs (4 mm diameter) taken from the advancing edges of 10 day-old pure cultures ofisolated fungus andincubated for 7 days. The fungicidal activities of MIC of methanol extracts were studied after immersing fungal blocks in solution for 1, 3, 6, 12 and 24 hours respectively. The media amended with methanol and recommended fungicide were considered as negative and positive control respectively.
Results: Based on morphological characteristics and rDNA spacer sequences, isolates were identified as F. proliferatum. This is the first record of flower bud wilt disease in Dendrobium sp. in Sri Lanka. Results showed that radial growth of fungus was significantly impaired (P<0.05.) by the addition of the extracts in the culture medium used. The test fungus differed in their reaction to the different extracts but on the whole, growth inhibition increased with the concentration of each extract. The most active extracts, 25% methanol extracts from sweet flag shows a marked effect with inhibition values of 77% against F. proliferatum whereas those from wild basil inhibited the growth by 55%. Out of four plants extract screened, sweet flag showed more than 80% fungal inhibition after 12 hour immersion and other extracts could not exceed 60% inhibition after any exposure time.
Conclusion: The present study concludes disease found in Dendrobium orchid was caused by F. proliferatum and Methanol extracts of Sweet flag contain antifungal constituents for the control of F. proliferatum.