Authors
S.
Sen
and
R.
Acharya
,
Molecular and Applied Mycology and Plant Pathology Laboratory, Department of Botany, University of Calcutta, India
;
A.
Saha
,
Department of Botany, University of North Bengal, Raja Rammohanpur, Darjeeling, West Bengal, India
; and
K.
Acharya
,
Molecular and Applied Mycology and Plant Pathology Lab. Department of Botany, University of Calcutta, India
Cymbidium spp. is an orchid of great horticultural value cultivated extensively in Eastern Himalaya, India. Since 1995, growers have experienced huge crop losses in every monsoon month because of pseudobulb rot. Pseudobulbs initially turned soft and pulpy followed by oozing of a dark brown liquid with a foul odor (early phase). With increasing severity, the bulbs and roots lose weight as the internal tissues gradually disintegrate (middle phase). Finally, the bulb becomes hollow, fibrous, and dry causing death of the plant (later phase). Surveys from 2002 to 2005 showed that disease incidence ranged from 60 to 100%. Rotted tissue was plated on nutrient agar and potato dextrose agar media. Three organisms were consistently isolated from 50 samples collected from 30 different localities. They were identified as Erwinia carotovora (2), Fusarium oxysporum (3), and Mucor hiemalis f. sp. hiemalis (1) and were predominant at the earlier, middle, and later stages of disease, respectively. Identifications were further confirmed by the Agricultural Research Institute (ARI), Pune, India. Pseudobulbs were surface sterilized with 0.5% sodium hypochlorite for 1 min, washed by sterile distilled water, and dipped separately into three different spore/cell suspensions (105 CFU/ml) for 1 min. Another set of sterilized bulbs was dipped first into E. carotovora, then into F. oxysporum 12 days later, and then into M. hiemalis f. sp. hiemalis 15 days after the second dip. For the control set, bulbs were dipped into sterile distilled water. Samples were incubated aseptically at 20°C with a relative humidity of 80%, and all inoculated bulbs were evaluated for disease 47 days after the first inoculation. When samples were inoculated separately, E. carotovora exhibited maximum (70%) tissue disintegration followed by F. oxysporum (30%) and M. hiemalis f. sp. hiemalis (10%), but none of the individual pathogens caused 100% tissue disintegration. Complete destruction was observed after 47 days of first inoculation when these three pathogens were inoculated consecutively according to their serial occurrence. It is an interesting report on host-pathogen combination as three pathogens act in sequence toward ultimate demolition of the host. We report this rot as a synergistic activity of three pathogens to cause an uncontrolled epidemic disease of Cymbidium spp.
References: (1) J. C. Gilman. Page 37 in: A Manual of Soil Fungi. Iowa State College Press. Ames, IA, 1945. (2) J. G. Holt. Page 469 in: Bergey's Manual of Systematic Bacteriology. Vol. I. Williams and Wilkins. Baltimore/London, 1984, (3) C. V. Subramaniam. Page 657 in: Hyphomycetes. Indian Council of Agricultural Research (ICAR). New Delhi, 1971.
