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Fungi Causing Rots in Onions at Storage and Market

M.A.M. Ara
Department of Plant Pathology, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

M.L. Khatun
Department of Plant Pathology, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

M. Ashrafuzzaman
Department of Plant Pathology, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

Abstract/ Executive Summary:

Five varieties of diseased onion samples, four local (Taherpuri, Faridpuri, Kalashnagari, and Zitka) and one Indian (Pusa Red) were collected from different markets of Mymensingh, to isolate and identify the associated fungi. Five different fungal species belonging to three genera were repeatedly isolated. Healthy looking and diseased onions were kept at room temperature. The population of associated fungi increased proportionately with the duration of storage period. The isolated fungi were Aspergillus niger, Aspergillus flavus, Penicillium spp., Fusarium oxysporum and Fusarium moniliforme. Healthy onion bulbs were inoculated (with injury and without injury) with these fungi. Individual fungi produced distinguishable symptom of disease in incubation chamber at 22-23ºC temperature. After 30 days of inoculation, bulbs showed comparatively low amount of disease development than those at 60 and 90 days. Maximum disease development was found in the variety Pusa Red (with injury) and it was minimum in Zitka variety (without injury).

Keywords: Fungi, Onion, Disease development

Location: Department of Plant Pathology, Bangladesh Agricultural University

Start Date: 00-00-2008

End Date: 00-00-2008

Program Area: Pest Management

Commodity: Onion

Objectives:

1. To identify the pathogenic fungal isolates obtained from the naturally infected bulbs and varietal differences in susceptibility of different onion varieties.

Methodology:

Collection of samples - Onion samples 1Kg/20 Kg bags were collected randomly from retailer shops of different areas of Mymensingh. The samples were pooled together to form one composite sample for a market. The onions were closely inspected and sorted out depending symptoms of infection. Isolation - Isolation of associated fungi with the symptom bearing onions was done by employing usual tissue planting method on both PDA medium and moist chamber maintaining asceptic conditions incubating at room temperature for 3-7 days. Identification of fungi - Each diseased onion sample (with petridish) was observed under a steriomicroscope at 10X and 25X magnifications in order to record the fungi. Most of the associated microorganisms were detected by observing their growth characters on the incubated diseased onion on blotter and PDA following the keys outlined by Barnett. For confirmation temporary slides were prepared from the pure cultured fungal colony and observed under a compound microscope, then identifie. Identified pure cultures were indicated as isolates. Pathogenicity test - Pathogenicity tests of the isolates were done to be certain about the disease causing ability of the organism(s) by inoculating the isolates in healthy onions by with or without pricking and then incubating at 22-23º C and observed for 7- 90 days. Incase of disease development symptoms were observed and pathogens were re-isolated and identified satisfying Koch’s postulates. The % disease developments were recorded in the test onions by eye estimation.

Source of Information: J. Bangladesh Agril. Univ. 6(2): 245–251, 2008; ISSN 1810-3030

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Fungi associated with storage diseases of marketed onions belonging to cultivars Taherpuri, Faridpuri, Kalashnagari, Zitka and Pusa Red, these were isolated, identified and confirmed through infectivity test. Five species of 3 genera of fungi were isolated and identified from collected diseased samples. These were Aspergillus niger, Aspergillus flavus, Penicillium spp., Fusarium oxysporum and Fusarium moniliforme. Among these fungi Aspergillus niger, Aspergillus flavus and Penicillium spp. were more predominant in stored onion bulbs, whereas Fusarium oxysporum and Fusarium moniliforme were occasionally found in stored onion bulbs. Aspergillus niger is the cause of black mould of onion, which is primarily a post harvest disease. Fungi (Aspergillus niger and Penicillium spp.) were isolated from bulbs that showed typical disease symptoms. Percent disease development was examined for five onion cultivars through artificial inoculation to healthy onion bulbs through with and without injury. The test bulbs were observed for 90 days. Inoculation was done separately with 5 fungi viz. Aspergillus niger, A. flavus, Penicillium spp., Fusarium oxysporum and F. moniliforme. Observation of disease development was made at 30 days after inoculation and then at 30 days interval up to 90 days. Highest disease development was found after 90 days of inoculation than other two shorter durations in all the test varieties of onion. The disease development was significantly different among the varieties. The cultivar Pusa Red was found more susceptible to disease than the bulbs of other cultivars and the cultivar Zitka was found rather resistant. The varietal susceptibility of 11 onion cultivars to Fusarium oxysporum f.sp. cepae was investigated in the field, at harvesting and after storage. This finding emphasize the importance of injury to the bulb, incurred before or during storage and this was the key factor in storage disease development causing storage loss. The results of this present study indicate that the fungal diseases (black mould rot, blue mould rot and Fusarium bulb rot are serious problem of onion bulb in Bangladesh and loss due to these diseases may be minimized by taking measures in the field, during transit and in storage so that the bulbs are not injured.

Knowledge Management: Journal