IDENTIFICATION OF GARLIC LEAF BLIGHT: FIRST RECORD IN BANGLADESH
Recently, a new disease appears on garlic plants in Bangladesh, showing leaf blight like symptoms killing the entire leaves progressively from the tip. An investigation was conducted to identify the disease and its causal pathogen. The disease was identified as Stemphylium blight which was confirmed through Koch’s postulate. This is the first record of the disease in Bangladesh.
Garlic (Allium sativum L.) is the second most widely consumed allium in the world. It is one of the major spices in Bangladesh. In the country, annual garlic production area is about 66,000 ha, where the production is 430000 metric tons and at an average yield of 6.55 mt/ha during 2013-2014 crop season (Krishi Diary, 2015). Total production as well as per hectare yield of garlic in Bangladesh is very low compared to other garlic producing countries like China where the production is 59 million metric tons annually with an average yield of 12.3 mt/ha (Krishi Diary, 2015).
In Bangladesh, recently, a new disease appears on garlic plants, showing leaf blight like symptoms killing the leaves progressively from the tip (Fig. 1). It is one of the major diseases of garlic in the country. It seems to be responsible for low production and yield of the crop. In 2015, garlic plants in several commercial fields of Bogra, Natore, Rajshahi and Faridpur districts, and at the research field of Spices Research Centre of Bangladesh Agricultural Research Institute (BARI), Bogra exhibited symptoms of the blight on the garlic leaves. Early symptoms are observed as white spots, 1-3 mm diameter, which enlarges to produce sunken purple lesions, extending whole leaves withered as described by Zheng et al. (2008) from China. In China leaf blight of garlic is caused by different species of Stemphylium causing severe damage of the crop. However, the causal agent of the leaf blight like disease appears in Bangladesh has not yet been identified. The present piece of research was undertaken to identify the causal pathogen of garlic leaf blight disease in the country.
The experiment was conducted at Spices Research Centre, BARI, Bogra, Bangladesh during 2014-15 to identify the causal pathogen of garlic leaf blight disease. To isolate the causal fungus associated with garlic leaf blight like symptoms infected leaves of garlic (Fig. 1) were collected from the field and cut into small pieces. The pieces were surface sterilized with 0.5% sodium hypochlorite solution for 1 minute, rinsed with sterilized distilled water for three times, soaked with sterilized filter paper and placed on Petri dishes containing sterilized moist Whitman filter paper No. 2. The leaf pieces in Petri dishes were incubated at 24 ±1C for 7 days at room te4mperature (28±2C). After incubation, profuse sporulation of the causal fungus developed on the incubated leaf pieces as observed under a stereo dissecting microscope and identified as Stemphylium sp. based on morphological characteristics (Ellis 1971).
To confirm the causal of the fungus of the disease, Koch’s postulate was performed by inoculating healthy garlic plants with spore suspension of the isolated fungus. Spores of Stemphylium sp. grown in Petri dishes were collected and suspended in 100 ml beaker containing sterile distilled water (Fig. 2). The concentration of spore was more than 70 per microscopic field. Three months old apparently healthy garlic plants grown in earthen pots (Fi9g. 3) were inoculated with the spore suspensions using a camel hair brush under sterile conditions. After inoculation, plants were covered with polyethylene bags to maintain proper humid condition. After 48 hr, the bags were removed and plants were kept under field conditions until symptoms appeared. The blight symptoms on inoculated plants (Fig. 4) were observed after 4 days of inoculation which was similar to the blight symptom appeared in the field (Fig. 1). The fungus was reisolated from lesions of the inoculated plants to complete Koch’s postulate.
The results of the Koch’s postulate confirmed that leaf blight of garlic occurs in Bangladesh which is caused by Stemphylium sp. This is the first record of Stemphylium leaf blight disease of garlic in the country. However, Rao and Pavgi (1975) first reported the disease from India. Shishkoff and Lorbeer (1987, 1989) also reported that leaf blight of garlic is caused by Stemphylium in the USA. Available reports reveal that the disease is widespread in Asia (Rao and Pavgi 1975, Zeng et al. 2008). Recently it has been reported on garlic from South Africa (Aveling and Naude 1992), Spain (Melero 1993) Brazil (Mehta 1998) and Australia (Suheri and Price 2000).
Aveling, T. A. S. and Naude, S. P. 1992. First report of Stemphylium vesicarium on garlic in South Africa. Pl. Dis. 76:426.
Ellis, M. B. 1971. Dematiaceous Hypomycete. Kew, London, UK. Commonwealth Mycological Institute, Kew, England.
Krishi Diary. 2015. Agriculture Information Service, Khamarbari, Farmgate, Dhaka-1215. p. 15.
Mehta, Y. R. 1998. Severe outbreak of Stemphylium leaf blight, a new disease of cotton in Brazil. Pl. Dis., 82: 333–336.
Melero, E. V. J. M. (1993). First report in Spain of two leaf spots of garlic caused by Stemphylium vesicarium. Pl. Dis. 77:952.
Rao, N. N., and Pavgi, M.S. (1975). Stemphylium leaf blight of onion. Mycopathologica 56:113–118.
Shishkoff, N. and Lorbeer, J.W. 1987. A new leaf disease of onion in New York caused by Stemphylium vesicarium. Phytopathology 77: 1713
Shishkoff, N. and Lorbeer, J.W. 1989. Etiology of Stemphylium leaf blight of onion. Phytopathology 79: 301-304
Suheri, H., and Price, T.V. 2000. Stemphylium leaf blight of garlic (Allium sativum) in Australia. Australian. Plant Path. 29:192–199.
Zheng, L., Huang, J. B., and Hsiang, T. 2008. First report of leaf blight of garlic (Allium sativum) caused by Stemphylium solani in China. Plant Pathol. 57: 380.