ReviewAn insight into the problem of bacterial wilt in Capsicum spp. With special reference to India
Introduction
The Capsicum genus has a rich diversity and there are 38 accepted species. Of these, there are five major cultivated species viz., Capsicum annuum, Capsicum chinense, Capsicum baccatum, Capsicum frutescens and Capsicum pubescens (Chhapekar et al., 2016). Hot pepper (Capsicum annuum var. annuum L.) and sweet pepper (Capsicum annuum L. var. grossum Sendt.) are the two species which are grown at several locations in India and consumed as spice and vegetable crop, respectively. Peppers, both hot and sweet are native of Mexico while India and Guatemala are considered as the secondary centre of diversity for hot and sweet pepper, respectively (Bukasov, 1930; Kraft et al., 2014).
In the world, hot and sweet peppers (green) are cultivated over an area of 1.9 million hectares with a production of 36.77 million tonnes. India is the largest producer, consumer and exporter of hot pepper in the world. In 2018, the area under hot and sweet pepper (green) in India was 9634 ha with a production of 79,668 tonnes (FAO, 2018). India shares 40% of the total world's hot pepper production and top ranked in terms of international trade and exports 18.40% (red chilli) of its total production. India exports excellent amount of chilli to Malaysia (~30%) followed by Bangladesh (~20%), Sri Lanka (15%), USA (9%), and UAE (8%) [FAO, 2014].
There are several diseases like bacterial wilt, anthracnose, wet rot, Phytophthora rot, leaf curl and powdery mildew which infect pepper crop worldwide (Dhaliwal, 2015). However, bacterial wilt is one of the most important diseases which is widely distributed among the pepper growing areas of the world (Hayward, 1991; Denny, 2006). The disease is most commonly seen in the coastal regions and foothills of India. The pathogen Ralstonia solanacearum Smith (Yabuuchi et al., 1995) earlier known as Pseudomonas solanacearum Smith or Burkholderia solanacearum Smith causing bacterial wilt disease of Capsicum spp., is known to infect several agricultural crops. It is difficult to assess the loss in the yield due to bacterial wilt as it varies from crop to crop, cultivar, climate, soil type, the presence of root-knot nematodes and strain pathogenicity. Bacterial wilt is ranked as one of the most important plant diseases in the entire world as it causes cent per cent yield losses in solanaceous vegetables (Jyothi et al., 2012).
The pathogen enters the plant through root wounds or penetrate the plant at sites of secondary root emergence and spreads to the plant through the vascular system. R. solanacearum remains in the deeper layers of soil, grows endophytically, moves with water and also have an association with the weeds (Wang and Lin, 2005). Due to soil-borne nature of the pathogen, the commonly used management practices like soil treatments, crop rotation and adjustment of planting time are not much successful. Hence, the phenomenon of host plant resistance is effective, economic, long-lasting and environmentally safe for soil born diseases. Due to the lack of compiled information with regard to bacterial wilt in peppers in India, this review article has been planned to give an insight into the various aspects of this disease.
Section snippets
History and distribution of the disease in India
The bacterial wilt of solanaceous crops is a world-wide crisis and the origin of this disease is uncertain and is mislaid into ancient times. The record of Pseudomonas solanacearum is not clear and it is difficult to say whether this pathogen arose on a single continent or it appeared separate on different locations on the related ancestors of modern plants (Buddenhagen and Kelman, 1964). According to reports, the disease first occurred in the virgin soils of Indonesia, Central America and
Structure and classification of pathogen
The pathogen Ralstonia solanacearum is an aerobic, gram-negative bacteria with two membranous structure. The cell wall consists of peptidoglycan. It is a non-spore-forming, rod-shaped with the dimension of 0.5–0.7 μm × 1.5–2.0 μm having a single polar flagellum (Sneath et al., 1986). The genome size is of 5.8 megabases (Mb) which are arranged into two replicons: (a) 3.7-Mb chromosome and (b) 2.1-Mb megaplasmid (Salanoubat et al., 2002). Many proteins are encoded by the genome which determines
Symptoms and disease incidence
The disease occurs in the scattered pattern in the field. The first signs of the disease are described as the wilting of new leaves of the plant mostly during the hottest part of the day and behaves normally during cooler hours of the day. Under high temperature and rainfall conditions, complete plant shows wilting and suddenly the plant dies (Cerkauskas, 2004). However, under unfavourable climatic conditions, the disease develops slowly and initially the plant shows stunted growth. The first
Detection of the bacterium
Detection of the bacterium is done by using certain culture-based, microbiological and molecular methods. The stem-streaming test, use of semi-selective (SMSA) and Triphenyl Tetrazolium Chloride (TZC) medium (Denny, 2006), immunodiagnostic assays, fatty acid methyl ester analysis, carbon source analysis (BIOLOG™ kits), Enzyme-Linked Immuno Sorbent Assay (ELISA), and nucleic acid-based methods like multiplex-polymerase chain reaction amplification (Multiplex-PCR) (Tran et al., 2016) and remote
Host range and transmission
The bacterium has a very broad range of hosts and infects about 200 plant species belonging to 54 families (Denny, 2006). Majority of the hosts are dicot plants with few exceptions like banana and plantains. Most of the economically important host plants are found in Solanaceae or nightshade family (Stevenson et al., 2001). It is reported to infect vegetables, fruits, ornamental , medicinal, aromatic as well as many weed plants (Table 1).
The infected plant material especially the propagative
Integrated pest management strategy
RSSC causing bacterial wilt disease in Capsicum spp. is considered as the second most important bacterial pathogen. The destructive outbursts carried out by the emergence of new races of the bacterium have put a threat to the sustainable pepper production and are of serious concern to other crops throughout the world. Being soil-borne pathogen, it cannot be controlled by using any single strategy. Therefore, an integrated approach utilizing physical, cultural, biological and chemical methods
Conclusion and future prospects
Bacterial wilt is an important soil-borne disease of all solanaceous crops. The pathogen is distributed throughout the pepper growing areas of the world and causes huge losses to the crop. The management of this disease is very difficult as it is both soil and seed borne besides also transmit through contaminated water and weed hosts (Wang and Lin, 2005). Several control methods using physical, cultural, biological and chemical means have been tried but effective methods have not been suggested
Declaration of competing interest
The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors are highly thankful to the National Coordinator, NAHEP-ICAR, New Delhi and Project Coordinator, CAAST-NAHEP, ICAR, New Delhi for their encouragement to prepare this manuscript. Authors are also thankful to Dr. Prajya Mishra, Principal, SCVB Government College Palampur, Himachal Pradesh, India for editing the manuscript.
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