Review on Virus Diseases of Okra
摘要
Okra (Abelmoschus esculentus L. Moench) belonging to the family Malvaceae is an important and extensively consumed vegetable crop of the tropical and subtropical region of the world. India is the largest producer of okra in world, followed by Nigeria, Sudan, Pakistan, Cameroon, Iraq, and Ghana. The crop is attacked by a number of biotic stresses, such as fungi, bacteria, phytoplasma, viruses, nematodes, and insect pests. Among the biotic stresses, viruses belonging to Begomovirus, Tymovirus, Ilarvirus, Potyvirus, and Tospovirus are major production constraints. The yield loss of vegetable due to viruses has been estimated up to 20–30%, and may increase up to 80–90% in case of a severe infestation at the early stage of infection. Globally, okra is known to be susceptible to at least 34 begomoviruses; of which, yellow vein mosaic disease (YVMD), okra enation leaf curl disease (OELCD) in India, okra leaf curl disease in Africa and the middle east, and okra yellow mottle crinkle disease in Mexico and Brazil are most severely affecting its production in terms of yield and fruit quality. Begomoviruses infecting okra are known to be associated with satellite DNA, beta satellites (Tolecusatellidae), or alpha satellites (Alphasatellidae). Beta satellites have been involved in pathogenicity; however, alpha satellites have not been shown to modulate the symptoms of the diseases. The wide diversity among begomoviruses associated with mixed infections is supposedly assisting in recombination and pseudorecombination events, leading to the frequent emergence of novel begomoviruses, having devastating effects on the okra. Recombination has played a significant role in the evolution of begomoviruses, and the sustainability of okra production is at potential risk in many okra growing countries. The tobacco streak virus, a pollen-borne emerging virus, is currently of significance to okra production and quality of fruits. Rapid and accurate identification of viruses is essential in order to adopt appropriate disease control strategies. For the diagnosis of begomoviruses infecting okra, PCR based daignosis using degenerate primers or gene specific primers need to be applied. Another amplification method “rolling circle amplification” (RCA) has been used to amplify multiple helper viruses as well as their recombinants. The current approaches of loop-mediated isothermal amplification polymerase chain reaction (LAMP) assay and next generation sequencing (NGS) are currently powerful techiniques for multiple virus detection in okra. A number of short-term strategies were developed to control this disease by reducing the vector population in the field. Long-term strategies involve the development of resistant okra cultivars either through conventional or non-conventional means. Durable resistance to begomoviruses poses a serious challenge to both breeders and pathologists as these viruses are highly diverse and constantly generate new forms via recombination. Additional sources of resistance to these viral diseases may be available in wild species like A. manihot, A. crinitus, A. angulosus, including certain landraces of A. tetraphyllus, marker-assisted recurrent selection (MARS), and genome-wide selection (GWS) to be effectively used for broad-spectrum resistance building.