TRANSMISSION OF COWPEA APHID BORNE MOSAIC VIRUS ...

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CHAPTER TWO 2.0 LITERATURE REVIEW 2.1 Diseases of Cowpea Viral diseases of cowpea (Vigna unguiculata), varies in terms of the severity depending on host cultivar and virus strain. (Aboul-Ata et al 1982). Diseased cowpea plants show variable amounts of dark green vein banding or interveinal chlorosis, leaf distortion, blistering and stunting (Van Kammen, 1973). Viruses that may be related cause mosaic diseases of adzuki bean (Phaseolus angu/aris) and asparagus bean (Vsesquipedalis). (Adegbite, and Amusa, 2008). 2.1.1 Incidence of viral diseases on cultivated cowpea Cowpea is greatly attacked by wide array of diseases of biological origin especially viruses which cause devastating effects and are a real constraint to increased yield of cowpea in several countries (Brunt,et el 1990). Of all the known viruses of cowpea, cowpea aphid-borne mosaic virus is responsible for the major viral disease of cowpea in Nigeria (Shoyinka et aI., 1997). In reference to a three year survey for the incidence and distribution of cowpea viruses, detected viruses in 390 out of 649 cowpea collected from all agro ecological zones in Nigeria using ELISA, CABMV had the highest incidence and was the most prevalent of all the virus detected, (Shoyinka et aI., 1997). 2.1.2 Symptoms of viral diseases of cowpea The majority of the viral diseases of cowpea lead to overall stunting, reduction in leaf size, mottling, mosaic, leaf chlorosis, leaf distortion, leaf curling, vein clearing, necrotic local lesion and death (Akinjogunla, 2005). Typical symptoms of viral infections are stunting, mottling or foliage and deformed pod.
2.1.3 Cowpea aphid borne mosaic virus (CABMV) Cowpea aphid borne mosaic virus (CABMV), a member of the Potyvirus genus, a distinctive virus with flexuous filamentous particles c. 750 nm long. It is seed-borne in cowpea, has a wide experimental host range, is transmitted by several common species of aphid, and occurs in many countries where cowpea is grown, (Bock, 1974) It causes a severe mosaic of cowpea (Vigna unguiculata), the severity depending on host cultivar and virus strain. Diseased cowpea plants show variable amounts of dark green vein banding or interveinal chlorosis, leaf distortion, blistering and stunting, (Thottapilly and Rossel, 1992) Viruses that may be related cause mosaic diseases of adzuki bean (Phaseolus angularis) and asparagus bean (V. sesquipedalis), (Tsuchizaki et al 1970). It occurs in Africa (Kenya, Uganda and Nigeria), Europe (Italy, and probably Rumania), and Asia (India, Iran, Japan, and probably China). Viruses that are probably related occur in the USA (Florida) and in the south-west Pacific area, (Bock et aI1974). It can cause a yield loss of 13 - 87% under field conditions depending upon crop susceptibility, virus strain and the environmental conditions, (Bashir,et el,1996). CABMV has spread world-wide through the exchange of virus-infected germplasm material. The virus-infected seed provides the initial inoculums and aphids are responsible for the secondary spread of the disease under field conditions, (Taiwo, 2003). The virus symptoms vary with the cowpea genotype and virus strain. Excellent sources of resistance are available for the breeding of resistant cultivars. Resistance in cowpea is conferred by either a dominant or a recessive gene. (Thottapilly and Rossel, 1992). 2.2 Spread of viral Diseases on Cultivated Cowpea Viruses can be transmitted by insects, primarily Aphids, leafhoppers, and thrips, sometimes whiteflies can transmit viruses. Mites, fungi and nematodes can occasionally transmit viruses (Bock et el 1974). However, there are two types of viral transmission which could be persistent or nonpersistent. By a vector, the virus is transmitted in the stylet-borne, non-persistent manner by Aphis
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