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The nature and severity of the symptoms induced by CABMV vary with host cultivars, virus strain and the time of infection (Rossel and Thottappilly, 1985). Natural infection of cowpea causes various mosaics, mottling, interveinal chlorosis and vein-banding (Bock and Conti, 1974). The European (type) strain causes a severe distorting mosaic in cowpea (Lovisolo and Conti, 1966); the African (neotype) strain induces irregular angular broken mosaic; the African mild strain induces a very mild mottle with little or no effect on plant growth; the African vein-banding strain induces a broad dark-green vein-banding (Bock, 1973). The CABMV variant from Australia caused variable mosaic symptoms on cowpea (Mali and Thottappilly, 1986). In the inoculated plants of Vigna cylindrica and Vigna unguiculata, the vein-banding strain of CABMV reported from Nigeria induced vein prominence which was followed on the next trifoliate leaf at least by the characteristic vein-banding symptoms; the subsequent trifoliate leaves either developed vein-banding or mosaic. Systemic mosaic symptoms were induced in Vigna unguiculata cvs New Era, Arlington and Crimson. Varying degrees of susceptibility were exhibited by Vigna unguiculata cv. Ife Brown. Some infected plants were killed; those surviving were severely infected with necrosis of stem and leaves; some developed mild mosaic, while others developed no symptoms and the virus could not then be recovered. In Phaseolus vulgaris cv. LB64003 the virus induced mild to severe systemic mosaic that was accompanied by general stunting. In P. vulgaris cv. H65008, the virus induced local necrotic lesions on inoculated leaves, followed by systemic flecky mosaic, leaf drooping and the eventual death of the plants. Systemic chlorotic mottle was induced in Cajanus cajan.
CABMV symptoms observed on cowpea under field conditions were extremely variable. Factors such as genetic variability of cowpea cultivars grown and stage of growth at the time of infection, influence the type and severity of symptoms produced. Apart from the characteristic vein-banding symptoms that distinguish CABMV from other virus symptoms, conspicuous mild mosaic and mosaic mottle are sometimes observed on plants infected by CABMV under field conditions, depending upon the cowpea cultivars grown (Shoyinka et al., 1997).
Symptoms of the CABMV isolate from Iran on cowpea cv. Early Ramshorn were mosaic, leaf deformation, puckering and stunting (Kaiser and Mossahebi, 1975), whereas cowpea plants infected by the Moroccan isolate (CABMV-Mor) showed mosaic pattern, leaf bumping and distortion, and serious stunting, which resulted in extreme yield reduction (Fisher and Lockhart, 1976). Mazyad et al. (1981) observed severe mosaic on field-grown cowpea plants, whereas yellow mottling with slight distortion of leaflets was observed on plants from Egypt inoculated with CABMV. Bashir and Hampton (1996b) recorded variable symptoms when they tested 51 cowpea genotypes against seven different CABMV isolates. Each isolate induced a wide range of symptoms in plants of susceptible genotypes. The characteristic symptoms of CABMV appeared as vein-banding, interveinal chlorosis, distortion, blistering and stunting of leaves. Some isolates induced necrotic local lesions, sometimes but not always followed by systemic spread of the virus in the form of mosaic pattern. In their host test experiment (Bashir and Hampton, 1996b), CABMV-Mor induced necrotic lesions in the inoculated plants of germplasm accession PI 218123 followed by whole plant necrosis; these plants were therefore said to exhibit lethal susceptibility (LS). In some genotypes symptom expression was delayed. These genotypes remained symptomless for 2-3 weeks, and then developed mild to severe systemic mosaic at a later stage.
Williams (1975) observed widespread mottling, interveinal chlorosis and vein-banding when the field-grown cowpea plants were infected with the CABMV-Nigeria isolate. As the disease developed, leaf cupping occurred; later, leaves became further distorted and developed necrotic lesions. Infected plants remained stunted and bushy, and flowering was retarded or inhibited. Thottappilly and Rossel (1997) reported prominent mosaic, mottle and stunting due to CABMV in bambara groundnut (Vigna subterranea) in Nigeria. Recently Pappu et al. (1997) reported mosaic type symptoms induced in field-grown sesame (Sesamum indicum) plants by a CABMV-SES isolate from the USA.
CABMV isolates obtained from yellow passionfruit plants were able to systemically infect yellow passionfruit (P. edulis f. flavicarpa), Nicotiana. benthamiana, N. clevelandii, bean cv. Preto 153, and cowpea cvs. Pitiúba and Clay (Nascimento et al., 2006). Woodiness symptoms were reproduced in plants of yellow passionfruit (including the fruit), accompanied by severe mosaic and leaf distortion. Both cowpea cultivars displayed mild mosaic symptoms when infected by isolates PB-Alh, PB-Cnd, PE-Bcs1, and PE-Bcs2, and severe mosaic symptoms when infected by other isolates. The bean cultivar Preto 153 reacted in a similar way, except that isolates PE-Ptr and SE-Nps also induced mild symptoms.
Symptoms resulting from seedborne infection
Seedborne infection is expressed in the primary leaves which show vein-clearing, vein-yellowing, diffused chlorotic spots or patches, or an intense chlorosis (Phatak, 1974; Bashir, 1992). Later, in trifoliate leaves, the symptoms are usually more distinct and include vein-yellowing, or variable degrees of yellow mosaic with or without dark-green, or somewhat irregular vein-banding and blistering.
Because CABMV spreads under field conditions through increased aphid populations, availability of susceptible host plants and the presence of initial infection foci through seedborne infection, appropriate control can be achieved by limiting these three factors. CABMV is transmitted by several species of aphid in a non-persistent manner; therefore the use of insecticides may not be an appropriate approach. However, chemical control of aphids at proper times may be useful in keeping the vector population at a low level to avoid secondary spread of the disease. Until genetic engineering is further refined, breeding for host-plant resistance remains the most practical approach for the control of CABMV (Rossel and Thottappilly, 1985).
Excellent sources of resistance to CABMV have been identified among cowpea germplasms in the USA (Taiwo et al., 1982), Brazil (Lima et al., 1981), Nigeria (Williams, 1977; Ladipo and Allen, 1979a), Tanzania (Patel et al. 1982), Iran (Kaiser and Mossahebi, 1975), India (Mali et al., 1981), Zambia (Kannaiyan et al., 1987) and Senegal (Ndiaye et al., 1993).
Cisse et al. (1997) reported an extra early maturing cowpea line PI 596353 that was not only resistant to CABMV but also to the aphid vector (A. craccivora) and bacterial blight disease. Bashir and Hampton (1996c) tested 51 cowpea lines by mechanical inoculation under greenhouse conditions against seven CABMV geographical diverse isolates, and identified TVU-410, TVU-1582 and TVU-1593 as immune to all seven isolates. Several immune, resistant and tolerant genotypes were identified against individual isolates. Mligo (1989) developed a cowpea cultivar VU 1-1 through breeding resistance to CABMV and bacterial blight. Ladipo and Allen (1979b) identified some cowpea lines resistant to CABMV and A. craccivora, and to two other cowpea viruses (southern bean mosaic virus and cowpea mosaic virus). Mih et al. (1991) reported TVU-15656 to be a highly resistant line to both CABMV and cucumber mosaic virus. Sources of CABMV resistance, including those with combined resistance against several distinct viruses (Allen, 1980, 1983) have now been widely utilized in cowpea breeding, both in Nigeria (Singh et al. 1987) and elsewhere in Africa (Kannaiyan and Haciwa, 1993).
Three types of resistance to CABMV are recognized: immunity, hypersensitive resistance and viral tolerance; immunity is more common than others (Bashir and Hampton, 1996c). Resistance is also expressed as development of very mild mosaic without adverse effects on plant growth (latent infection) (Patel. et al., 1982) and also by tolerance in which systemic infection occurs without the appearance of symptoms (Ladipo and Allen, 1979a; Bashir and Hampton, 1996c). Several cowpea lines with isolate-specific resistance have been identified (Bashir and Hampton, 1996c).
No cowpea genotypes have been found to possess resistance to both CABMV and BlCMV; however, cowpea genotypes including TVUs 22, 410, 1582, 1593, 612, 1453, 1948, 2331, 2480, 2657, 2740, 3433, Big Boy, Corona and Serodo have been identified as useful differentials (Ladipo and Allen, 1979a; Patel et al., 1982; Taiwo et al., 1982; Bashir and Hampton, 1996b, c).
To date, no sources of resistance to CABMV have been identified in Passiflora.
Genetics of Resistance
Inheritance of resistance in cowpea to CABMV is governed by a single dominant or recessive gene (Taiwo et al., 1981; Fisher and Kyle, 1994, 1996), sometimes in association with minor and/or modifier genes (Patel et al., 1982). Provvidenti et al. (1983) reported that resistance to CABMV and BlCMV in common bean (Phaseolus vulgaris) was conferred independently by a single dominant factor that appears to be closely linked. Systemic resistance to CABMV in P. vulgaris cv. Great Northern 1140 (GN1140) is conditioned by a dominant allele which has been designated Cam2. Under some environmental conditions, a recessive allele at an unlinked locus Cam3 also controls a resistant response to CABMV (Fisher and Kyle, 1996). Monogenic dominant resistance to CABMV in P. vulgaris cv. Black Turtle Soup was reported by Fisher and Kyle (1994).
The virus can be controlled through cultural practices which include early sowing and intercropping of cowpea with cereals, possibly leading to decreased virus incidence (Kannaiyan and Haciwa, 1993). The use of virus-free seed is potentially important, particularly in preventing spread to new areas (Zettler and Evans, 1972).
Production of Virus-Free Seed
The production of virus-free seed is a potential measure for the control of CABMV, particularly if certified seed can be produced in areas where the virus is not known to occur (Zettler and Evans, 1972). Field inspection and roguing of diseased plants may help to eliminate seedborne inoculum, but because there is evidence that CABMV may also occur and occasionally be seed-transmitted in symptomless plants (Aboul-Ata et al., 1982; Bashir and Hampton, 1996c), there is potential value in implementing a rapid indexing procedure for the detection of CABMV in seedlots. It is well established that the level of seed transmission varies with cultivar (Bock and Conti, 1974; Kaiser and Mossahebi, 1975; Ladipo, 1977; Aboul-Ata et al., 1982). Cowpea genotypes resistant to seed transmission have been identified (Ladipo, 1977; Mali et al., 1981, 1983; Bashir and Hampton, 1994). Because the detection of virus in seed depends partly on the sensitivity of the assay (Konate and Neya, 1996), the incidence of seed transmission in some cultivars might be sufficiently low to control the disease significantly. As the minimum level of seedborne inoculum at which an epidemic may be initiated is still unknown, selection for resistance to seed transmission could prove a useful strategy for CABMV control (Aboul-Ata et al., 1982).
Resistance to Seed Transmission
The identification of cowpea cultivars/lines that prevent seed transmission of the virus or permit only minimum seed transmission would help to decrease the transmission rate. Cowpea lines resistant to seed transmission have been identified (Ladipo, 1977; Mali et al., 1983).
Control Through Seed Certification
Certification against seedborne viruses such as CABMV is one of the methods which minimizes their spread and it must be used in the production of certified seed. The seed certification programme should be started at the basic level of the germplasm collection available to the plant breeders, and continue through the subsequent development of varieties. Moreover, such programmes must also take into consideration other means by which a particular seedborne virus may be disseminated in the standing crop. The major method of monitoring the presence of seedborne viruses, i.e., visual inspection, should be followed in the standing crop. A seed certification programme for CABMV is being practised at IITA, Ibadan, Nigeria. Samples of 1000 seeds are planted and the resulting seedlings are examined visually for the presence of seed-transmitted viruses. Differential hosts and serological tests are used to facilitate identification where appropriate. Seed lots showing 2% seed transmission or greater are not distributed; those with less than 2% seed transmission are so indicated and the recipient is advised. ELISA is the most reliable method for virus detection from seed and plant tissue (Hamilton, 1983).
Resistance to Aphid Vector
Cowpea cultivars possessing resistance or tolerance to the CABMV aphid vector (Aphis craccivora) have been identified (Chari et al., 1976; Singh and Allen, 1979, 1980), but it is not known whether their use would limit the rate of CABMV spread.
Chemical Control of Virus Vector
Although control of disease through chemical treatment of aphids in the case of potyviruses is not effective, certain insecticides may possibly have potential in controlling CABMV. Organophosphates and carbamate had no effect. Atiri et al. (1987) found synthetic pyrethroid cypermethrin restricts the acquisition and inoculation of the virus, and protects against its transmission; however, the initial virus introduction was not prevented by these synthetic pyrethroids when the incidence of incoming alatae aphids was high, and virus incidence was higher in sprayed plots relative to unsprayed controls (Roberts et al., 1993).
Raheja and Leleji (1974) reported the complete loss of a cowpea crop in northern Nigeria resulting from CABMV attack under irrigated field conditions. Kaiser et al. (1968) reported 44-80% seed yield reduction in cowpea in greenhouse studies. A yield loss of 13-87% due to natural infection of cowpea by CABMV was reported in Iran (Kaiser and Mossahebi, 1975), and 48-60% loss in cowpea was reported in Zambia (Kannaiyan and Haciwa, 1993).
Passionfruit woodiness caused by CABMV is a devastating disease, and one of the main limiting factors to passionfruit yield in South America and Africa. The complete absence of control measures forces growers in Brazil, the world's largest producer of passionfruit, to grow passionfruit as an annual crop, replanting it every year, despite the plant’s potential to remain productive for many years.