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Leaves of infected plants show dark-green mottles, with dark-green spotting, patching and banding adjacent to the veins. Mottling is most conspicuous on young leaves. Affected leaves are generally smaller in size and show varying degrees of distortion. Plants infected early are usually stunted, show dark-green streaks on the stems and branches, and most of their flowers drop before bearing fruits. The few and small fruits which are produced are occasionally mottled and/or distorted.
The severity of symptoms depends upon the cultivars and the duration of infection.
The only effective, economic and feasible control of ChiVMV appears to be the planting of resistant or tolerant cultivars.
Resistant Crop Cultivars
Cultivars/lines of Capsicum annuum and C. frutescens that are resistant or tolerant to ChiVMV have been identified from a breeding programme initiated in 1986 at the Asian Vegetable Research and Development Center (AVRDC) (Yoon et al., 1989) and complemented since 1989 by the pepper virus sub-network of the Asian Vegetable Network, funded by the Asian Development Bank (AVRDC, 1990b, c, 1991, 1992, 1993). The resistances of these cultivars/lines are now being incorporated into AVRDC's promising breeding lines.
ChiVMV resistance is governed by a single recessive gene (Chew and Ong, 1990). Nineteen ChiVMV-resistant breeding lines with good horticultural characteristics have been developed in Malaysia by crossing adapted susceptible genotypes with resistant sources. Researchers are incorporating cucumber mosaic cucumovirus (CMV) resistance genes into existing ChiVMV-resistant progenies from the breeding lines. These multiple virus-resistant lines could be ready for distribution to farmers within a few years (AVRDC, 1993).
Cultural Practices (Vector Reduction)
Intercropping of maize with Capsicum species and planting maize as a barrier crop reduce vector populations and virus incidence (Roff and Ho, 1991; Kittipakorn et al., 1994).
To reduce aphid populations and virus incidence, to delay virus infection, and to increase the yield of capsicums, reflective mulches of locally produced aluminium-painted or silver-coated polyethylene sheets have been applied by farmers in Malaysia and Taiwan (Chen and Chen, 1980, 1983; Ong, 1984). This practice is usually accompanied by weekly (or more frequent) applications of insecticide.
Experimentally, the use of mineral oil alone or in combination with reflective mulches has been shown to delay virus infection and to reduce virus incidence (Su, 1982; Cheng and Tung, 1987). Use of skimmed milk sprays and colour-bait techniques, (e.g. yellow water-troughs which attract aphids away from the crop and kill them by drowning), has been shown to reduce the number of aphids and incidence of virus infection and to increase the yield of capsicums (Kittipakorn et al., 1994; Ong, 1984). Covering the seedbeds with 80 mesh Nylon or cheesecloth protects plants from the aphids and so can eliminate the virus completely in seedlings; thus it can delay virus infection and reduce virus incidence in the field (Cheng and Tung, 1987; Fujisawa et al., 1990).
For capsicum plants grown under shelter, the use of plastic mulch and yellow or white sticky traps has been shown significantly to reduce the aphid populations (Cheng and Tung, 1987; Kuo and Wu, 1989).
An integrated management package to control virus diseases of Capsicum species has been developed in Malaysia (Green, 1992) utilizing silvery reflective sheets, insect-proof cages for raising seedlings, prophylactic insecticide application and regular (fortnightly) foliar fertilizer treatments.
ChiVMV is widely distributed and greatly reduces crop yield in capsicum-growing areas of peninsular Malaysia. Yield reduction as high as 60% results from early infection (Ong and Ting, 1977; Ong et al., 1980). Infected plants yield fewer flowers, resulting in a reduced number of smaller and lighter fruit being produced.