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Symptoms first show at the top of the plant. Apical leaves bulge slightly toward the base at the ends of the lobes and in the areas between the main veins. These areas are light green to bluish, but as the leaf ages the zone expands to include the entire leaf blade, which then darkens to a deep bluish-green. In addition to the dark green or bluish colour of infected leaves, other symptoms include severe epinasty and stunting, particularly when plants are infected early in the growing season. Leaves roll under, become leathery in texture, and may show reddened petioles and veins. Affected plants are usually dwarfed and stems exhibit a distinctive zigzag pattern of growth. Symptoms do not appear on organs existing prior to the infection. Remission may occur if plants are cut back and allowed to regrow (Delalande, 1970; Cauquil and Follin, 1983).
Triple hybrid cotton lines, originating from Côte d'Ivoire and arising from introductions of gerrmplasm from the USA, have shown good disease resistance. Crosses of Gossypium hirsutum, G. arboreum and G. raimondii are resistant to the disease, with G. arboreum serving as the primary source of resistance. Additional crosses have been made with different African and American varieties, using these triple hybrids as parents to produce HAR G198-9/BJA-610-1186 which gives good yields and has high fibre quality. One selection from Chad, SRI-F4, has been successfully cultivated since 1974 and has been commonly grown throughout Africa. Although it is resistant under natural conditions, graft inoculation has shown that it is susceptible to the disease (Cauquil, 1981;Cauquil and Follin, 1983).
The most important cultural control is the absolute eradication of infected cotton from fields at the end of the growing season or prior to early-season spring migrations of the aphid vector. In addition, avoidance of infection of cotton at the early developmental stages may be accomplished with early planting. Good cultural practices, particularly tillage to control weeds which might serve as hosts of the aphid vector, and fertilizer application to promote the growth of a vigorous plant, reportedly helps combat the effects of the disease. High density (70-80,000 plants/ha) planting of cotton is also recommended.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
During the epidemic years, disease incidence regularly reached 25-30% in some fields, and overall production was reduced by an estimated 3-6%. The disease is now known to occur in Benin, Chad, Cameroon, Côte d'Ivoire and Zaire. Although the impact of the disease in certain areas has diminished since the 1966-68 epidemic when cultivation of tolerant or resistant varieties was implemented, the disease remains a problem in most cotton producing areas of Africa.
Approximately 3-4% of the affected plants die, while about 6% of the plants recover from the symptoms. The severity and degree of impact of the disease depends on plant age at the time of infection.
Early season infection <50 days post-emergence) results in complete loss of yield in susceptible cotton varieties, whereas infection occurring 100 days post-emergence results in yield losses of 15-20%. Yield losses from blue disease are due to the production of fewer and smaller flowers and increased boll shedding. Infected plants exhibit a low seed index and reduced fibre quality, particularly reduced fibre length and decreased fibre resistance.