Epidemiology of Maize Streak Disease

Abstract

Maize streak disease virus is spread by several species of leafhoppers that belong to the genus Cicadulina (23, 77, 107, 108, 117). No other method of spread is known (78, 108, 124). Consequently, the epidemiology of maize streak disease is largely an expre,ssion of the biology, phenology, and dispersal behavior of Cicadulina species in relationship to the virus and its hosts. The virulent form of streak disease in maize, strain maize streak A (62), causes severe losses in irrigated maize crops in many parts of Africa. The first sign of the disease is the appearance of pale, circular spots 0.5-2.0 mm in diameter on the lowest exposed portions of the youngest leaves of a maize plant. As the leaves grow the numbers of spots increase and some elongate. Leaves fully affected are character­ ized by a pronounced chlorosis, which is confined to narrow broken stripes along the veins (108). The stripes are caused through failure of the chloroplasts to develop in tissues surrounding the vascular bundles. The primary veins are less affected than the secondary and tertiary veins, so that there tend to be pronounced groups of five to seven parallel streaks on leaves (62). Only new growth develops these symptoms and there are normally green leaves at the base of the diseased plants. These enable the stage of the plant at time of infection to be estimated. Plants infected at an early stage of growth become stunted and produce poor cobs. Photosynthesis is reduced, respiration is increased, and there is a reduction in leaf length and height of plants (A. C. Whitwell, unpublished data). With increasing age at time of infection, the rate of decline in maize yield falls logarithmically (133). Plants infected less than a week after germination produce no yield, at 3 weeks about half yield, and at 8 weeks nearly full yield of maize. During an epidemic nearly every plant within a field of maize may be infected within 8 weeks of germination, and then it is not worth harvesting. As many as SO hectares of irrigated maize with streak disease have sometimes been destroyed in this way (85).