MSV and WDV cause serious disease in maize and wheat crops (Van Rensburg & Kuhn, 1977; Rose, 1978; Mzira, 1984; Kim et al., 1989; Lindsten et al., 1980). Streak disease affects barley, wheat, oats, sugarcane, pearl millet (Pennisetum typhoides) and finger millet (Eleusine coracana) crops in southern and East Africa, but is of minor economic importance (Soto & Buddenhagen, 1982). The other members of the Mastrevirus genus mainly affect wild grasses and have little economic impact (Greber, 1989). Again, MSV is epidemiologically the best studied virus of this subgroup.
Yield loss in maize due to maize streak disease is directly related to time of infection - infected seedlings produce no yield or are killed, while plants infected at the 2nd, 4th, 6th, 8th, and 10th leaf stages lose approximately 55%, 45%, 40%, 33%, and 25% in grain weight loss, respectively (Bock, 1982).
MSV epidemics are more severe in tropical regions (Rose, 1978; Autrey & Ricaud, 1983). Virus spread between crops is facilitated by successive cropping and the presence of wild grasses as reservoirs of both virus and vectors (Autrey & Ricaud, 1983), as well as the wide variety of plant species that leafhoppers feed on, the ability of the vectors to transmit MSV persistently, and the insects' often considerable migration distances (Rose, 1978). In the warm wet season, the fecund, long-bodied form of C. mbila is produced. This morph flies less than 10 metres, and only isolated pockets of disease develop. However, with the onset of crop maturity or under drought conditions - which cause the food plants of leafhoppers to dry out - the stronger-flying, short-bodied form of C. mbila is produced. Extensive migration into irrigated crops occurs, spreading disease over great distances and resulting in widespread crop failure (Rose, 1978).
Disease avoidance can be practiced by adjusting planting dates to avoid migrating leafhoppers landing on young plants (Rose, 1978). The vector can be controlled by applying systemic insecticides (eg. Carbofuran) to the planting furrow during maize planting (Mzira, 1984).
However, the development and use of streak-resistant cultivars is probably the most effective and economically viable means of preventing streak epidemics. Naturally occurring resistance to MSV has been found in maize on Reunion Island (where MSV has long been endemic). The resistance appeared to be simply inherited and was rapidly fixed in breeding, being easily transferred to other maize lines (Soto & Buddenhagen, 1982). The incorporated resistance resulted in lowered disease incidence and reduced disease severity (Soto & Buddenhagen, 1982; Bock, 1982; Damsteegt, 1983). More recently, it has been shown that resistance to MSV in maize is quantitatively inherited, with relatively small numbers of genes involved (Kim et al., 1989). Approximately five pairs of different alleles, both dominant and recessive, confer MSV resistance in maize. (G. J. M. A. Gorter, pers. comm. to E. Rybicki). Thus, it is envisaged that simple recurrent selection or modified back-cross breeding methods could be used to breed for MSV resistance in Africa (Kim et al., 1989).