A plant and field study of BYDV-PAV and -MAV distribution on maize in France

The distribution of BYD viruses PAV and MAV in maize was studied in 1996 an d 1997 at plant and field levels in western France. Distribution of viruses was studied in individual maize plants grown in a glasshouse and inoculate d in the laboratory by viruliferous Rhopalosiphum padi (PAV) and Sitobion a venae (MAV). In a first experiment, we studied the influence of the growth stage of maize when inoculated on the success of infection. For both viruse s 100 % infection were obtained for maize plants inoculated at 2, 4 and 6 l eaf stages. In case of inoculations at an older stage, the percentage of in fection decreased drastically for PAV and more slowly for MAV. In a second experiment the kinetics of PAV and MAV was studied in maize plants inoculat ed at 4 leaf stage. PAV was detected more frequently than MAV in roots and both viruses develop mainly in upper leaves. Leaves 8 and 9 were the more c onsistantly infected by PAV and could be used for field sampling. The distr ibution of MAV and PAV was studied weekly in a maize field during spring an d summer 1997, by regular sampling several hundreds of plants. The results showed clearly a lack of secondary spread of the viruses:50-80 % of the pla nts were infected in the few days following the infestation of the maize cr op by the alate aphid vectors at mid-June, this percentage of infected plan ts remained unchanged during summer.