ACQUISITION OF POTATO LEAFROLL VIRUS BY MYZUS-PERSICAE FROM SECONDARILY-INFECTED POTATO PLANTS OF DIFFERENT GENOTYPES

The acquisition of potato leafroll virus (PLRV) by Myzus persicae nymp hs from the top leaves of potato plants was studied throughout a growi ng season in relation to the antigen titre in those leaves and the fee ding behaviour of the aphid. Secondarily-infected plants of eight pota to genotypes with different levels of field resistance served as virus sources. Early in the growing season, plants were efficient sources f or virus acquisition. The amount of viral antigen detected in M. persi cae nymphs fed on the top leaves was strongly correlated with the titr es of viral antigen in these leaves. Virus acquisition from the top le aves of older potato plants was markedly impaired and could not be cor related with their virus titre. With increasing age of the potato plan ts and the development of virus symptoms, the virus titre in the leave s declined and the initial weak correlation between the virus titre an d field resistance ratings disappeared. Thus, screening secondarily-in fected potato plants for field resistance to PLRV based on the concent ration of viral antigen in leaves or in aphids fed on them should be a voided later in the growing season. The feeding rate of M. persicae, m easured by the number of honeydew droplets excreted, did not account f or the reduced uptake of virus from older plants since it was not infl uenced by the age of the plant. Throughout the growing season, the fee ding rate of M. persicae nymphs on PLRV-infected plants was higher on genotypes with low levels of field resistance to PLRV than on genotype s with high ones.