The effects of supplying the fertilizer nitrogen (N) as a recommended quantity of ammonium nitrate or as a commonly used dose of poultry manure on yield of sugarbeet infected with Beet mild yellowing virus (BMYV) or Beet yellows virus (BYV) were studied in field experiments at IACR-Broom's Barn in 1990, 1991 and 1992. Three N fertilizer treatments comprising Zero (N0), standard rate of 110 kg N/ha (N1) and poultry manure equivalent to c. 300 kg/ha of available N (N2) were applied to plots which were uninoculated or were subsequently inoculated with either BMYV or BYV. Averaged over virus treatments, N1 increased sugar yields by 23% relative to N0: there was no further increase when N2 was applied. When averaged over N treatments, early virus yellows infection reduced the sugar yields by 23%. Generally there was no significant interaction between N supply and virus infection. There was no evidence that the large N supply could reduce the yield effect of virus yellows infection, as had previously been thought. Crops infected from late July produced similar yields to uninoculated controls. The main effect of virus yellows was to reduce the efficiency of radiation conversion even when account was taken of the light intercepted by yellow foliage. Whilst the N2 treatment helped to maintain a green leaf cover throughout the season on virus yellows infected crops, it had no effect on virus replication. Beet processing quality was impaired by increasing the N supply and by virus infection, but again there were generally no significant interactions between infection and N rate.