. When sweetpotato chlorotic stunt crinivirus (SPCSV) and sweetpotato feathery mottle potyvirus (SPFMV)
infect sweetpotato [Ipomoea batatas (L.) Lam.], they interact synergistically and cause sweetpotato virus disease (SPVD),
a major constraint to food productivity in east Africa. The genetic basis of resistance to these diseases was investigated
in 15 sweetpotato diallel families (1352 genotypes) in Uganda, and in two families of the same diallel at the International
Potato Center (CIP), Lima, Peru. Graft inoculation with SPCSV and SPFMV resulted in severe SPVD symptoms in all
the families in Uganda. The distribution of SPVD scores was skewed toward highly susceptible categories (SPVD scores
4 and 5), eliminating almost all the resistant genotypes (scores 1 and 2). Likewise, when two promising diallel families
(‘Tanzania’ x ‘Bikilamaliya’ and ‘Tanzania’ x ‘Wagabolige’) were graft inoculated with SPCSV and SPFMV at CIP,
severe SPVD was observed in most of the progenies. Individual inoculation of these two families with SPCSV or SPFMV,
and Mendelian segregation analysis for resistant vs. susceptible categories led us to hypothesize that resistance to SPCSV
and SPFMV was conditioned by two separate recessive genes inherited in a hexasomic or tetradisomic manner.
Subsequent molecular marker studies yielded two genetic markers associated with resistance to SPCSV and SPFMV. The
AFLP and RAPD markers linked to SPCSV and SPFMV resistance explained 70% and 72% of the variation in resistance,
respectively. We propose naming these genes as spcsv1 and spfmv1. Our results also suggest that, in the presence of both
of these viruses, additional genes mediate oligogenic or multigenic horizontal (quantitative) effects in the progenies
studied for resistance to SPVD.