We have shown that Mi-mediated aphid resistance
is isolate-speciÞc, and it may not extend to all aphid
species. Our data indicate that tomato varieties carrying
Mi are susceptible to green peach aphid isolates
collected from three different host plant species. Either
virulence is widespreadamonggreen peach aphid
populations, or Mi is not active against this species.
Preliminary experiments with the bean aphid, Aphis
fabae Scopoli, and the melon aphid, Aphis gossypii
Glover, also suggest that Mi may not be effective
against these aphid species (D. Holtzman, F.L.G., and
S. Seah, unpublished data).
Our data also indicate that tomato varieties carrying
Mi are resistant to some but not all isolates of the
potato aphid. Furthermore, our Þndings demonstrate
that virulent or resistance-breaking potato aphid isolates
are widespread, and can retain their virulence