Before TYLCV-DNA probes became available, selection for
TYLCV resistance was based on the absence (or attenuation) of
symptoms in infested fields and in amelioration of yields. Today,
we realize that all tomato cultivars and lines currently available
are, at best, tolerant to TYLCV; although yields and symptoms
have much improved compared with susceptible cultivars, plants
contain various concentrations of virus (12). To the best of our
knowledge, none of the tomato cultivars are resistant (no symptoms,
no virus). The only wild tomato species reported to be resistant
upon whitefly-mediated inoculation was L. chilense LA1969
(21). Mapping and introgression of resistance from L. chilense has
led to a tolerant line, in which virus replication and movement is
slower than in the susceptible near-isogenic line (13). We have
initiated a breeding program with the ultimate goal of obtaining
cultivars totally resistant to whitefly-mediated inoculation. Screening
of accessions of wild tomato species reported to be resistant to
TYLCV has shown that susceptible, tolerant, and resistant individuals
can be found within each accession.