In this work, the cultivar Marmande was used as the susceptible parent and line 825, a TMV-
homozygous resistant (Tm-2a/Tm-2a) version of Marmande, was used as the resistant parent. Herein,
Marmande and line 825 are respectively referred to as 'Marmande Susceptible'and 'Marmande Resistant'.
Crosses were made between 'Marmande Resistant' and 'Marmande Susceptible', and the F
1
hybrid was selfed
and backcrossed to 'Marmande Susceptible'. 'Marmande Susceptible' servedas the female parent in each
cross. Seeds from crosses between different plants were collected separately and self-pollinated seed was
harvested individually from plants in parent and F1 generations and saved as families.
Both cotyledons of 2-wk old plants from different generations were inoculated with a TMV isolate of
strain 0 in a greenhouse chamber at 20-21oC. Immediately after inoculation, plants of each line were placed in
two growth-chambers at 30-31 and 20-21oC (control) with a 14-hr photoperiod. The final disease record was
taken 6-12 wk after inoculation.
At 30-31oC, 63 to 72% of the F1 plants reacted systemically (Table 1). The F1 progenies showed only the
local reaction at 20-21oC, as was expected. None of the self-pollinate progenies of the resistant parents
developed systemic symptoms at either temperature regime.
When F2 and BC1F1 lines derived from individual F1 plants with each of the phenotypes occurring at high
temperature (local reaction and systemic necrosis) were inoculated and incubated at 30-31oC, three classes -
local reaction, systemic necrosis, and mosaic - were observed. In each of these generations, the incidence of
systemic necrosis in lines derived from necrotic (systemic reaction) and non-necrotic (local reaction) F1 plants
was found to be similar. At 20-21oC, the F2 and BC1F1 lines segregated for local reaction and mosaic in ratios of
3:1 and 1:1, respectively, as was expected. Thus, no evidence was obtained that the occurrence of two types of
symptoms in plants of genotype Tm-2a/+ at high temperature was the result of genetic differences in the
parental material. Furthermore, segregation for local reaction and systemic necrosis was also found to occur in
vegetatively propagated populations (clones), each derived from a different F1 (Tm-2a/+) plant.
The mechanism of localization of TMV infection in Tm-2a/+ plants could be overcome, i.e., systemic
necrosis occurred, if larger numbers of virus particles were introduced into the cotyledons by repeated
inoculations prior to their transfer to high temperature. It is therefore concluded that the absence of systemic
necrosis in some of the Tm-2a/+ plants which were inoculated with TMV only once was due to the fact that the
amount of virus introduced into the cotyledons by rubbing was less than the minimum required for the
development of systemic symptoms.
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