Clayberg, C. D.
Genetic resistance to tobacco mosaic virus has undeniable importance for varietal breeding. For this reason the following study was made comparing susceptibility levels of two known genes for resistance to the disease.
In summer 1958 plants of the genotypes Tm\1/Tm\1 (PI235673) and Tm\2/Tm\2 (P. G. Smith) were selfed and crossed to the susceptible variety Fireball. The next winter six plants each of these genotypes were grown: Tm\2/Tm\2, Tm\2/+, Tm\1/Tm\1, Tm\1/+, and +/+. One month after starting these from seed, five plants of each class were inoculated with virus by rubbing on the upper surface of two lower leaves per plant an expressate from freshly macerated leaves of young, vigorously growing tomato plants severely infected with a wild green strain of tobacco-mosaic virus.
Four weeks were allowed to elapse for adequate multiplication of the virus. The plants were then observed for degree of symptom expression. Following this, 1 1/2 grams of fresh weight were taken separately from the growing tip of each plant and thoroughly ground up with 5.8 cc. distilled water. The supernatant liquid was applied to leaves of Nicotiana glutinosa to measure infectivity of the inoculated plants. For every tomato plant two drops (ca. 0.1 cc.) were applied to each of three N. glutinosa leaves. The dropa were spread evenly over the leaf surface using a metal spatula. Equipment was decontaminated between inoculations involving different plants by washing in strong detergent solution and rinsing in 95% ethanol.
The results obtained upon counting lesions five to six days after inoculation are shown in the following table.
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Total lesions (three leaves)
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59L57 59L58 59L60 59L61 59L62
Plants Tm\2/Tm\2 Tm\2/+ Tm\1/Tm\1 Tm\1/+ +/+
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Uninoculated
Control 0 0 0 0 2
1 0 0 0 50 63
2 0 1 38 84 25
3 1 0 0 92 32
4 0 0 77 97 161
5 0 0 22 180 116
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Ave. plants 1-5 0.2 0.2 27.4* 100.6 79.4
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* Average for infected plants only = 45.7
Apparently Tm\2 is completely dominant and confers a level of resistance
almost equal to immunity. In contrast Tm\1 appears to show no dominance in
this test for resistance to virus build up in the host, although all infected
plants of 59L61 had mild to moderate mottling symptoms as compared with the
severe mottling in 59L62. The inoculum used here may have been more virulent
than that of Holmes (1954), which would account for my failure to observe
dominance of Tm\1.Two plants of 59L60 escaped infection, and the remaining plants of this family seemed to have higher resistance than either 59L61 or 59L62. Nevertheless, between-plant variability, as well as within-plant variability measured by differences among the three tobacco leaves infected for each tomato plant, cautions one against basing much quantitative interpretation on these results. No mottling or other sywtoms were exhibited by infected plants of 59L57 or 59L58 or any of the uninoculated controls.