This study of resistance to the root-knot-nematode disease of tobacco (Meloidogyne incognita) was begun in 1935. Collections of Nicotiana tabacum were obtained from Mexico, Central and South America and tested for resistance. TI 706 was selected as the best source of root knot resistance within the cultivated species. Resistance was carried through the original TI 706 X flue-cured cross and four backcrosses to susceptible flue-cured tobacco. The 706 resistance proved to be intermediate with respect to dominance. F₂ populations from crosses between resistant lines and susceptible flue-cured varieties had from 3 to 5% of highly resistant individuals. TI 706 plants branched freely, flowered early, and had small leaves. During the backcross program all undesirable growth characters were eliminated except small leaf size. The best resistant selections had much larger leaves than 706, but in no case were leaves equal in size to the flue-cured parents. In an attempt to break the apparent linkage between small leaf size and resistance, a breeding line, RK 42, was crossed with alloploid N. sylvestris × N. tomentosiformis. Following this cross, very high root knot resistance was recovered in the F₁ and F₂ generations, and simple monogenic, dominant segregation was indicated. In subsequent F₂ generations, from backcrosses to susceptible flue-cured tobacco, there ·was a deficiency of resistant plants, indicating modifier gene action. The leaf size problem was completely eliminated. Measurements of resistant F₃ plants showed that these had broader and larger leaves than flue-cured variety 402. The cross with alloploid N. sylvestris × N. tomentosiformis changed the expression of TI 706 root knot resistance from an intermediate to a dominant and the yield of resistant plants in the F₂ from 5 per cent or less to over 60 per cent, and eliminated all linkage between root knot resistance and small leaf size. During the progress of this investigation the level of recoverable root knot resistance has been raised twice. First, out of TI 706 × flue-cured and backcrossed to flue-cured, stable lines were isolated that were more highly resistant than the 706 parent. Second, when a resistant line was crossed with alloploid N. sylvestris × N, tomentosiformis and backcrossed to flue-cured, genotypes with still higher resistance were recovered. Investigation of the root knot resistance of species of Nicotiana indicated that N. repanda and N. megslosiphon were immune. However, resistance transferred from N. megslosiphon into the tobacco genome appeared to be similar to that already obtained from TI 706. Other crosses with N. plumbaginifolia also failed to yield superior root knot resistance.

(Full article published with kind permission from "Tobacco International")