Gilbert, J. C. and D. C. McGuire
Additional information on the inheritance of resistance to severe galling from the root knot nematode, Meloidogyne incognita, has been obtained. This year's data confirm earlier tests which suggested a single dominant gene for gall resistance in derivatives of L. peruvianum, P.I. 128657.
Tests of progeny of 19 segregating selections produced 922 resistant to 339 susceptibles, which fits the 3:1 ratio at the .05 level. Backcrosses of two resistant F1's (HES 4969 x Oahu and 4846 x Homestead) to their susceptible parents produced 109 resistant to 110 susceptible seedlings as a total of both tests. Previous tests have shown 4969 and 4846 to breed true for resistance.
In past work at the University of Hawaii, progeny tests were made only from the horticulturally most promising plants. When segregating lines were sampled in that way, there was a consistent failure to recover the expected proportion of homozygous resistant segregates from the plants which were phenotypically gall resistant. In some tests, no true breeding lines were found while in other tests one might be found in every 10 or 15 selections. Because of the distinct possibility that selections based on horticultural characters might be biased in favor of the heterozygotes, all of the phenotypically resistant plants in the segregating lines 5149, 5153 and 5273 were progeny-tested. After retesting the apparently true breeding segregates, the three lines showed 26 true breeding lines and 56 which were still segregating for gall resistance. This further confirms the single major gene hypothesis. Since this resistance is not effective against all species of root knot nematodes but rather to Meloidogyne incognita (the common species in Hawaii and the southern part of the U.S.A.), the symbol Mi is assigned to the gene for this type of root knot resistance in tomato.