Inheritance of the resistant reaction of the lettuce cultivar 'Grand Rapids' to the southern root-knot nematode Meloidogyne incognita (Kofoid & White) Chitwood

Resistance to the southern root-knot nematode Meloidogyne incognita Chitwoo d would be an important attribute of lettuce Lactuca sativa L. cultivars ad apted to both protected and field cultivation in tropical regions. 'Grand R apids' and a few other cultivars are reported to be resistant to this nemat ode. In this paper, we studied the inheritance of the resistant reaction of 'Grand Rapids' (P-2) in a cross with a standard nematode-susceptible culti var Regma-71 (P-1). F-1 (Regina-71 x Grand Rapids) and F-2 seed were obtain ed, and inoculated along with the parental cultivars with different races o f M. incognita to evaluate nematode resistance. Broad sense heritability es timates for the number of galls and of eggmasses per root system, gall size and gall index were generally in the order of 0.5 or higher. Class distrib utions of these variables over generations P-1, P-2, F-1 and F-2 were in ag reement with simulated theoretical distributions based on monogenic inherit ance models. F-3 families were obtained from randomly sampled F-2 plants an d tested for reaction to the nematode. The frequency ratio of homozygous re sistant, segregating and homozygous susceptible F-3 families did not differ from the 1:2:1 ratio expected from monogenic inheritance. M. incognita res istance appears to be under control of a single gene locus. The Grand Rapid s allele (for which the symbol Me is proposed) is responsible for the resis tant reaction, and shows high (though incomplete) penetrance, variable expr essivity and predominantly additive gene action.