Genetic Relationships for Resistance to Heterodera glycines Races 3 and 5 in Soybean.

Heterodera glycines is the most damaging root pathogen of soybean in the United States. Plant introduction (PI) 90.763 is resistant to both races 3 and 5, whereas PI 424.595 is resistant only to race 5. Genetic relationships in these PI lines for resistance to H. glycines are not known. Crosses between PI 90.763 and PI 424.595 and susceptible cv. Essex were studied in the F, F, and F generations to compare the genes involved in resistance to race 3 with those for resistance to race 5. The F plants also were studied separately for reaction to the two races by dividing roots into separate pots. Plants were screened using conventional techniques and classified as either resistant or susceptible based on an index of parasitism. Data were analyzed using X(2) to determine goodness of fit between observed and expected genetic ratios. The cross PI 90.763 x Essex segregated 3 resistant: 13 susceptible plants in the F generation, which indicated one dominant and one recessive gene, conditioned resistance to race 3. All progenies of the cross PI 424.595 x Essex, involving both susceptibles parents, were susceptible, indicating no genetic interaction. The cross of PI 90.763 x PI 424.595 showed monogenic inheritance with a dominant gene in PI 90.763. The divided root study of the cross PI 90.763 x Essex indicated that all race 5 resistant F plants also were resistant to race 3, whereas, in the cross PI 90.763 x PI 424.595, some of the race 5 resistant plants were susceptible to race 3. Similarly, all plants susceptible to race 3 also were susceptible to race 5 in cross PI 90.763 x Essex, but not for PI 90.763 x PI 424.595. The results indicated that the dominant resistance gene and one of the recessive resistance genes in PI 90.763 which conditioned resistance to race 5 also imparted resistance to race 3. The additional recessive gene that controlled resistance to race 5 in PI 424.595 was ineffective against race 3.