Characteristics of natural resistance to the infective Frankia strain type AgI-WD1 were investigated in different progenies of the host species Alnus glutinosa (L.) Gaertn. with the aid of nodulation experiments. This strain type of Frankia was abundant in the soil of an A. glutinosa stand at Voorne, The Netherlands. Soil suspensions from this site produced high numbers of ineffective root nodules on some of the A. glutinosa seedlings used in biotests, but a significant number of the seedlings did not produce ineffective nodules. Resistance (R) or susceptibility (S) of the seedlings to frankia strain type AgI-WD1, as expressed by absence or presence of ineffective root nodules, proved to be persistent in cloned plants of each phenotype. Both R and S-seedlings were susceptible were susceptible to effective Frankia strains in the soil suspensions. Variation in nodulation time was examined among cloned plants of S-seedlings selected for this trait. Average nodulation times of three selected clones were 11, 26 and 39 d, respectively. Relative growth rates of cloned plants showed no relationship with nodulation time. Both R and S phenotypes were present in all seed-lots of A. glutinosa under study, but the frequency of R phenotypes of seed-lots from different locations ranged from 24 to 100%. R-frequencies of seed-lots of nine trees from a small population of A. glutinosa segregated into two distinct frequency classes which suggested genotypic differentiation of the R-trait among these trees. A significant correlation was found between R-frequencies of seedlots from the same trees over successive years.
