The results of the evaluations performed in each population at two different times (CV1 and CV2) are listed in Table II. The results of the first evaluation were inconsistent with 3:1 segregation, as confirmed by the chi-square test (Table III), a fact possibly explained by the occurrence of resistance in adult plants, as mentioned by Mignucci and Lim (1980). When the first evaluation was performed, the plants were still at the seedlingstage, when the individuals tested may present susceptibility, even if carrying the resistance gene. Furthermore, according to Mignucci and Lim (1980), after the formation of the second 3-parted leaf, the symptoms tend to disappear and the same plants that appear to be susceptible in the young-plant stage may present a resistance reaction to the disease when adult.
Based on prior studies conducted in the United States, which pointed out that the genetic control of soybean resistance to powdery mildew is ruled by major genes (Grau and Laurence, 1975), the chi-square test was applied to each population evaluated and to each evaluation. Table III presents the results of the chi-square test for both evaluations, with 3:1 segregation being considered. It was confirmed that population 7 presented a high level of probability to accept 3:1 segregation, indicating that soybean resistance to powdery mildew is ruled by a single gene with a dominant effect, with two alleles. On the other hand, populations 1, 2, 4, and 5 also presented 3:1 segregation (0.70
The second evaluation of the level of leaf infection with M. diffusa was performed when the plants had already flowered. By comparing the data obtained in the first and second evaluations (Table II), it can be seen that the studied plants presented adult-plant resistance, as suggested by Mignucci and Lim (1980).
The results of the heterogeneity test are presented in Table IV. The non-significance of this test indicates that the results obtained by applying the chi-square test to the data for the eight populations studied are consistent, confirming that the genetic control of soybean resistance to powdery mildew is associated with a single gene with a dominant effect.